______________________________________________________________________
3 Basic concepts [basic]
______________________________________________________________________
1 [Note: this clause presents the basic concepts of the C++ language.
It explains the difference between an object and a name and how they
relate to the notion of an lvalue. It introduces the concepts of a
declaration and a definition and presents C++'s notion of type, scope,
linkage, and storage duration. The mechanisms for starting and termi-
nating a program are discussed. Finally, this clause presents the
fundamental types of the language and lists the ways of constructing
compound types from these.
2 This clause does not cover concepts that affect only a single part of
the language. Such concepts are discussed in the relevant clauses. ]
3 An entity is a value, object, subobject, base class subobject, array
element, variable, function, instance of a function, enumerator, type,
class member, template, or namespace.
4 A name is a use of an identifier (_lex.name_) that denotes an entity
or label (_stmt.goto_, _stmt.label_). A variable is introduced by the
declaration of an object. The variable's name denotes the object.
5 Every name that denotes an entity is introduced by a declaration.
Every name that denotes a label is introduced either by a goto state-
ment (_stmt.goto_) or a labeled-statement (_stmt.label_).
6 Some names denote types, classes, enumerations, or templates. In gen-
eral, it is necessary to determine whether or not a name denotes one
of these entities before parsing the program that contains it. The
process that determines this is called name lookup (_basic.lookup_).
7 Two names are the same if
--they are identifiers composed of the same character sequence; or
--they are the names of overloaded operator functions formed with the
same operator; or
--they are the names of user-defined conversion functions formed with
the same type.
8 An identifier used in more than one translation unit can potentially
refer to the same entity in these translation units depending on the
linkage (_basic.link_) of the identifier specified in each translation
unit.
3.1 Declarations and definitions [basic.def]
1 A declaration (clause _dcl.dcl_) introduces names into a translation
unit or redeclares names introduced by previous declarations. A dec-
laration specifies the interpretation and attributes of these names.
2 A declaration is a definition unless it declares a function without
specifying the function's body (_dcl.fct.def_), it contains the extern
specifier (_dcl.stc_) or a linkage-specification1) (_dcl.link_) and
neither an initializer nor a function-body, it declares a static data
member in a class declaration (_class.static_), it is a class name
declaration (_class.name_), or it is a typedef declaration (_dcl.type-
def_), a using-declaration (_namespace.udecl_), or a using-directive
(_namespace.udir_).
3 [Example: all but one of the following are definitions:
int a; // defines a
extern const int c = 1; // defines c
int f(int x) { return x+a; } // defines f and defines x
struct S { int a; int b; }; // defines S, S::a, and S::b
struct X { // defines X
int x; // defines nonstatic data member x
static int y; // declares static data member y
X(): x(0) { } // defines a constructor of X
};
int X::y = 1; // defines X::y
enum { up, down }; // defines up and down
namespace N { int d; } // defines N and N::d
namespace N1 = N; // defines N1
X anX; // defines anX
whereas these are just declarations:
extern int a; // declares a
extern const int c; // declares c
int f(int); // declares f
struct S; // declares S
typedef int Int; // declares Int
extern X anotherX; // declares anotherX
using N::d; // declares N::d
--end example]
4 [Note: in some circumstances, C++ implementations implicitly define
the default constructor (_class.ctor_), copy constructor
(_class.copy_), assignment operator (_class.copy_), or destructor
(_class.dtor_) member functions. [Example: given
_________________________
1) Appearing inside the braced-enclosed declaration-seq in a linkage-
specification does not affect whether a declaration is a definition.
struct C {
string s; // string is the standard library class (clause _lib.strings_)
};
int main()
{
C a;
C b = a;
b = a;
}
the implementation will implicitly define functions to make the defi-
nition of C equivalent to
struct C {
string s;
C(): s() { }
C(const C& x): s(x.s) { }
C& operator=(const C& x) { s = x.s; return *this; }
~C() { }
};
--end example] --end note]
5 [Note: a class name can also be implicitly declared by an elaborated-
type-specifier (_basic.scope.pdecl_). ]
6 A program is ill-formed if the definition of any object gives the
object an incomplete type (_basic.types_).
3.2 One definition rule [basic.def.odr]
1 No translation unit shall contain more than one definition of any
variable, function, class type, enumeration type or template.
2 An expression is potentially evaluated unless either it is the operand
of the sizeof operator (_expr.sizeof_), or it is the operand of the
typeid operator and does not designate an lvalue of polymorphic class
type (_expr.typeid_). An object or non-overloaded function is used if
its name appears in a potentially-evaluated expression. A virtual
member function is used if it is not pure. An overloaded function is
used if it is selected by overload resolution when referred to from a
potentially-evaluated expression. [Note: this covers calls to named
functions (_expr.call_), operator overloading (clause _over_), user-
defined conversions (_class.conv.fct_), allocation function for place-
ment new (_expr.new_), as well as non-default initialization
(_dcl.init_). A copy constructor is used even if the call is actually
elided by the implementation. ] An allocation or deallocation func-
tion for a class is used by a new expression appearing in a poten-
tially-evaluated expression as specified in _expr.new_ and
_class.free_. A deallocation function for a class is used by a delete
expression appearing in a potentially-evaluated expression as speci-
fied in _expr.delete_ and _class.free_. A copy-assignment function
for a class is used by an implicitly-defined copy-assignment function
for another class as specified in _class.copy_. A default constructor
for a class is used by default initialization as specified in
_dcl.init_. A constructor for a class is used as specified in
_dcl.init_. A destructor for a class is used as specified in
_class.dtor_.
3 Every program shall contain exactly one definition of every non-inline
function or object that is used in that program; no diagnostic
required. The definition can appear explicitly in the program, it can
be found in the standard or a user-defined library, or (when appropri-
ate) it is implicitly defined (see _class.ctor_, _class.dtor_ and
_class.copy_). An inline function shall be defined in every transla-
tion unit in which it is used.
4 Exactly one definition of a class is required in a translation unit if
the class is used in a way that requires the class type to be com-
plete. [Example: the following complete translation unit is well-
formed, even though it never defines X:
struct X; // declare X as a struct type
struct X* x1; // use X in pointer formation
X* x2; // use X in pointer formation
--end example] [Note: the rules for declarations and expressions
describe in which contexts complete class types are required. A class
type T must be complete if:
--an object of type T is defined (_basic.def_, _expr.new_), or
--an lvalue-to-rvalue conversion is applied to an lvalue referring to
an object of type T (_conv.lval_), or
--an expression is converted (either implicitly or explicitly) to type
T (clause _conv_, _expr.type.conv_, _expr.dynamic.cast_,
_expr.static.cast_, _expr.cast_), or
--an expression that is not a null pointer constant, and has type
other than void *, is converted to the type pointer to T or refer-
ence to T using an implicit conversion (clause _conv_), a
dynamic_cast (_expr.dynamic.cast_) or a static_cast
(_expr.static.cast_), or
--a class member access operator is applied to an expression of type T
(_expr.ref_), or
--the typeid operator (_expr.typeid_) or the sizeof operator
(_expr.sizeof_) is applied to an operand of type T, or
--a function with a return type or argument type of type T is defined
(_basic.def_) or called (_expr.call_), or
--an lvalue of type T is assigned to (_expr.ass_). ]
5 There can be more than one definition of a class type (clause
_class_), enumeration type (_dcl.enum_), inline function with external
linkage (_dcl.fct.spec_), class template (clause _temp_), non-static
function template (_temp.fct_), static data member of a class template
(_temp.static_), member function template (_temp.mem.func_), or tem-
plate specialization for which some template parameters are not
specified (_temp.spec_, _temp.class.spec_) in a program provided that
each definition appears in a different translation unit, and provided
the definitions satisfy the following requirements. Given such an
entity named D defined in more than one translation unit, then
--each definition of D shall consist of the same sequence of tokens;
and
--in each definition of D, corresponding names, looked up according to
_basic.lookup_, shall refer to an entity defined within the defini-
tion of D, or shall refer to the same entity, after overload resolu-
tion (_over.match_) and after matching of partial template special-
ization (_temp.over_), except that a name can refer to a const
object with internal or no linkage if the object has the same inte-
gral or enumeration type in all definitions of D, and the object is
initialized with a constant expression (_expr.const_), and the value
(but not the address) of the object is used, and the object has the
same value in all definitions of D; and
--in each definition of D, the overloaded operators referred to, the
implicit calls to conversion functions, constructors, operator new
functions and operator delete functions, shall refer to the same
function, or to a function defined within the definition of D; and
--in each definition of D, a default argument used by an (implicit or
explicit) function call is treated as if its token sequence were
present in the definition of D; that is, the default argument is
subject to the three requirements described above (and, if the
default argument has sub-expressions with default arguments, this
requirement applies recursively).2)
--if D is a class with an implicitly-declared constructor
(_class.ctor_), it is as if the constructor was implicitly defined
in every translation unit where it is used, and the implicit defini-
tion in every translation unit shall call the same constructor for a
base class or a class member of D. [Example:
_________________________
2) _dcl.fct.default_ describes how default argument names are looked
up.
// translation unit 1:
struct X {
X(int);
X(int, int);
};
X::X(int = 0) { }
class D: public X { };
D d2; // X(int) called by D()
// translation unit 2:
struct X {
X(int);
X(int, int);
};
X::X(int = 0, int = 0) { }
class D: public X { }; // X(int, int) called by D();
// D()'s implicit definition
// violates the ODR
--end example] If D is a template, and is defined in more than one
translation unit, then the last four requirements from the list
above shall apply to names from the template's enclosing scope used
in the template definition (_temp.nondep_), and also to dependent
names at the point of instantiation (_temp.dep_). If the defini-
tions of D satisfy all these requirements, then the program shall
behave as if there were a single definition of D. If the defini-
tions of D do not satisfy these requirements, then the behavior is
undefined.
3.3 Declarative regions and scopes [basic.scope]
1 Every name is introduced in some portion of program text called a
declarative region, which is the largest part of the program in which
that name is valid, that is, in which that name may be used as an
unqualified name to refer to the same entity. In general, each par-
ticular name is valid only within some possibly discontiguous portion
of program text called its scope. To determine the scope of a decla-
ration, it is sometimes convenient to refer to the potential scope of
a declaration. The scope of a declaration is the same as its poten-
tial scope unless the potential scope contains another declaration of
the same name. In that case, the potential scope of the declaration
in the inner (contained) declarative region is excluded from the scope
of the declaration in the outer (containing) declarative region.
2 [Example: in
int j = 24;
int main()
{
int i = j, j;
j = 42;
}
the identifier j is declared twice as a name (and used twice). The
declarative region of the first j includes the entire example. The
potential scope of the first j begins immediately after that j and
extends to the end of the program, but its (actual) scope excludes the
text between the , and the }. The declarative region of the second
declaration of j (the j immediately before the semicolon) includes all
the text between { and }, but its potential scope excludes the decla-
ration of i. The scope of the second declaration of j is the same as
its potential scope. ]
3 The names declared by a declaration are introduced into the scope in
which the declaration occurs, except that the presence of a friend
specifier (_class.friend_), certain uses of the elaborated-type-speci-
fier (_basic.scope.pdecl_), and using-directives (_namespace.udir_)
alter this general behavior.
4 Given a set of declarations in a single declarative region, each of
which specifies the same unqualified name,
--they shall all refer to the same entity, or all refer to functions
and function templates; or
--exactly one declaration shall declare a class name or enumeration
name that is not a typedef name and the other declarations shall all
refer to the same object or enumerator, or all refer to functions
and function templates; in this case the class name or enumeration
name is hidden (_basic.scope.hiding_). [Note: a namespace name or a
class template name must be unique in its declarative region
(_namespace.alias_, clause _temp_). ]
[Note: these restrictions apply to the declarative region into which a
name is introduced, which is not necessarily the same as the region in
which the declaration occurs. In particular, elaborated-type-speci-
fiers (_basic.scope.pdecl_) and friend declarations (_class.friend_)
may introduce a (possibly not visible) name into an enclosing names-
pace; these restrictions apply to that region. Local extern declara-
tions (_basic.link_) may introduce a name into the declarative region
where the declaration appears and also introduce a (possibly not visi-
ble) name into an enclosing namespace; these restrictions apply to
both regions. ]
5 [Note: the name lookup rules are summarized in _basic.lookup_. ]
3.3.1 Point of declaration [basic.scope.pdecl]
1 The point of declaration for a name is immediately after its complete
declarator (clause _dcl.decl_) and before its initializer (if any),
except as noted below. [Example:
int x = 12;
{ int x = x; }
Here the second x is initialized with its own (indeterminate) value.
]
2 [Note: a nonlocal name remains visible up to the point of declaration
of the local name that hides it. [Example:
const int i = 2;
{ int i[i]; }
declares a local array of two integers. ] ]
3 The point of declaration for an enumerator is immediately after its
enumerator-definition. [Example:
const int x = 12;
{ enum { x = x }; }
Here, the enumerator x is initialized with the value of the constant
x, namely 12. ]
4 After the point of declaration of a class member, the member name can
be looked up in the scope of its class. [Note: this is true even if
the class is an incomplete class. For example,
struct X {
enum E { z = 16 };
int b[X::z]; // OK
};
--end note]
5 The point of declaration of a class first declared in an elaborated-
type-specifier is as follows:
--for an elaborated-type-specifier of the form
class-key identifier ;
the elaborated-type-specifier declares the identifier to be a class-
name in the scope that contains the declaration, otherwise
--for an elaborated-type-specifier of the form
class-key identifier
if the elaborated-type-specifier is used in the decl-specifier-seq
or parameter-declaration-clause of a function defined in namespace
scope, the identifier is declared as a class-name in the namespace
that contains the declaration; otherwise, except as a friend decla-
ration, the identifier is declared in the smallest non-class, non-
function-prototype scope that contains the declaration. [Note: if
the elaborated-type-specifier designates an enumeration, the identi-
fier must refer to an already declared enum-name. If the identifier
in the elaborated-type-specifier is a qualified-id, it must refer to
an already declared class-name or enum-name. See
_basic.lookup.elab_. ]
6 [Note: friend declarations refer to functions or classes that are mem-
bers of the nearest enclosing namespace, but they do not introduce new
names into that namespace (_namespace.memdef_). Function declarations
at block scope and object declarations with the extern specifier at
block scope refer to delarations that are members of an enclosing
namespace, but they do not introduce new names into that scope. ]
7 [Note: For point of instantiation of a template, see _temp.inst_. ]
3.3.2 Local scope [basic.scope.local]
1 A name declared in a block (_stmt.block_) is local to that block. Its
potential scope begins at its point of declaration
(_basic.scope.pdecl_) and ends at the end of its declarative region.
2 The potential scope of a function parameter name in a function defini-
tion (_dcl.fct.def_) begins at its point of declaration. If the func-
tion has a function try-block the potential scope of a parameter ends
at the end of the last associated handler, else it ends at the end of
the outermost block of the function definition. A parameter name
shall not be redeclared in the outermost block of the function defini-
tion nor in the outermost block of any handler associated with a func-
tion try-block .
3 The name in a catch exception-declaration is local to the handler and
shall not be redeclared in the outermost block of the handler.
4 Names declared in the for-init-statement, and in the condition of if,
while, for, and switch statements are local to the if, while, for, or
switch statement (including the controlled statement), and shall not
be redeclared in a subsequent condition of that statement nor in the
outermost block (or, for the if statement, any of the outermost
blocks) of the controlled statement; see _stmt.select_.
3.3.3 Function prototype scope [basic.scope.proto]
1 In a function declaration, or in any function declarator except the
declarator of a function definition (_dcl.fct.def_), names of parame-
ters (if supplied) have function prototype scope, which terminates at
the end of the nearest enclosing function declarator.
3.3.4 Function scope [basic.funscope]
1 Labels (_stmt.label_) have function scope and may be used anywhere in
the function in which they are declared. Only labels have function
scope.
3.3.5 Namespace scope [basic.scope.namespace]
1 The declarative region of a namespace-definition is its namespace-
body. The potential scope denoted by an original-namespace-name is
the concatenation of the declarative regions established by each of
the namespace-definitions in the same declarative region with that
original-namespace-name. Entities declared in a namespace-body are
said to be members of the namespace, and names introduced by these
declarations into the declarative region of the namespace are said to
be member names of the namespace. A namespace member name has names-
pace scope. Its potential scope includes its namespace from the
name's point of declaration (_basic.scope.pdecl_) onwards; and for
each using-directive (_namespace.udir_) that nominates the member's
namespace, the member's potential scope includes that portion of the
potential scope of the using-directive that follows the member's point
of declaration. [Example:
namespace N {
int i;
int g(int a) { return a; }
int j();
void q();
}
namespace { int l=1; }
// the potential scope of l is from its point of declaration
// to the end of the translation unit
namespace N {
int g(char a) // overloads N::g(int)
{
return l+a; // l is from unnamed namespace
}
int i; // error: duplicate definition
int j(); // OK: duplicate function declaration
int j() // OK: definition of N::j()
{
return g(i); // calls N::g(int)
}
int q(); // error: different return type
}
--end example]
2 A namespace member can also be referred to after the :: scope resolu-
tion operator (_expr.prim_) applied to the name of its namespace or
the name of a namespace which nominates the member's namespace in a
using-directive; see _namespace.qual_.
3 A name declared outside all named or unnamed namespaces (_basic.names-
pace_), blocks (_stmt.block_), function declarations (_dcl.fct_),
function definitions (_dcl.fct.def_) and classes (clause _class_) has
global namespace scope (also called global scope). The potential
scope of such a name begins at its point of declaration
(_basic.scope.pdecl_) and ends at the end of the translation unit that
is its declarative region. Names declared in the global namespace
scope are said to be global.
3.3.6 Class scope [basic.scope.class]
1 The following rules describe the scope of names declared in classes.
1)The potential scope of a name declared in a class consists not only
of the declarative region following the name's declarator, but also
of all function bodies, default arguments, and constructor ctor-ini-
tializers in that class (including such things in nested classes).
2)A name N used in a class S shall refer to the same declaration in
its context and when re-evaluated in the completed scope of S. No
diagnostic is required for a violation of this rule.
3)If reordering member declarations in a class yields an alternate
valid program under (1) and (2), the program is ill-formed, no diag-
nostic is required.
4)A name declared within a member function hides a declaration of the
same name whose scope extends to or past the end of the member func-
tion's class.
5)The potential scope of a declaration that extends to or past the end
of a class definition also extends to the regions defined by its
member definitions, even if the members are defined lexically out-
side the class (this includes static data member definitions, nested
class definitions, member function definitions (including the member
function body and, for constructor functions (_class.ctor_), the
ctor-initializer (_class.base.init_)) and any portion of the
declarator part of such definitions which follows the identifier,
including a parameter-declaration-clause and any default arguments
(_dcl.fct.default_). [Example:
typedef int c;
enum { i = 1 };
class X {
char v[i]; // error: i refers to ::i
// but when reevaluated is X::i
int f() { return sizeof(c); } // OK: X::c
char c;
enum { i = 2 };
};
typedef char* T;
struct Y {
T a; // error: T refers to ::T
// but when reevaluated is Y::T
typedef long T;
T b;
};
typedef int I;
class D {
typedef I I; // error, even though no reordering involved
};
--end example]
2 The name of a class member shall only be used as follows:
--in the scope of its class (as described above) or a class derived
(clause _class.derived_) from its class,
--after the . operator applied to an expression of the type of its
class (_expr.ref_) or a class derived from its class,
--after the -> operator applied to a pointer to an object of its class
(_expr.ref_) or a class derived from its class,
--after the :: scope resolution operator (_expr.prim_) applied to the
name of its class or a class derived from its class.
3.3.7 Name hiding [basic.scope.hiding]
1 A name can be hidden by an explicit declaration of that same name in a
nested declarative region or derived class (_class.member.lookup_).
2 A class name (_class.name_) or enumeration name (_dcl.enum_) can be
hidden by the name of an object, function, or enumerator declared in
the same scope. If a class or enumeration name and an object, func-
tion, or enumerator are declared in the same scope (in any order) with
the same name, the class or enumeration name is hidden wherever the
object, function, or enumerator name is visible.
3 In a member function definition, the declaration of a local name hides
the declaration of a member of the class with the same name; see
_basic.scope.class_. The declaration of a member in a derived class
(clause _class.derived_) hides the declaration of a member of a base
class of the same name; see _class.member.lookup_.
4 During the lookup of a name qualified by a namespace name, declara-
tions that would otherwise be made visible by a using-directive can be
hidden by declarations with the same name in the namespace containing
the using-directive; see (_namespace.qual_).
5 If a name is in scope and is not hidden it is said to be visible.
3.4 Name lookup [basic.lookup]
1 The name lookup rules apply uniformly to all names (including typedef-
names (_dcl.typedef_), namespace-names (_basic.namespace_) and class-
names (_class.name_)) wherever the grammar allows such names in the
context discussed by a particular rule. Name lookup associates the
use of a name with a declaration (_basic.def_) of that name. Name
lookup shall find an unambiguous declaration for the name (see
_class.member.lookup_). Name lookup may associate more than one dec-
laration with a name if it finds the name to be a function name; the
declarations are said to form a set of overloaded functions
(_over.load_). Overload resolution (_over.match_) takes place after
name lookup has succeeded. The access rules (clause _class.access_)
are considered only once name lookup and function overload resolution
(if applicable) have succeeded. Only after name lookup, function
overload resolution (if applicable) and access checking have succeeded
are the attributes introduced by the name's declaration used further
in expression processing (clause _expr_).
2 A name "looked up in the context of an expression" is looked up as an
unqualified name in the scope where the expression is found.
3 Because the name of a class is inserted in its class scope (clause
_class_), the name of a class is also considered a member of that
class for the purposes of name hiding and lookup.
4 [Note: _basic.link_ discusses linkage issues. The notions of scope,
point of declaration and name hiding are discussed in _basic.scope_.
]
3.4.1 Unqualified name lookup [basic.lookup.unqual]
1 In all the cases listed in _basic.lookup.unqual_, the scopes are
searched for a declaration in the order listed in each of the respec-
tive categories; name lookup ends as soon as a declaration is found
for the name. If no declaration is found, the program is ill-formed.
2 The declarations from the namespace nominated by a using-directive
become visible in a namespace enclosing the using-directive; see
_namespace.udir_. For the purpose of the unqualified name lookup
rules described in _basic.lookup.unqual_, the declarations from the
namespace nominated by the using-directive are considered members of
that enclosing namespace.
3 The lookup for an unqualified name used as the postfix-expression of a
function call is described in _basic.lookup.koenig_. [Note: for pur-
poses of determining (during parsing) whether an expression is a post-
fix-expression for a function call, the usual name lookup rules apply.
The rules in _basic.lookup.koenig_ have no effect on the syntactic
interpretation of an expression. For example,
typedef int f;
struct A {
friend void f(A &);
operator int();
void g(A a) {
f(a);
}
};
The expression f(a) is a cast-expression equivalent to int(a).
Because the expression is not a function call, the argument-dependent
name lookup (_basic.lookup.koenig_) does not apply and the friend
function f is not found. ]
4 A name used in global scope, outside of any function, class or user-
declared namespace, shall be declared before its use in global scope.
5 A name used in a user-declared namespace outside of the definition of
any function or class shall be declared before its use in that names-
pace or before its use in a namespace enclosing its namespace.
6 A name used in the definition of a function3) that is a member of
namespace N (where, only for the purpose of exposition, N could repre-
sent the global scope) shall be declared before its use in the block
in which it is used or in one of its enclosing blocks (_stmt.block_)
or, shall be declared before its use in namespace N or, if N is a
nested namespace, shall be declared before its use in one of N's
enclosing namespaces. [Example:
_________________________
3) This refers to unqualified names following the function declarator;
such a name may be used as a type or as a default argument name in the
parameter-declaration-clause, or may be used in the function body.
namespace A {
namespace N {
void f();
}
}
void A::N::f() {
i = 5;
// The following scopes are searched for a declaration of i:
// 1) outermost block scope of A::N::f, before the use of i
// 2) scope of namespace N
// 3) scope of namespace A
// 4) global scope, before the definition of A::N::f
}
--end example]
7 A name used in the definition of a class X outside of a member func-
tion body or nested class definition4) shall be declared in one of the
following ways:
--before its use in class X or be a member of a base class of X
(_class.member.lookup_), or
--if X is a nested class of class Y (_class.nest_), before the defini-
tion of X in Y, or shall be a member of a base class of Y (this
lookup applies in turn to Y's enclosing classes, starting with the
innermost enclosing class),5) or
--if X is a local class (_class.local_) or is a nested class of a
local class, before the definition of class X in a block enclosing
the definition of class X, or
--if X is a member of namespace N, or is a nested class of a class
that is a member of N, or is a local class or a nested class within
a local class of a function that is a member of N, before the defi-
nition of class X in namespace N or in one of N's enclosing names-
paces.
[Example:
_________________________
4) This refers to unqualified names following the class name; such a
name may be used in the base-clause or may be used in the class defi-
nition.
5) This lookup applies whether the definition of X is nested within
Y's definition or whether X's definition appears in a namespace scope
enclosing Y's definition (_class.nest_).
namespace M {
class B { };
}
namespace N {
class Y : public M::B {
class X {
int a[i];
};
};
}
// The following scopes are searched for a declaration of i:
// 1) scope of class N::Y::X, before the use of i
// 2) scope of class N::Y, before the definition of N::Y::X
// 3) scope of N::Y's base class M::B
// 4) scope of namespace N, before the definition of N::Y
// 5) global scope, before the definition of N
--end example] [Note: when looking for a prior declaration of a class
or function introduced by a friend declaration, scopes outside of the
innermost enclosing namespace scope are not considered; see _names-
pace.memdef_. ] [Note: _basic.scope.class_ further describes the
restrictions on the use of names in a class definition. _class.nest_
further describes the restrictions on the use of names in nested class
definitions. _class.local_ further describes the restrictions on the
use of names in local class definitions. ]
8 A name used in the definition of a function that is a member function
(_class.mfct_)6) of class X shall be declared in one of the following
ways:
--before its use in the block in which it is used or in an enclosing
block (_stmt.block_), or
--shall be a member of class X or be a member of a base class of X
(_class.member.lookup_), or
--if X is a nested class of class Y (_class.nest_), shall be a member
of Y, or shall be a member of a base class of Y (this lookup applies
in turn to Y's enclosing classes, starting with the innermost
enclosing class),7) or
--if X is a local class (_class.local_) or is a nested class of a
local class, before the definition of class X in a block enclosing
the definition of class X, or
_________________________
6) That is, an unqualified name following the function declarator;
such a name may be used as a type or as a default argument name in the
parameter-declaration-clause, or may be used in the function body, or,
if the function is a constructor, may be used in the expression of a
mem-initializer.
7) This lookup applies whether the member function is defined within
the definition of class X or whether the member function is defined in
a namespace scope enclosing X's definition.
--if X is a member of namespace N, or is a nested class of a class
that is a member of N, or is a local class or a nested class within
a local class of a function that is a member of N, before the member
function definition, in namespace N or in one of N's enclosing
namespaces.
[Example:
class B { };
namespace M {
namespace N {
class X : public B {
void f();
};
}
}
void M::N::X::f() {
i = 16;
}
// The following scopes are searched for a declaration of i:
// 1) outermost block scope of M::N::X::f, before the use of i
// 2) scope of class M::N::X
// 3) scope of M::N::X's base class B
// 4) scope of namespace M::N
// 5) scope of namespace M
// 6) global scope, before the definition of M::N::X::f
--end example] [Note: _class.mfct_ and _class.static_ further
describe the restrictions on the use of names in member function defi-
nitions. _class.nest_ further describes the restrictions on the use
of names in the scope of nested classes. _class.local_ further
describes the restrictions on the use of names in local class defini-
tions. ]
9 Name lookup for a name used in the definition of a friend function
(_class.friend_) defined inline in the class granting friendship shall
proceed as described for lookup in member function definitions. If
the friend function is not defined in the class granting friendship,
name lookup in the friend function definition shall proceed as
described for lookup in namespace member function definitions.
10In a friend declaration naming a member function, a name used in the
function declarator and not part of a template-argument in a template-
id is first looked up in the scope of the member function's class. If
it is not found, or if the name is part of a template-argument in a
template-id, the look up is as described for unqualified names in the
definition of the class granting friendship. [Example:
struct A {
typedef int AT;
void f1(AT);
void f2(float);
};
struct B {
typedef float BT;
friend void A::f1(AT); // parameter type is A::AT
friend void A::f2(BT); // parameter type is B::BT
};
--end example]
11During the lookup for a name used as a default argument
(_dcl.fct.default_) in a function parameter-declaration-clause or used
in the expression of a mem-initializer for a constructor
(_class.base.init_), the function parameter names are visible and hide
the names of entities declared in the block, class or namespace scopes
containing the function declaration. [Note: _dcl.fct.default_ further
describes the restrictions on the use of names in default arguments.
_class.base.init_ further describes the restrictions on the use of
names in a ctor-initializer. ]
12A name used in the definition of a static data member of class X
(_class.static.data_) (after the qualified-id of the static member) is
looked up as if the name was used in a member function of X. [Note:
_class.static.data_ further describes the restrictions on the use of
names in the definition of a static data member. ]
13A name used in the handler for a function-try-block (clause _except_)
is looked up as if the name was used in the outermost block of the
function definition. In particular, the function parameter names
shall not be redeclared in the exception-declaration nor in the outer-
most block of a handler for the function-try-block. Names declared in
the outermost block of the function definition are not found when
looked up in the scope of a handler for the function-try-block.
[Note: but function parameter names are found. ]
14[Note: the rules for name lookup in template definitions are described
in _temp.res_. ]
3.4.2 Argument-dependent name lookup [basic.lookup.koenig]
1 When an unqualified name is used as the postfix-expression in a func-
tion call (_expr.call_), other namespaces not considered during the
usual unqualified lookup (_basic.lookup.unqual_) may be searched, and
namespace-scope friend function declarations (_class.friend_) not oth-
erwise visible may be found. These modifications to the search depend
on the types of the arguments (and for template template arguments,
the namespace of the template argument).
2 For each argument type T in the function call, there is a set of zero
or more associated namespaces and a set of zero or more associated
classes to be considered. The sets of namespaces and classes is
determined entirely by the types of the function arguments (and the
namespace of any template template argument). Typedef names and
using-declarations used to specify the types do not contribute to this
set. The sets of namespaces and classes are determined in the follow-
ing way:
--If T is a fundamental type, its associated sets of namespaces and
classes are both empty.
--If T is a class type, its associated classes are the class itself
and its direct and indirect base classes. Its associated namespaces
are the namespaces in which its associated classes are defined.
--If T is a union or enumeration type, its associated namespace is the
namespace in which it is defined. If it is a class member, its
associated class is the member's class; else it has no associated
class.
--If T is a pointer to U or an array of U, its associated namespaces
and classes are those associated with U.
--If T is a function type, its associated namespaces and classes are
those associated with the function parameter types and those associ-
ated with the return type.
--If T is a pointer to a member function of a class X, its associated
namespaces and classes are those associated with the function param-
eter types and return type, together with those associated with X.
--If T is a pointer to a data member of class X, its associated names-
paces and classes are those associated with the member type together
with those associated with X.
--If T is a template-id, its associated namespaces and classes are the
namespace in which the template is defined; for member templates,
the member template's class; the namespaces and classes associated
with the types of the template arguments provided for template type
parameters (excluding template template parameters); the namespaces
in which any template template arguments are defined; and the
classes in which any member templates used as template template
arguments are defined. [Note: non-type template arguments do not
contribute to the set of associated namespaces. ]
If the ordinary unqualified lookup of the name finds the declaration
of a class member function, the associated namespaces and classes are
not considered. Otherwise the set of declarations found by the lookup
of the function name is the union of the set of declarations found
using ordinary unqualified lookup and the set of declarations found in
the namespaces and classes associated with the argument types. [Exam-
ple:
namespace NS {
class T { };
void f(T);
}
NS::T parm;
int main() {
f(parm); // OK: calls NS::f
}
--end example]
3 When considering an associated namespace, the lookup is the same as
the lookup performed when the associated namespace is used as a quali-
fier (_namespace.qual_) except that:
--Any using-directives in the associated namespace are ignored.
--Any namespace-scope friend functions declared in associated classes
are visible within their respective namespaces even if they are not
visible during an ordinary lookup (_class.friend_).
3.4.3 Qualified name lookup [basic.lookup.qual]
1 The name of a class or namespace member can be referred to after the
:: scope resolution operator (_expr.prim_) applied to a nested-name-
specifier that nominates its class or namespace. During the lookup
for a name preceding the :: scope resolution operator, object, func-
tion, and enumerator names are ignored. If the name found is not a
class-name (clause _class_) or namespace-name (_namespace.def_), the
program is ill-formed. [Example:
class A {
public:
static int n;
};
int main()
{
int A;
A::n = 42; // OK
A b; // ill-formed: A does not name a type
}
--end example]
2 [Note: Multiply qualified names, such as N1::N2::N3::n, can be used to
refer to members of nested classes (_class.nest_) or members of nested
namespaces. ]
3 In a declaration in which the declarator-id is a qualified-id, names
used before the qualified-id being declared are looked up in the
defining namespace scope; names following the qualified-id are looked
up in the scope of the member's class or namespace. [Example:
class X { };
class C {
class X { };
static const int number = 50;
static X arr[number];
};
X C::arr[number]; // ill-formed:
// equivalent to: ::X C::arr[C::number];
// not to: C::X C::arr[C::number];
--end example]
4 A name prefixed by the unary scope operator :: (_expr.prim_) is looked
up in global scope, in the translation unit where it is used. The
name shall be declared in global namespace scope or shall be a name
whose declaration is visible in global scope because of a using-direc-
tive (_namespace.qual_). The use of :: allows a global name to be
referred to even if its identifier has been hidden (_basic.scope.hid-
ing_).
5 If a pseudo-destructor-name (_expr.pseudo_) contains a nested-name-
specifier, the type-names are looked up as types in the scope desig-
nated by the nested-name-specifier. In a qualified-id of the form:
::opt nested-name-specifier ~ class-name
where the nested-name-specifier designates a namespace scope, and in a
qualified-id of the form:
::opt nested-name-specifier class-name :: ~ class-name
the class-names are looked up as types in the scope designated by the
nested-name-specifier. [Example:
struct C {
typedef int I;
};
typedef int I1, I2;
extern int* p;
extern int* q;
p->C::I::~I(); // I is looked up in the scope of C
q->I1::~I2(); // I2 is looked up in the scope of
// the postfix-expression
struct A {
~A();
};
typedef A AB;
int main()
{
AB *p;
p->AB::~AB(); // explicitly calls the destructor for A
}
--end example] [Note: _basic.lookup.classref_ describes how name
lookup proceeds after the . and -> operators. ]
3.4.3.1 Class members [class.qual]
1 If the nested-name-specifier of a qualified-id nominates a class, the
name specified after the nested-name-specifier is looked up in the
scope of the class (_class.member.lookup_), except for the cases
listed below. The name shall represent one or more members of that
class or of one of its base classes (clause _class.derived_). [Note:
a class member can be referred to using a qualified-id at any point in
its potential scope (_basic.scope.class_). ] The exceptions to the
name lookup rule above are the following:
--a destructor name is looked up as specified in _basic.lookup.qual_;
--a conversion-type-id of an operator-function-id is looked up both in
the scope of the class and in the context in which the entire post-
fix-expression occurs and shall refer to the same type in both con-
texts;
--the template-arguments of a template-id are looked up in the context
in which the entire postfix-expression occurs.
2 A class member name hidden by a name in a nested declarative region or
by the name of a derived class member can still be found if qualified
by the name of its class followed by the :: operator.
3.4.3.2 Namespace members [namespace.qual]
1 If the nested-name-specifier of a qualified-id nominates a namespace,
the name specified after the nested-name-specifier is looked up in the
scope of the namespace, except that the template-arguments of a tem-
plate-id are looked up in the context in which the entire postfix-
expression occurs.
2 Given X::m (where X is a user-declared namespace), or given ::m (where
X is the global namespace), let S be the set of all declarations of m
in X and in the transitive closure of all namespaces nominated by
using-directives in X and its used namespaces, except that using-
directives are ignored in any namespace, including X, directly con-
taining one or more declarations of m. No namespace is searched more
than once in the lookup of a name. If S is the empty set, the program
is ill-formed. Otherwise, if S has exactly one member, or if the con-
text of the reference is a using-declaration (_namespace.udecl_), S is
the required set of declarations of m. Otherwise if the use of m is
not one that allows a unique declaration to be chosen from S, the pro-
gram is ill-formed. [Example:
int x;
namespace Y {
void f(float);
void h(int);
}
namespace Z {
void h(double);
}
namespace A {
using namespace Y;
void f(int);
void g(int);
int i;
}
namespace B {
using namespace Z;
void f(char);
int i;
}
namespace AB {
using namespace A;
using namespace B;
void g();
}
void h()
{
AB::g(); // g is declared directly in AB,
// therefore S is { AB::g() } and AB::g() is chosen
AB::f(1); // f is not declared directly in AB so the rules are
// applied recursively to A and B;
// namespace Y is not searched and Y::f(float)
// is not considered;
// S is { A::f(int), B::f(char) } and overload
// resolution chooses A::f(int)
AB::f('c'); // as above but resolution chooses B::f(char)
AB::x++; // x is not declared directly in AB, and
// is not declared in A or B, so the rules are
// applied recursively to Y and Z,
// S is { } so the program is ill-formed
AB::i++; // i is not declared directly in AB so the rules are
// applied recursively to A and B,
// S is { A::i, B::i } so the use is ambiguous
// and the program is ill-formed
AB::h(16.8); // h is not declared directly in AB and
// not declared directly in A or B so the rules are
// applied recursively to Y and Z,
// S is { Y::h(int), Z::h(double) } and overload
// resolution chooses Z::h(double)
}
3 The same declaration found more than once is not an ambiguity (because
it is still a unique declaration). For example:
namespace A {
int a;
}
namespace B {
using namespace A;
}
namespace C {
using namespace A;
}
namespace BC {
using namespace B;
using namespace C;
}
void f()
{
BC::a++; // OK: S is { A::a, A::a }
}
namespace D {
using A::a;
}
namespace BD {
using namespace B;
using namespace D;
}
void g()
{
BD::a++; // OK: S is { A::a, A::a }
}
4 Because each referenced namespace is searched at most once, the fol-
lowing is well-defined:
namespace B {
int b;
}
namespace A {
using namespace B;
int a;
}
namespace B {
using namespace A;
}
void f()
{
A::a++; // OK: a declared directly in A, S is { A::a }
B::a++; // OK: both A and B searched (once), S is { A::a }
A::b++; // OK: both A and B searched (once), S is { B::b }
B::b++; // OK: b declared directly in B, S is { B::b }
}
--end example]
5 During the lookup of a qualified namespace member name, if the lookup
finds more than one declaration of the member, and if one declaration
introduces a class name or enumeration name and the other declarations
either introduce the same object, the same enumerator or a set of
functions, the non-type name hides the class or enumeration name if
and only if the declarations are from the same namespace; otherwise
(the declarations are from different namespaces), the program is ill-
formed. [Example:
namespace A {
struct x { };
int x;
int y;
}
namespace B {
struct y {};
}
namespace C {
using namespace A;
using namespace B;
int i = C::x; // OK, A::x (of type int)
int j = C::y; // ambiguous, A::y or B::y
}
--end example]
6 In a declaration for a namespace member in which the declarator-id is
a qualified-id, given that the qualified-id for the namespace member
has the form
nested-name-specifier unqualified-id
the unqualified-id shall name a member of the namespace designated by
the nested-name-specifier. [Example:
namespace A {
namespace B {
void f1(int);
}
using namespace B;
}
void A::f1(int) { } // ill-formed, f1 is not a member of A
--end example] However, in such namespace member declarations, the
nested-name-specifier may rely on using-directives to implicitly pro-
vide the initial part of the nested-name-specifier. [Example:
namespace A {
namespace B {
void f1(int);
}
}
namespace C {
namespace D {
void f1(int);
}
}
using namespace A;
using namespace C::D;
void B::f1(int){} // OK, defines A::B::f1(int)
--end example]
3.4.4 Elaborated type specifiers [basic.lookup.elab]
1 An elaborated-type-specifier may be used to refer to a previously
declared class-name or enum-name even though the name has been hidden
by a non-type declaration (_basic.scope.hiding_). The class-name or
enum-name in the elaborated-type-specifier may either be a simple
identifer or be a qualified-id.
2 If the name in the elaborated-type-specifier is a simple identifer,
and unless the elaborated-type-specifier has the following form:
class-key identifier ;
the identifier is looked up according to _basic.lookup.unqual_ but
ignoring any non-type names that have been declared. If this name
lookup finds a typedef-name, the elaborated-type-specifier is ill-
formed. If the elaborated-type-specifier refers to an enum-name and
this lookup does not find a previously declared enum-name, the elabo-
rated-type-specifier is ill-formed. If the elaborated-type-specifier
refers to an class-name and this lookup does not find a previously
declared class-name, or if the elaborated-type-specifier has the form:
class-key identifier ;
the elaborated-type-specifier is a declaration that introduces the
class-name as described in _basic.scope.pdecl_.
3 If the name is a qualified-id, the name is looked up according its
qualifications, as described in _basic.lookup.qual_, but ignoring any
non-type names that have been declared. If this name lookup finds a
typedef-name, the elaborated-type-specifier is ill-formed. If this
name lookup does not find a previously declared class-name or enum-
name, the elaborated-type-specifier is ill-formed. [Example:
struct Node {
struct Node* Next; // OK: Refers to Node at global scope
struct Data* Data; // OK: Declares type Data
// at global scope and member Data
};
struct Data {
struct Node* Node; // OK: Refers to Node at global scope
friend struct ::Glob; // error: Glob is not declared
// cannot introduce a qualified type (_dcl.type.elab_)
friend struct Glob; // OK: Refers to (as yet) undeclared Glob
// at global scope.
/* ... */
};
struct Base {
struct Data; // OK: Declares nested Data
struct ::Data* thatData; // OK: Refers to ::Data
struct Base::Data* thisData; // OK: Refers to nested Data
friend class ::Data; // OK: global Data is a friend
friend class Data; // OK: nested Data is a friend
struct Data { /* ... */ }; // Defines nested Data
struct Data; // OK: Redeclares nested Data
};
struct Data; // OK: Redeclares Data at global scope
struct ::Data; // error: cannot introduce a qualified type (_dcl.type.elab_)
struct Base::Data; // error: cannot introduce a qualified type (_dcl.type.elab_)
struct Base::Datum; // error: Datum undefined
struct Base::Data* pBase; // OK: refers to nested Data
--end example]
3.4.5 Class member access [basic.lookup.classref]
1 In a class member access expression (_expr.ref_), if the . or -> token
is immediately followed by an identifier followed by a <, the identi-
fier must be looked up to determine whether the < is the beginning of
a template argument list (_temp.names_) or a less-than operator. The
identifier is first looked up in the class of the object expression.
If the identifier is not found, it is then looked up in the context of
the entire postfix-expression and shall name a class or function tem-
plate. If the lookup in the class of the object expression finds a
template, the name is also looked up in the context of the entire
postfix-expression and
--if the name is not found, the name found in the class of the object
expression is used, otherwise
--if the name is found in the context of the entire postfix-expression
and does not name a class template, the name found in the class of
the object expression is used, otherwise
--if the name found is a class template, it must refer to the same
entity as the one found in the class of the object expression, oth-
erwise the program is ill-formed.
2 If the id-expression in a class member access (_expr.ref_) is an
unqualified-id, and the type of the object expression is of a class
type C (or of pointer to a class type C), the unqualified-id is looked
up in the scope of class C. If the type of the object expression is
of pointer to scalar type, the unqualified-id is looked up in the con-
text of the complete postfix-expression.
3 If the unqualified-id is ~type-name, and the type of the object
expression is of a class type C (or of pointer to a class type C), the
type-name is looked up in the context of the entire postfix-expression
and in the scope of class C. The type-name shall refer to a class-
name. If type-name is found in both contexts, the name shall refer to
the same class type. If the type of the object expression is of
scalar type, the type-name is looked up in the scope of the complete
postfix-expression.
4 If the id-expression in a class member access is a qualified-id of the
form
class-name-or-namespace-name::...
the class-name-or-namespace-name following the . or -> operator is
looked up both in the context of the entire postfix-expression and in
the scope of the class of the object expression. If the name is found
only in the scope of the class of the object expression, the name
shall refer to a class-name. If the name is found only in the context
of the entire postfix-expression, the name shall refer to a class-name
or namespace-name. If the name is found in both contexts, the class-
name-or-namespace-name shall refer to the same entity. [Note: the
result of looking up the class-name-or-namespace-name is not required
to be a unique base class of the class type of the object expression,
as long as the entity or entities named by the qualified-id are mem-
bers of the class type of the object expression and are not ambiguous
according to _class.member.lookup_.
struct A {
int a;
};
struct B: virtual A { };
struct C: B { };
struct D: B { };
struct E: public C, public D { };
struct F: public A { };
void f() {
E e;
e.B::a = 0; // OK, only one A::a in E
F f;
f.A::a = 1; // OK, A::a is a member of F
}
--end note]
5 If the qualified-id has the form
::class-name-or-namespace-name::...
the class-name-or-namespace-name is looked up in global scope as a
class-name or namespace-name.
6 If the nested-name-specifier contains a class template-id
(_temp.names_), its template-arguments are evaluated in the context in
which the entire postfix-expression occurs.
7 If the id-expression is a conversion-function-id, its conversion-type-
id shall denote the same type in both the context in which the entire
postfix-expression occurs and in the context of the class of the
object expression (or the class pointed to by the pointer expression).
3.4.6 Using-directives and namespace aliases [basic.lookup.udir]
1 When looking up a namespace-name in a using-directive or namespace-
alias-definition, only namespace names are considered.
3.5 Program and linkage [basic.link]
1 A program consists of one or more translation units (clause _lex_)
linked together. A translation unit consists of a sequence of decla-
rations.
translation-unit:
declaration-seqopt
2 A name is said to have linkage when it might denote the same object,
reference, function, type, template, namespace or value as a name
introduced by a declaration in another scope:
--When a name has external linkage, the entity it denotes can be
referred to by names from scopes of other translation units or from
other scopes of the same translation unit.
--When a name has internal linkage, the entity it denotes can be
referred to by names from other scopes in the same translation unit.
--When a name has no linkage, the entity it denotes cannot be referred
to by names from other scopes.
3 A name having namespace scope (_basic.scope.namespace_) has internal
linkage if it is the name of
--an object, reference, function or function template that is
explicitly declared static or,
--an object or reference that is explicitly declared const and neither
explicitly declared extern nor previously declared to have external
linkage; or
--a data member of an anonymous union.
4 A name having namespace scope has external linkage if it is the name
of
--an object or reference, unless it has internal linkage; or
--a function, unless it has internal linkage; or
--a named class (clause _class_), or an unnamed class defined in a
typedef declaration in which the class has the typedef name for
linkage purposes (_dcl.typedef_); or
--a named enumeration (_dcl.enum_), or an unnamed enumeration defined
in a typedef declaration in which the enumeration has the typedef
name for linkage purposes (_dcl.typedef_); or
--an enumerator belonging to an enumeration with external linkage; or
--a template, unless it is a function template that has internal link-
age (clause _temp_); or
--a namespace (_basic.namespace_), unless it is declared within an
unnamed namespace.
5 In addition, a member function, static data member, class or enumera-
tion of class scope has external linkage if the name of the class has
external linkage.
6 The name of a function declared in block scope, and the name of an
object declared by a block scope extern declaration, have linkage. If
there is a visible declaration of an entity with linkage having the
same name and type, ignoring entities declared outside the innermost
enclosing namespace scope, the block scope declaration declares that
same entity and receives the linkage of the previous declaration. If
there is more than one such matching entity, the program is ill-
formed. Otherwise, if no matching entity is found, the block scope
entity receives external linkage. [Example:
static void f();
static int i = 0; //1
void g() {
extern void f(); // internal linkage
int i; //2: i has no linkage
{
extern void f(); // internal linkage
extern int i; //3: external linkage
}
}
There are three objects named i in this program. The object with
internal linkage introduced by the declaration in global scope (line
//1), the object with automatic storage duration and no linkage intro-
duced by the declaration on line //2, and the object with static stor-
age duration and external linkage introduced by the declaration on
line //3. ]
7 When a block scope declaration of an entity with linkage is not found
to refer to some other declaration, then that entity is a member of
the innermost enclosing namespace. However such a declaration does
not introduce the member name in its namespace scope. [Example:
namespace X {
void p()
{
q(); // error: q not yet declared
extern void q(); // q is a member of namespace X
}
void middle()
{
q(); // error: q not yet declared
}
void q() { /* ... */ } // definition of X::q
}
void q() { /* ... */ } // some other, unrelated q
--end example]
8 Names not covered by these rules have no linkage. Moreover, except as
noted, a name declared in a local scope (_basic.scope.local_) has no
linkage. A name with no linkage (notably, the name of a class or enu-
meration declared in a local scope (_basic.scope.local_)) shall not be
used to declare an entity with linkage. If a declaration uses a type-
def name, it is the linkage of the type name to which the typedef
refers that is considered. [Example:
void f()
{
struct A { int x; }; // no linkage
extern A a; // ill-formed
typedef A B;
extern B b; // ill-formed
}
--end example] This implies that names with no linkage cannot be used
as template arguments (_temp.arg_).
9 Two names that are the same (clause _basic_) and that are declared in
different scopes shall denote the same object, reference, function,
type, enumerator, template or namespace if
--both names have external linkage or else both names have internal
linkage and are declared in the same translation unit; and
--both names refer to members of the same namespace or to members, not
by inheritance, of the same class; and
--when both names denote functions, the function types are identical
for purposes of overloading; and
--when both names denote function templates, the signatures
(_temp.over.link_) are the same.
10After all adjustments of types (during which typedefs (_dcl.typedef_)
are replaced by their definitions), the types specified by all decla-
rations referring to a given object or function shall be identical,
except that declarations for an array object can specify array types
that differ by the presence or absence of a major array bound
(_dcl.array_). A violation of this rule on type identity does not
require a diagnostic.
11[Note: linkage to non-C++ declarations can be achieved using a link-
age-specification (_dcl.link_). ]
3.6 Start and termination [basic.start]
3.6.1 Main function [basic.start.main]
1 A program shall contain a global function called main, which is the
designated start of the program. It is implementation-defined whether
a program in a freestanding environment is required to define a main
function. [Note: in a freestanding environment, start-up and termina-
tion is implementation-defined; start-up contains the execution of
constructors for objects of namespace scope with static storage dura-
tion; termination contains the execution of destructors for objects
with static storage duration. ]
2 An implementation shall not predefine the main function. This func-
tion shall not be overloaded. It shall have a return type of type
int, but otherwise its type is implementation-defined. All implemen-
tations shall allow both of the following definitions of main:
int main() { /* ... */ }
and
int main(int argc, char* argv[]) { /* ... */ }
In the latter form argc shall be the number of arguments passed to the
program from the environment in which the program is run. If argc is
nonzero these arguments shall be supplied in argv[0] through
argv[argc-1] as pointers to the initial characters of null-terminated
multibyte strings (NTMBSs) (_lib.multibyte.strings_) and argv[0] shall
be the pointer to the initial character of a NTMBS that represents the
name used to invoke the program or "". The value of argc shall be
nonnegative. The value of argv[argc] shall be 0. [Note: it is recom-
mended that any further (optional) parameters be added after argv. ]
3 The function main shall not be used (_basic.def.odr_) within a pro-
gram. The linkage (_basic.link_) of main is implementation-defined.
A program that declares main to be inline or static is ill-formed.
The name main is not otherwise reserved. [Example: member functions,
classes, and enumerations can be called main, as can entities in other
namespaces. ]
4 Calling the function
void exit(int);
declared in <cstdlib> (_lib.support.start.term_) terminates the pro-
gram without leaving the current block and hence without destroying
any objects with automatic storage duration (_class.dtor_). If exit
is called to end a program during the destruction of an object with
static storage duration, the program has undefined behavior.
5 A return statement in main has the effect of leaving the main function
(destroying any objects with automatic storage duration) and calling
exit with the return value as the argument. If control reaches the
end of main without encountering a return statement, the effect is
that of executing
return 0;
3.6.2 Initialization of non-local objects [basic.start.init]
1 The storage for objects with static storage duration
(_basic.stc.static_) shall be zero-initialized (_dcl.init_) before any
other initialization takes place. Zero-initialization and initializa-
tion with a constant expression are collectively called static ini-
tialization; all other initialization is dynamic initialization.
Objects of POD types (_basic.types_) with static storage duration ini-
tialized with constant expressions (_expr.const_) shall be initialized
before any dynamic initialization takes place. Objects with static
storage duration defined in namespace scope in the same translation
unit and dynamically initialized shall be initialized in the order in
which their definition appears in the translation unit. [Note:
_dcl.init.aggr_ describes the order in which aggregate members are
initialized. The initialization of local static objects is described
in _stmt.dcl_. ]
2 An implementation is permitted to perform the initialization of an
object of namespace scope with static storage duration as a static
initialization even if such initialization is not required to be done
statically, provided that
--the dynamic version of the initialization does not change the value
of any other object of namespace scope with static storage duration
prior to its initialization, and
--the static version of the initialization produces the same value in
the initialized object as would be produced by the dynamic initial-
ization if all objects not required to be initialized statically
were initialized dynamically.
[Note: as a consequence, if the initialization of an object obj1
refers to an object obj2 of namespace scope with static storage dura-
tion potentially requiring dynamic initialization and defined later in
the same translation unit, it is unspecified whether the value of obj2
used will be the value of the fully initialized obj2 (because obj2 was
statically initialized) or will be the value of obj2 merely zero-ini-
tialized. For example,
inline double fd() { return 1.0; }
extern double d1;
double d2 = d1; // unspecified:
// may be statically initialized to 0.0 or
// dynamically initialized to 1.0
double d1 = fd(); // may be initialized statically to 1.0
--end note]
3 It is implementation-defined whether or not the dynamic initialization
(_dcl.init_, _class.static_, _class.ctor_, _class.expl.init_) of an
object of namespace scope is done before the first statement of main.
If the initialization is deferred to some point in time after the
first statement of main, it shall occur before the first use of any
function or object defined in the same translation unit as the object
to be initialized.8) [Example:
// - File 1 -
#include "a.h"
#include "b.h"
B b;
A::A(){
b.Use();
}
// - File 2 -
#include "a.h"
A a;
// - File 3 -
#include "a.h"
#include "b.h"
extern A a;
extern B b;
int main() {
a.Use();
b.Use();
}
It is implementation-defined whether either a or b is initialized
before main is entered or whether the initializations are delayed
until a is first used in main. In particular, if a is initialized
before main is entered, it is not guaranteed that b will be initial-
ized before it is used by the initialization of a, that is, before
A::A is called. If, however, a is initialized at some point after the
first statement of main, b will be initialized prior to its use in
A::A. ]
4 If construction or destruction of a non-local static object ends in
throwing an uncaught exception, the result is to call terminate
(_lib.terminate_).
_________________________
8) An object defined in namespace scope having initialization with
side-effects must be initialized even if it is not used (_ba-
sic.stc.static_).
3.6.3 Termination [basic.start.term]
1 Destructors (_class.dtor_) for initialized objects of static storage
duration (declared at block scope or at namespace scope) are called as
a result of returning from main and as a result of calling exit
(_lib.support.start.term_). These objects are destroyed in the
reverse order of the completion of their constructor or of the comple-
tion of their dynamic initialization. If an object is initialized
statically, the object is destroyed in the same order as if the object
was dynamically initialized. For an object of array or class type,
all subobjects of that object are destroyed before any local object
with static storage duration initialized during the construction of
the subobjects is destroyed.
2 If a function contains a local object of static storage duration that
has been destroyed and the function is called during the destruction
of an object with static storage duration, the program has undefined
behavior if the flow of control passes through the definition of the
previously destroyed local object.
3 If a function is registered with atexit (see <cstdlib>, _lib.sup-
port.start.term_) then following the call to exit, any objects with
static storage duration initialized prior to the registration of that
function shall not be destroyed until the registered function is
called from the termination process and has completed. For an object
with static storage duration constructed after a function is regis-
tered with atexit, then following the call to exit, the registered
function is not called until the execution of the object's destructor
has completed. If atexit is called during the construction of an
object, the complete object to which it belongs shall be destroyed
before the registered function is called.
4 Calling the function
void abort();
declared in <cstdlib> terminates the program without executing
destructors for objects of automatic or static storage duration and
without calling the functions passed to atexit().
3.7 Storage duration [basic.stc]
1 Storage duration is the property of an object that defines the minimum
potential lifetime of the storage containing the object. The storage
duration is determined by the construct used to create the object and
is one of the following:
--static storage duration
--automatic storage duration
--dynamic storage duration
2 Static and automatic storage durations are associated with objects
introduced by declarations (_basic.def_) and implicitly created by the
implementation (_class.temporary_). The dynamic storage duration is
associated with objects created with operator new (_expr.new_).
3 The storage class specifiers static and auto are related to storage
duration as described below.
4 The storage duration categories apply to references as well. The
lifetime of a reference is its storage duration.
3.7.1 Static storage duration [basic.stc.static]
1 All objects which neither have dynamic storage duration nor are local
have static storage duration. The storage for these objects shall
last for the duration of the program (_basic.start.init_,
_basic.start.term_).
2 If an object of static storage duration has initialization or a
destructor with side effects, it shall not be eliminated even if it
appears to be unused, except that a class object or its copy may be
eliminated as specified in _class.copy_.
3 The keyword static can be used to declare a local variable with static
storage duration. [Note: _stmt.dcl_ describes the initialization of
local static variables; _basic.start.term_ describes the destruction
of local static variables. ]
4 The keyword static applied to a class data member in a class defini-
tion gives the data member static storage duration.
3.7.2 Automatic storage duration [basic.stc.auto]
1 Local objects explicitly declared auto or register or not explicitly
declared static or extern have automatic storage duration. The stor-
age for these objects lasts until the block in which they are created
exits.
2 [Note: these objects are initialized and destroyed as described in
_stmt.dcl_. ]
3 If a named automatic object has initialization or a destructor with
side effects, it shall not be destroyed before the end of its block,
nor shall it be eliminated as an optimization even if it appears to be
unused, except that a class object or its copy may be eliminated as
specified in _class.copy_.
3.7.3 Dynamic storage duration [basic.stc.dynamic]
1 Objects can be created dynamically during program execution
(_intro.execution_), using new-expressions (_expr.new_), and destroyed
using delete-expressions (_expr.delete_). A C++ implementation pro-
vides access to, and management of, dynamic storage via the global
allocation functions operator new and operator new[] and the global
deallocation functions operator delete and operator delete[].
2 The library provides default definitions for the global allocation and
deallocation functions. Some global allocation and deallocation func-
tions are replaceable (_lib.new.delete_). A C++ program shall provide
at most one definition of a replaceable allocation or deallocation
function. Any such function definition replaces the default version
provided in the library (_lib.replacement.functions_). The following
allocation and deallocation functions (_lib.support.dynamic_) are
implicitly declared in global scope in each translation unit of a pro-
gram
void* operator new(std::size_t) throw(std::bad_alloc);
void* operator new[](std::size_t) throw(std::bad_alloc);
void operator delete(void*) throw();
void operator delete[](void*) throw();
These implicit declarations introduce only the function names operator
new, operator new[], operator delete, operator delete[]. [Note: the
implicit declarations do not introduce the names std, std::bad_alloc,
and std::size_t, or any other names that the library uses to declare
these names. Thus, a new-expression, delete-expression or function
call that refers to one of these functions without including the
header <new> is well-formed. However, referring to std,
std::bad_alloc, and std::size_t is ill-formed unless the name has been
declared by including the appropriate header. ] Allocation and/or
deallocation functions can also be declared and defined for any class
(_class.free_).
3 Any allocation and/or deallocation functions defined in a C++ program
shall conform to the semantics specified in _basic.stc.dynamic.alloca-
tion_ and _basic.stc.dynamic.deallocation_.
3.7.3.1 Allocation functions [basic.stc.dynamic.allocation]
1 An allocation function shall be a class member function or a global
function; a program is ill-formed if an allocation function is
declared in a namespace scope other than global scope or declared
static in global scope. The return type shall be void*. The first
parameter shall have type size_t (_lib.support.types_). The first
parameter shall not have an associated default argument
(_dcl.fct.default_). The value of the first parameter shall be inter-
preted as the requested size of the allocation. An allocation func-
tion can be a function template. Such a template shall declare its
return type and first parameter as specified above (that is, template
parameter types shall not be used in the return type and first parame-
ter type). Template allocation functions shall have two or more
parameters.
2 The allocation function attempts to allocate the requested amount of
storage. If it is successful, it shall return the address of the
start of a block of storage whose length in bytes shall be at least as
large as the requested size. There are no constraints on the contents
of the allocated storage on return from the allocation function. The
order, contiguity, and initial value of storage allocated by succes-
sive calls to an allocation function is unspecified. The pointer
returned shall be suitably aligned so that it can be converted to a
pointer of any complete object type and then used to access the object
or array in the storage allocated (until the storage is explicitly
deallocated by a call to a corresponding deallocation function). If
the size of the space requested is zero, the value returned shall not
be a null pointer value (_conv.ptr_). The results of dereferencing a
pointer returned as a request for zero size are undefined.9)
3 An allocation function that fails to allocate storage can invoke the
currently installed new_handler (_lib.new.handler_), if any. [Note: A
program-supplied allocation function can obtain the address of the
currently installed new_handler using the set_new_handler function
(_lib.set.new.handler_). ] If an allocation function declared with an
empty exception-specification (_except.spec_), throw(), fails to allo-
cate storage, it shall return a null pointer. Any other allocation
function that fails to allocate storage shall only indicate failure by
throwing an exception of class std::bad_alloc (_lib.bad.alloc_) or a
class derived from std::bad_alloc.
4 A global allocation function is only called as the result of a new
expression (_expr.new_), or called directly using the function call
syntax (_expr.call_), or called indirectly through calls to the func-
tions in the C++ standard library. [Note: in particular, a global
allocation function is not called to allocate storage for objects with
static storage duration (_basic.stc.static_), for objects of type
type_info (_expr.typeid_), for the copy of an object thrown by a throw
expression (_except.throw_). ]
3.7.3.2 Deallocation functions [basic.stc.dynamic.deallocation]
1 Deallocation functions shall be class member functions or global func-
tions; a program is ill-formed if deallocation functions are declared
in a namespace scope other than global scope or declared static in
global scope.
2 Each deallocation function shall return void and its first parameter
shall be void*. A deallocation function can have more than one param-
eter. If a class T has a member deallocation function named operator
delete with exactly one parameter, then that function is a usual (non-
placement) deallocation function. If class T does not declare such an
operator delete but does declare a member deallocation function named
operator delete with exactly two parameters, the second of which has
type std::size_t (_lib.support.types_), then this function is a usual
deallocation function. Similarly, if a class T has a member dealloca-
tion function named operator delete[] with exactly one parameter, then
that function is a usual (non-placement) deallocation function. If
class T does not declare such an operator delete[] but does declare a
member deallocation function named operator delete[] with exactly two
parameters, the second of which has type std::size_t, then this func-
tion is a usual deallocation function. A deallocation function can be
an instance of a function template. Neither the first parameter nor
_________________________
9) The intent is to have operator new() implementable by calling mal-
loc() or calloc(), so the rules are substantially the same. C++ dif-
fers from C in requiring a zero request to return a non-null pointer.
the return type shall depend on a template parameter. [Note: that is,
a deallocation function template shall have a first parameter of type
void* and a return type of void (as specified above). ] A dealloca-
tion function template shall have two or more function parameters. A
template instance is never a usual deallocation function, regardless
of its signature.
3 The value of the first argument supplied to one of the deallocation
functions provided in the standard library may be a null pointer
value; if so, the call to the deallocation function has no effect.
Otherwise, the value supplied to operator delete(void*) in the stan-
dard library shall be one of the values returned by a previous invoca-
tion of either operator new(size_t) or operator new(size_t, const
std::nothrow_t&) in the standard library, and the value supplied to
operator delete[](void*) in the standard library shall be one of the
values returned by a previous invocation of either operator
new[](size_t) or operator new[](size_t, const std::nothrow_t&) in the
standard library.
4 If the argument given to a deallocation function in the standard
library is a pointer that is not the null pointer value (_conv.ptr_),
the deallocation function shall deallocate the storage referenced by
the pointer, rendering invalid all pointers referring to any part of
the deallocated storage. The effect of using an invalid pointer value
(including passing it to a deallocation function) is undefined.10)
3.7.4 Duration of sub-objects [basic.stc.inherit]
1 The storage duration of member subobjects, base class subobjects and
array elements is that of their complete object (_intro.object_).
3.8 Object Lifetime [basic.life]
1 The lifetime of an object is a runtime property of the object. The
lifetime of an object of type T begins when:
--storage with the proper alignment and size for type T is obtained,
and
--if T is a class type with a non-trivial constructor (_class.ctor_),
the constructor call has completed.
The lifetime of an object of type T ends when:
--if T is a class type with a non-trivial destructor (_class.dtor_),
the destructor call starts, or
--the storage which the object occupies is reused or released.
_________________________
10) On some implementations, it causes a system-generated runtime
fault.
2 [Note: the lifetime of an array object or of an object of type
(_basic.types_) starts as soon as storage with proper size and align-
ment is obtained, and its lifetime ends when the storage which the
array or object occupies is reused or released. _class.base.init_
describes the lifetime of base and member subobjects. ]
3 The properties ascribed to objects throughout this International Stan-
dard apply for a given object only during its lifetime. [Note: in
particular, before the lifetime of an object starts and after its
lifetime ends there are significant restrictions on the use of the
object, as described below, in _class.base.init_ and in _class.cdtor_.
Also, the behavior of an object under construction and destruction
might not be the same as the behavior of an object whose lifetime has
started and not ended. _class.base.init_ and _class.cdtor_ describe
the behavior of objects during the construction and destruction
phases. ]
4 A program may end the lifetime of any object by reusing the storage
which the object occupies or by explicitly calling the destructor for
an object of a class type with a non-trivial destructor. For an
object of a class type with a non-trivial destructor, the program is
not required to call the destructor explicitly before the storage
which the object occupies is reused or released; however, if there is
no explicit call to the destructor or if a delete-expression
(_expr.delete_) is not used to release the storage, the destructor
shall not be implicitly called and any program that depends on the
side effects produced by the destructor has undefined behavior.
5 Before the lifetime of an object has started but after the storage
which the object will occupy has been allocated11) or, after the life-
time of an object has ended and before the storage which the object
occupied is reused or released, any pointer that refers to the storage
location where the object will be or was located may be used but only
in limited ways. Such a pointer refers to allocated storage
(_basic.stc.dynamic.deallocation_), and using the pointer as if the
pointer were of type void*, is well-defined. Such a pointer may be
dereferenced but the resulting lvalue may only be used in limited
ways, as described below. If the object will be or was of a class
type with a non-trivial destructor, and the pointer is used as the
operand of a delete-expression, the program has undefined behavior.
If the object will be or was of a non-POD class type, the program has
undefined behavior if:
--the pointer is used to access a non-static data member or call a
non-static member function of the object, or
--the pointer is implicitly converted (_conv.ptr_) to a pointer to a
base class type, or
--the pointer is used as the operand of a static_cast
_________________________
11) For example, before the construction of a global object of non-POD
class type (_class.cdtor_).
(_expr.static.cast_) (except when the conversion is to void*, or to
void* and subsequently to char*, or unsigned char*).
--the pointer is used as the operand of a dynamic_cast
(_expr.dynamic.cast_). [Example:
struct B {
virtual void f();
void mutate();
virtual ~B();
};
struct D1 : B { void f(); };
struct D2 : B { void f(); };
void B::mutate() {
new (this) D2; // reuses storage - ends the lifetime of *this
f(); // undefined behavior
... = this; // OK, this points to valid memory
}
void g() {
void* p = malloc(sizeof(D1) + sizeof(D2));
B* pb = new (p) D1;
pb->mutate();
&pb; // OK: pb points to valid memory
void* q = pb; // OK: pb points to valid memory
pb->f(); // undefined behavior, lifetime of *pb has ended
}
--end example]
6 Similarly, before the lifetime of an object has started but after the
storage which the object will occupy has been allocated or, after the
lifetime of an object has ended and before the storage which the
object occupied is reused or released, any lvalue which refers to the
original object may be used but only in limited ways. Such an lvalue
refers to allocated storage (_basic.stc.dynamic.deallocation_), and
using the properties of the lvalue which do not depend on its value is
well-defined. If an lvalue-to-rvalue conversion (_conv.lval_) is
applied to such an lvalue, the program has undefined behavior; if the
original object will be or was of a non-POD class type, the program
has undefined behavior if:
--the lvalue is used to access a non-static data member or call a non-
static member function of the object, or
--the lvalue is implicitly converted (_conv.ptr_) to a reference to a
base class type, or
--the lvalue is used as the operand of a static_cast
(_expr.static.cast_) (except when the conversion is ultimately to
char& or unsigned char&), or
--the lvalue is used as the operand of a dynamic_cast
(_expr.dynamic.cast_) or as the operand of typeid.
7 If, after the lifetime of an object has ended and before the storage
which the object occupied is reused or released, a new object is cre-
ated at the storage location which the original object occupied, a
pointer that pointed to the original object, a reference that referred
to the original object, or the name of the original object will auto-
matically refer to the new object and, once the lifetime of the new
object has started, can be used to manipulate the new object, if:
--the storage for the new object exactly overlays the storage location
which the original object occupied, and
--the new object is of the same type as the original object (ignoring
the top-level cv-qualifiers), and
--the original object was a most derived object (_intro.object_) of
type T and the new object is a most derived object of type T (that
is, they are not base class subobjects). [Example:
struct C {
int i;
void f();
const C& operator=( const C& );
};
const C& C::operator=( const C& other)
{
if ( this != &other )
{
this->~C(); // lifetime of *this ends
new (this) C(other); // new object of type C created
f(); // well-defined
}
return *this;
}
C c1;
C c2;
c1 = c2; // well-defined
c1.f(); // well-defined; c1 refers to a new object of type C
--end example]
8 If a program ends the lifetime of an object of type T with static
(_basic.stc.static_) or automatic (_basic.stc.auto_) storage duration
and if T has a non-trivial destructor,12) the program must ensure that
an object of the original type occupies that same storage location
when the implicit destructor call takes place; otherwise the behavior
of the program is undefined. This is true even if the block is exited
with an exception. [Example:
_________________________
12) that is, an object for which a destructor will be called implicit-
ly -- either upon exit from the block for an object with automatic
storage duration or upon exit from the program for an object with
static storage duration.
class T { };
struct B {
~B();
};
void h() {
B b;
new (&b) T;
} // undefined behavior at block exit
--end example]
9 Creating a new object at the storage location that a const object with
static or automatic storage duration occupies or, at the storage loca-
tion that such a const object used to occupy before its lifetime ended
results in undefined behavior. [Example:
struct B {
B();
~B();
};
const B b;
void h() {
b.~B();
new (&b) const B; // undefined behavior
}
--end example]
3.9 Types [basic.types]
1 [Note: _basic.types_ and the subclauses thereof impose requirements on
implementations regarding the representation of types. There are two
kinds of types: fundamental types and compound types. Types describe
objects (_intro.object_), references (_dcl.ref_), or functions
(_dcl.fct_). ]
2 For any complete POD object type T, whether or not the object holds a
valid value of type T, the underlying bytes (_intro.memory_) making up
the object can be copied into an array of char or unsigned char.13) If
the content of the array of char or unsigned char is copied back into
the object, the object shall subsequently hold its original value.
[Example:
#define N sizeof(T)
char buf[N];
T obj; // obj initialized to its original value
memcpy(buf, &obj, N); // between these two calls to memcpy,
// obj might be modified
memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type
// holds its original value
--end example]
3 For any POD type T, if two pointers to T point to distinct T objects
obj1 and obj2, if the value of obj1 is copied into obj2, using the
_________________________
13) By using, for example, the library functions (_lib.headers_) mem-
cpy or memmove.
memcpy library function, obj2 shall subsequently hold the same value
as obj1. [Example:
T* t1p;
T* t2p;
// provided that t2p points to an initialized object ...
memcpy(t1p, t2p, sizeof(T)); // at this point, every subobject of POD type in *t1p
// contains the same value as the corresponding subobject in
// *t2p
--end example]
4 The object representation of an object of type T is the sequence of N
unsigned char objects taken up by the object of type T, where N equals
sizeof(T). The value representation of an object is the set of bits
that hold the value of type T. For POD types, the value representa-
tion is a set of bits in the object representation that determines a
value, which is one discrete element of an implementation-defined set
of values.14)
5 Object types have alignment requirements (_basic.fundamental_,
_basic.compound_). The alignment of a complete object type is an
implementation-defined integer value representing a number of bytes;
an object is allocated at an address that meets the alignment require-
ments of its object type.
6 A class that has been declared but not defined, or an array of unknown
size or of incomplete element type, is an incompletely-defined object
type.15) Incompletely-defined object types and the void types are
incomplete types (_basic.fundamental_). Objects shall not be defined
to have an incomplete type.
7 A class type (such as "class X") might be incomplete at one point in a
translation unit and complete later on; the type "class X" is the same
type at both points. The declared type of an array object might be an
array of incomplete class type and therefore incomplete; if the class
type is completed later on in the translation unit, the array type
becomes complete; the array type at those two points is the same type.
The declared type of an array object might be an array of unknown size
and therefore be incomplete at one point in a translation unit and
complete later on; the array types at those two points ("array of
unknown bound of T" and "array of N T") are different types. The type
of a pointer to array of unknown size, or of a type defined by a type-
def declaration to be an array of unknown size, cannot be completed.
[Example:
_________________________
14) The intent is that the memory model of C++ is compatible with that
of ISO/IEC 9899 Programming Language C.
15) The size and layout of an instance of an incompletely-defined ob-
ject type is unknown.
class X; // X is an incomplete type
extern X* xp; // xp is a pointer to an incomplete type
extern int arr[]; // the type of arr is incomplete
typedef int UNKA[]; // UNKA is an incomplete type
UNKA* arrp; // arrp is a pointer to an incomplete type
UNKA** arrpp;
void foo()
{
xp++; // ill-formed: X is incomplete
arrp++; // ill-formed: incomplete type
arrpp++; // OK: sizeof UNKA* is known
}
struct X { int i; }; // now X is a complete type
int arr[10]; // now the type of arr is complete
X x;
void bar()
{
xp = &x; // OK; type is ``pointer to X''
arrp = &arr; // ill-formed: different types
xp++; // OK: X is complete
arrp++; // ill-formed: UNKA can't be completed
}
--end example]
8 [Note: the rules for declarations and expressions describe in which
contexts incomplete types are prohibited. ]
9 An object type is a (possibly cv-qualified) type that is not a func-
tion type, not a reference type, and not a void type.
10Arithmetic types (_basic.fundamental_), enumeration types, pointer
types, and pointer to member types (_basic.compound_), and cv-quali-
fied versions of these types (_basic.type.qualifier_) are collectively
called scalar types. Scalar types, POD-struct types, POD-union types
(clause _class_), arrays of such types and cv-qualified versions of
these types (_basic.type.qualifier_) are collectively called POD
types.
11If two types T1 and T2 are the same type, then T1 and T2 are layout-
compatible types. [Note: Layout-compatible enumerations are described
in _dcl.enum_. Layout-compatible POD-structs and POD-unions are
described in _class.mem_. ]
3.9.1 Fundamental types [basic.fundamental]
1 Objects declared as characters char) shall be large enough to store
any member of the implementation's basic character set. If a charac-
ter from this set is stored in a character object, the integral value
of that character object is equal to the value of the single character
literal form of that character. It is implementation-defined whether
a char object can hold negative values. Characters can be explicitly
declared unsigned or signed. Plain char, signed char, and
unsigned char are three distinct types. A char, a signed char, and an
unsigned char occupy the same amount of storage and have the same
alignment requirements (_basic.types_); that is, they have the same
object representation. For character types, all bits of the object
representation participate in the value representation. For unsigned
character types, all possible bit patterns of the value representation
represent numbers. These requirements do not hold for other types.
In any particular implementation, a plain char object can take on
either the same values as a signed char or an unsigned char; which one
is implementation-defined.
2 There are four signed integer types: "signed char", "short int",
"int", and "long int." In this list, each type provides at least as
much storage as those preceding it in the list. Plain ints have the
natural size suggested by the architecture of the execution
environment16) ; the other signed integer types are provided to meet
special needs.
3 For each of the signed integer types, there exists a corresponding
(but different) unsigned integer type: "unsigned char", "unsigned
short int", "unsigned int", and "unsigned long int," each of which
occupies the same amount of storage and has the same alignment
requirements (_basic.types_) as the corresponding signed integer
type17) ; that is, each signed integer type has the same object repre-
sentation as its corresponding unsigned integer type. The range of
nonnegative values of a signed integer type is a subrange of the cor-
responding unsigned integer type, and the value representation of each
corresponding signed/unsigned type shall be the same.
4 Unsigned integers, declared unsigned, shall obey the laws of arith-
metic modulo 2n where n is the number of bits in the value representa-
tion of that particular size of integer.18)
5 Type wchar_t is a distinct type whose values can represent distinct
codes for all members of the largest extended character set specified
among the supported locales (_lib.locale_). Type wchar_t shall have
the same size, signedness, and alignment requirements (_basic.types_)
as one of the other integral types, called its underlying type.
6 Values of type bool are either true or false.19) [Note: there are no
signed, unsigned, short, or long bool types or values. ] As described
below, bool values behave as integral types. Values of type bool
_________________________
16) that is, large enough to contain any value in the range of INT_MIN
and INT_MAX, as defined in the header <climits>.
17) See _dcl.type.simple_ regarding the correspondence between types
and the sequences of type-specifiers that designate them.
18) This implies that unsigned arithmetic does not overflow because a
result that cannot be represented by the resulting unsigned integer
type is reduced modulo the number that is one greater than the largest
value that can be represented by the resulting unsigned integer type.
19) Using a bool value in ways described by this International Stan-
dard as ``undefined,'' such as by examining the value of an uninitial-
ized automatic variable, might cause it to behave as if is neither
true nor false.
participate in integral promotions (_conv.prom_).
7 Types bool, char, wchar_t, and the signed and unsigned integer types
are collectively called integral types.20) A synonym for integral type
is integer type. The representations of integral types shall define
values by use of a pure binary numeration system.21) [Example: this
International Standard permits 2's complement, 1's complement and
signed magnitude representations for integral types. ]
8 There are three floating point types: float, double, and long double.
The type double provides at least as much precision as float, and the
type long double provides at least as much precision as double. The
set of values of the type float is a subset of the set of values of
the type double; the set of values of the type double is a subset of
the set of values of the type long double. The value representation
of floating-point types is implementation-defined. Integral and
floating types are collectively called arithmetic types. Specializa-
tions of the standard template numeric_limits (_lib.support.limits_)
shall specify the maximum and minimum values of each arithmetic type
for an implementation.
9 The void type has an empty set of values. The void type is an incom-
plete type that cannot be completed. It is used as the return type
for functions that do not return a value. Any expression can be
explicitly converted to type cv void (_expr.cast_). An expression of
type void shall be used only as an expression statement (_stmt.expr_),
as an operand of a comma expression (_expr.comma_), as a second or
third operand of ?: (_expr.cond_), as the operand of typeid, or as the
expression in a return statement (_stmt.return_) for a function with
the return type void.
10[Note: even if the implementation defines two or more basic types to
have the same value representation, they are nevertheless different
types. ]
3.9.2 Compound types [basic.compound]
1 Compound types can be constructed in the following ways:
--arrays of objects of a given type, _dcl.array_;
--functions, which have parameters of given types and return void or
references or objects of a given type, _dcl.fct_;
_________________________
20) Therefore, enumerations (_dcl.enum_) are not integral; however,
enumerations can be promoted to int, unsigned int, long, or unsigned
long, as specified in _conv.prom_.
21) A positional representation for integers that uses the binary dig-
its 0 and 1, in which the values represented by successive bits are
additive, begin with 1, and are multiplied by successive integral pow-
er of 2, except perhaps for the bit with the highest position.
(Adapted from the American National Dictionary for Information Pro-
cessing Systems.)
--pointers to void or objects or functions (including static members
of classes) of a given type, _dcl.ptr_;
--references to objects or functions of a given type, _dcl.ref_;
--classes containing a sequence of objects of various types (clause
_class_), a set of types, enumerations and functions for manipulat-
ing these objects (_class.mfct_), and a set of restrictions on the
access to these entities (clause _class.access_);
--unions, which are classes capable of containing objects of different
types at different times, _class.union_;
--enumerations, which comprise a set of named constant values. Each
distinct enumeration constitutes a different enumerated type,
_dcl.enum_;
--pointers to non-static22) class members, which identify members of a
given type within objects of a given class, _dcl.mptr_.
2 These methods of constructing types can be applied recursively;
restrictions are mentioned in _dcl.ptr_, _dcl.array_, _dcl.fct_, and
_dcl.ref_.
3 A pointer to objects of type T is referred to as a "pointer to T."
[Example: a pointer to an object of type int is referred to as
"pointer to int" and a pointer to an object of class X is called a
"pointer to X." ] Except for pointers to static members, text refer-
ring to "pointers" does not apply to pointers to members. Pointers to
incomplete types are allowed although there are restrictions on what
can be done with them (_basic.types_). The value representation of
pointer types is implementation-defined. Pointers to cv-qualified and
cv-unqualified versions (_basic.type.qualifier_) of layout-compatible
types shall have the same value representation and alignment require-
ments (_basic.types_).
4 Objects of cv-qualified (_basic.type.qualifier_) or cv-unqualified
type void* (pointer to void), can be used to point to objects of
unknown type. A void* shall be able to hold any object pointer. A
cv-qualified or cv-unqualified (_basic.type.qualifier_) void* shall
have the same representation and alignment requirements as a cv-quali-
fied or cv-unqualified char*.
3.9.3 CV-qualifiers [basic.type.qualifier]
1 A type mentioned in _basic.fundamental_ and _basic.compound_ is a cv-
unqualified type. Each type which is a cv-unqualified complete or
incomplete object type or is void (_basic.types_) has three corre-
sponding cv-qualified versions of its type: a const-qualified version,
a volatile-qualified version, and a const-volatile-qualified version.
_________________________
22) Static class members are objects or functions, and pointers to
them are ordinary pointers to objects or functions.
The term object type (_intro.object_) includes the cv-qualifiers spec-
ified when the object is created. The presence of a const specifier
in a decl-specifier-seq declares an object of const-qualified object
type; such object is called a const object. The presence of a
volatile specifier in a decl-specifier-seq declares an object of
volatile-qualified object type; such object is called a volatile
object. The presence of both cv-qualifiers in a decl-specifier-seq
declares an object of const-volatile-qualified object type; such
object is called a const volatile object. The cv-qualified or cv-
unqualified versions of a type are distinct types; however, they shall
have the same representation and alignment requirements
(_basic.types_).23)
2 A compound type (_basic.compound_) is not cv-qualified by the cv-qual-
ifiers (if any) of the types from which it is compounded. Any cv-
qualifiers applied to an array type affect the array element type, not
the array type (_dcl.array_).
3 Each non-static, non-mutable, non-reference data member of a const-
qualified class object is const-qualified, each non-static, non-refer-
ence data member of a volatile-qualified class object is volatile-
qualified and similarly for members of a const-volatile class. See
_dcl.fct_ and _class.this_ regarding cv-qualified function types.
4 There is a (partial) ordering on cv-qualifiers, so that a type can be
said to be more cv-qualified than another. Table 1 shows the rela-
tions that constitute this ordering.
_________________________
23) The same representation and alignment requirements are meant to
imply interchangeability as arguments to functions, return values from
functions, and members of unions.
Table 1--relations on const and volatile
+----------+
x] .if 00h0]>85M .as 10 0]u-85M)'
x] .if 00d0]>35M .as 10 0]u-35M'
no cv-qualifier<const
x] .if 00h0]>85M .as 10 0]u-85M)'
x] .if 00d0]>35M .as 10 0]u-35M'
no cv-qualifier<volatile
x] .if 00h0]>85M .as 10 0]u-85M)'
x] .if 00d0]>35M .as 10 0]u-35M'
no cv-qualifier<const volatile
x] .if 00h0]>85M .as 10 0]u-85M)'
x] .if 00d0]>35M .as 10 0]u-35M'
const<const volatile
x] .if 00h0]>85M .as 10 0]u-85M)'
x] .if 00d0]>35M .as 10 0]u-35M'
volatile<const volatile
+----------+
5 In this International Standard, the notation cv (or cv1, cv2, etc.),
used in the description of types, represents an arbitrary set of cv-
qualifiers, i.e., one of {const}, {volatile}, {const, volatile}, or
the empty set. Cv-qualifiers applied to an array type attach to the
underlying element type, so the notation "cv T," where T is an array
type, refers to an array whose elements are so-qualified. Such array
types can be said to be more (or less) cv-qualified than other types
based on the cv-qualification of the underlying element types.
3.10 Lvalues and rvalues [basic.lval]
1 Every expression is either an lvalue or an rvalue.
2 An lvalue refers to an object or function. Some rvalue expressions--
those of class or cv-qualified class type--also refer to objects.24)
3 [Note: some built-in operators and function calls yield lvalues.
[Example: if E is an expression of pointer type, then *E is an lvalue
expression referring to the object or function to which E points. As
another example, the function
int& f();
yields an lvalue, so the call f() is an lvalue expression. ] ]
4 [Note: some built-in operators expect lvalue operands. [Example:
built-in assignment operators all expect their left hand operands to
_________________________
24) Expressions such as invocations of constructors and of functions
that return a class type refer to objects, and the implementation can
invoke a member function upon such objects, but the expressions are
not lvalues.
be lvalues. ] Other built-in operators yield rvalues, and some expect
them. [Example: the unary and binary + operators expect rvalue argu-
ments and yield rvalue results. ] The discussion of each built-in
operator in clause _expr_ indicates whether it expects lvalue operands
and whether it yields an lvalue. ]
5 The result of calling a function that does not return a reference is
an rvalue. User defined operators are functions, and whether such
operators expect or yield lvalues is determined by their parameter and
return types.
6 An expression which holds a temporary object resulting from a cast to
a nonreference type is an rvalue (this includes the explicit creation
of an object using functional notation (_expr.type.conv_)).
7 Whenever an lvalue appears in a context where an rvalue is expected,
the lvalue is converted to an rvalue; see _conv.lval_, _conv.array_,
and _conv.func_.
8 The discussion of reference initialization in _dcl.init.ref_ and of
temporaries in _class.temporary_ indicates the behavior of lvalues and
rvalues in other significant contexts.
9 Class rvalues can have cv-qualified types; non-class rvalues always
have cv-unqualified types. Rvalues shall always have complete types
or the void type; in addition to these types, lvalues can also have
incomplete types.
10An lvalue for an object is necessary in order to modify the object
except that an rvalue of class type can also be used to modify its
referent under certain circumstances. [Example: a member function
called for an object (_class.mfct_) can modify the object. ]
11Functions cannot be modified, but pointers to functions can be modifi-
able.
12A pointer to an incomplete type can be modifiable. At some point in
the program when the pointed to type is complete, the object at which
the pointer points can also be modified.
13The referent of a const-qualified expression shall not be modified
(through that expression), except that if it is of class type and has
a mutable component, that component can be modified (_dcl.type.cv_).
14If an expression can be used to modify the object to which it refers,
the expression is called modifiable. A program that attempts to mod-
ify an object through a nonmodifiable lvalue or rvalue expression is
ill-formed.
15If a program attempts to access the stored value of an object through
an lvalue of other than one of the following types the behavior is
undefined25):
_________________________
25) The intent of this list is to specify those circumstances in which
--the dynamic type of the object,
--a cv-qualified version of the dynamic type of the object,
--a type that is the signed or unsigned type corresponding to the
dynamic type of the object,
--a type that is the signed or unsigned type corresponding to a cv-
qualified version of the dynamic type of the object,
--an aggregate or union type that includes one of the aforementioned
types among its members (including, recursively, a member of a sub-
aggregate or contained union),
--a type that is a (possibly cv-qualified) base class type of the
dynamic type of the object,
--a char or unsigned char type.
_________________________
an object may or may not be aliased.