______________________________________________________________________
13 Overloading [over]
______________________________________________________________________
1 When two or more different declarations are specified for a single
name in the same scope, that name is said to be overloaded. By exten-
sion, two declarations in the same scope that declare the same name
but with different types are called overloaded declarations. Only
function declarations can be overloaded; object and type declarations
cannot be overloaded.
2 When an overloaded function name is used in a call, which overloaded
function declaration is being referenced is determined by comparing
the types of the arguments at the point of use with the types of the
parameters in the overloaded declarations that are visible at the
point of use. This function selection process is called overload res-
olution and is defined in _over.match_. [Example:
double abs(double);
int abs(int);
abs(1); // call abs(int);
abs(1.0); // call abs(double);
--end example]
13.1 Overloadable declarations [over.load]
1 Not all function declarations can be overloaded. Those that cannot be
overloaded are specified here. A program is ill-formed if it contains
two such non-overloadable declarations in the same scope. [Note: this
restriction applies to explicit declarations in a scope, and between
such declarations and declarations made through a using-declaration
(_namespace.udecl_). It does not apply to sets of functions fabri-
cated as a result of name lookup (e.g., because of using-directives)
or overload resolution (e.g., for operator functions). ]
2 Certain function declarations cannot be overloaded:
--Function declarations that differ only in the return type cannot be
overloaded.
--Member function declarations with the same name and the same parame-
ter types cannot be overloaded if any of them is a static member
function declaration (_class.static_). Likewise, member function
template declarations with the same name, the same parameter types,
and the same template parameter lists cannot be overloaded if any of
them is a static member function template declaration. The types of
the implicit object parameters constructed for the member functions
for the purpose of overload resolution (_over.match.funcs_) are not
considered when comparing parameter types for enforcement of this
rule. In contrast, if there is no static member function declara-
tion among a set of member function declarations with the same name
and the same parameter types, then these member function declara-
tions can be overloaded if they differ in the type of their implicit
object parameter. [Example: the following illustrates this distinc-
tion:
class X {
static void f();
void f(); // ill-formed
void f() const; // ill-formed
void f() const volatile; // ill-formed
void g();
void g() const; // OK: no static g
void g() const volatile; // OK: no static g
};
--end example]
3 [Note: as specified in _dcl.fct_, function declarations that have
equivalent parameter declarations declare the same function and there-
fore cannot be overloaded:
--Parameter declarations that differ only in the use of equivalent
typedef "types" are equivalent. A typedef is not a separate type,
but only a synonym for another type (_dcl.typedef_). [Example:
typedef int Int;
void f(int i);
void f(Int i); // OK: redeclaration of f(int)
void f(int i) { /* ... */ }
void f(Int i) { /* ... */ } // error: redefinition of f(int)
--end example]
Enumerations, on the other hand, are distinct types and can be used
to distinguish overloaded function declarations. [Example:
enum E { a };
void f(int i) { /* ... */ }
void f(E i) { /* ... */ }
--end example]
--Parameter declarations that differ only in a pointer * versus an
array [] are equivalent. That is, the array declaration is adjusted
to become a pointer declaration (_dcl.fct_). Only the second and
subsequent array dimensions are significant in parameter types
(_dcl.array_). [Example:
int f(char*);
int f(char[]); // same as f(char*);
int f(char[7]); // same as f(char*);
int f(char[9]); // same as f(char*);
int g(char(*)[10]);
int g(char[5][10]); // same as g(char(*)[10]);
int g(char[7][10]); // same as g(char(*)[10]);
int g(char(*)[20]); // different from g(char(*)[10]);
--end example]
--Parameter declarations that differ only in that one is a function
type and the other is a pointer to the same function type are equiv-
alent. That is, the function type is adjusted to become a pointer
to function type (_dcl.fct_). [Example:
void h(int());
void h(int (*)()); // redeclaration of h(int())
void h(int x()) { } // definition of h(int())
void h(int (*x)()) { } // ill-formed: redefinition of h(int())
]
--Parameter declarations that differ only in the presence or absence
of const and/or volatile are equivalent. That is, the const and
volatile type-specifiers for each parameter type are ignored when
determining which function is being declared, defined, or called.
[Example:
typedef const int cInt;
int f (int);
int f (const int); // redeclaration of f(int)
int f (int) { ... } // definition of f(int)
int f (cInt) { ... } // error: redefinition of f(int)
--end example]
Only the const and volatile type-specifiers at the outermost level
of the parameter type specification are ignored in this fashion;
const and volatile type-specifiers buried within a parameter type
specification are significant and can be used to distinguish over-
loaded function declarations.1) In particular, for any type T,
"pointer to T," "pointer to const T," and "pointer to volatile T"
are considered distinct parameter types, as are "reference to T,"
"reference to const T," and "reference to volatile T."
--Two parameter declarations that differ only in their default argu-
ments are equivalent. [Example: consider the following:
void f (int i, int j);
void f (int i, int j = 99); // OK: redeclaration of f(int, int)
void f (int i = 88, int j); // OK: redeclaration of f(int, int)
void f (); // OK: overloaded declaration of f
void prog ()
{
f (1, 2); // OK: call f(int, int)
f (1); // OK: call f(int, int)
f (); // Error: f(int, int) or f()?
}
--end example] --end note]
_________________________
1) When a parameter type includes a function type, such as in the case
of a parameter type that is a pointer to function, the const and
volatile type-specifiers at the outermost level of the parameter type
specifications for the inner function type are also ignored.
13.2 Declaration matching [over.dcl]
1 Two function declarations of the same name refer to the same function
if they are in the same scope and have equivalent parameter declara-
tions (_over.load_). A function member of a derived class is not in
the same scope as a function member of the same name in a base class.
[Example:
class B {
public:
int f(int);
};
class D : public B {
public:
int f(char*);
};
Here D::f(char*) hides B::f(int) rather than overloading it.
void h(D* pd)
{
pd->f(1); // error:
// D::f(char*) hides B::f(int)
pd->B::f(1); // OK
pd->f("Ben"); // OK, calls D::f
}
--end example]
2 A locally declared function is not in the same scope as a function in
a containing scope. [Example:
int f(char*);
void g()
{
extern f(int);
f("asdf"); // error: f(int) hides f(char*)
// so there is no f(char*) in this scope
}
void caller ()
{
extern void callee(int, int);
{
extern void callee(int); // hides callee(int, int)
callee(88, 99); // error: only callee(int) in scope
}
}
--end example]
3 Different versions of an overloaded member function can be given dif-
ferent access rules. [Example:
class buffer {
private:
char* p;
int size;
protected:
buffer(int s, char* store) { size = s; p = store; }
// ...
public:
buffer(int s) { p = new char[size = s]; }
// ...
};
--end example]
13.3 Overload resolution [over.match]
1 Overload resolution is a mechanism for selecting the best function to
call given a list of expressions that are to be the arguments of the
call and a set of candidate functions that can be called based on the
context of the call. The selection criteria for the best function are
the number of arguments, how well the arguments match the types of the
parameters of the candidate function, how well (for nonstatic member
functions) the object matches the implied object parameter, and cer-
tain other properties of the candidate function. [Note: the function
selected by overload resolution is not guaranteed to be appropriate
for the context. Other restrictions, such as the accessibility of the
function, can make its use in the calling context ill-formed. ]
2 Overload resolution selects the function to call in seven distinct
contexts within the language:
--invocation of a function named in the function call syntax
(_over.call.func_);
--invocation of a function call operator, a pointer-to-function con-
version function, a reference-to-pointer-to-function conversion
function, or a reference-to-function conversion function on a class
object named in the function call syntax (_over.call.object_);
--invocation of the operator referenced in an expression
(_over.match.oper_);
--invocation of a constructor for direct-initialization (_dcl.init_)
of a class object (_over.match.ctor_);
--invocation of a user-defined conversion for copy-initialization
(_dcl.init_) of a class object (_over.match.copy_);
--invocation of a conversion function for initialization of an object
of a nonclass type from an expression of class type
(_over.match.conv_); and
--invocation of a conversion function for conversion to an lvalue to
which a reference (_dcl.init.ref_) will be directly bound
(_over.match.ref_).
3 Each of these contexts defines the set of candidate functions and the
list of arguments in its own unique way. But, once the candidate
functions and argument lists have been identified, the selection of
the best function is the same in all cases:
--First, a subset of the candidate functions--those that have the
proper number of arguments and meet certain other conditions--is
selected to form a set of viable functions (_over.match.viable_).
--Then the best viable function is selected based on the implicit con-
version sequences (_over.best.ics_) needed to match each argument to
the corresponding parameter of each viable function.
4 If a best viable function exists and is unique, overload resolution
succeeds and produces it as the result. Otherwise overload resolution
fails and the invocation is ill-formed. When overload resolution suc-
ceeds, and the best viable function is not accessible (clause
_class.access_) in the context in which it is used, the program is
ill-formed.
13.3.1 Candidate functions and argument lists [over.match.funcs]
1 The subclauses of _over.match.funcs_ describe the set of candidate
functions and the argument list submitted to overload resolution in
each of the seven contexts in which overload resolution is used. The
source transformations and constructions defined in these subclauses
are only for the purpose of describing the overload resolution pro-
cess. An implementation is not required to use such transformations
and constructions.
2 The set of candidate functions can contain both member and non-member
functions to be resolved against the same argument list. So that
argument and parameter lists are comparable within this heterogeneous
set, a member function is considered to have an extra parameter,
called the implicit object parameter, which represents the object for
which the member function has been called. For the purposes of over-
load resolution, both static and non-static member functions have an
implicit object parameter, but constructors do not.
3 Similarly, when appropriate, the context can construct an argument
list that contains an implied object argument to denote the object to
be operated on. Since arguments and parameters are associated by
position within their respective lists, the convention is that the
implicit object parameter, if present, is always the first parameter
and the implied object argument, if present, is always the first argu-
ment.
4 For non-static member functions, the type of the implicit object
parameter is "reference to cv X" where X is the class of which the
function is a member and cv is the cv-qualification on the member
function declaration. [Example: for a const member function of class
X, the extra parameter is assumed to have type "reference to const X".
] For conversion functions, the function is considered to be a member
of the class of the implicit object argument for the purpose of defin-
ing the type of the implicit object parameter. For non-conversion
functions introduced by a using-declaration into a derived class, the
function is considered to be a member of the derived class for the
purpose of defining the type of the implicit object parameter. For
static member functions, the implicit object parameter is considered
to match any object (since if the function is selected, the object is
discarded). [Note: no actual type is established for the implicit
object parameter of a static member function, and no attempt will be
made to determine a conversion sequence for that parameter
(_over.match.best_). ]
5 During overload resolution, the implied object argument is indistin-
guishable from other arguments. The implicit object parameter, how-
ever, retains its identity since conversions on the corresponding
argument shall obey these additional rules:
--no temporary object can be introduced to hold the argument for the
implicit object parameter;
--no user-defined conversions can be applied to achieve a type match
with it; and
--even if the implicit object parameter is not const-qualified, an
rvalue temporary can be bound to the parameter as long as in all
other respects the temporary can be converted to the type of the
implicit object parameter.
6 Because only one user-defined conversion is allowed in an implicit
conversion sequence, special rules apply when selecting the best user-
defined conversion (_over.match.best_, _over.best.ics_). [Example:
class T {
public:
T();
// ...
};
class C : T {
public:
C(int);
// ...
};
T a = 1; // ill-formed: T(C(1)) not tried
--end example]
7 In each case where a candidate is a function template, candidate tem-
plate functions are generated using template argument deduction
(_temp.over_, _temp.deduct_). Those candidates are then handled as
candidate functions in the usual way.2) A given name can refer to one
or more function templates and also to a set of overloaded non-tem-
plate functions. In such a case, the candidate functions generated
from each function template are combined with the set of non-template
candidate functions.
_________________________
2) The process of argument deduction fully determines the parameter
types of the template functions, i.e., the parameters of template
functions contain no template parameter types. Therefore the template
functions can be treated as normal (non-template) functions for the
remainder of overload resolution.
13.3.1.1 Function call syntax [over.match.call]
1 Recall from _expr.call_, that a function call is a postfix-expression,
possibly nested arbitrarily deep in parentheses, followed by an
optional expression-list enclosed in parentheses:
(...(opt postfix-expression )...)opt (expression-listopt)
Overload resolution is required if the postfix-expression is the name
of a function, a function template (_temp.fct_), an object of class
type, or a set of pointers-to-function.
2 _over.call.func_ describes how overload resolution is used in the
first two of the above cases to determine the function to call.
_over.call.object_ describes how overload resolution is used in the
third of the above cases to determine the function to call.
3 The fourth case arises from a postfix-expression of the form &F, where
F names a set of overloaded functions. In the context of a function
call, the set of functions named by F shall contain only non-member
functions and static member functions3). And in this context using &F
behaves the same as using the name F by itself. Thus,
(&F)(expression-listopt) is simply (F)(expression-listopt), which is
discussed in _over.call.func_. (The resolution of &F in other con-
texts is described in _over.over_.)
13.3.1.1.1 Call to named function [over.call.func]
1 Of interest in _over.call.func_ are only those function calls in which
the postfix-expression ultimately contains a name that denotes one or
more functions that might be called. Such a postfix-expression, per-
haps nested arbitrarily deep in parentheses, has one of the following
forms:
postfix-expression:
postfix-expression . id-expression
postfix-expression -> id-expression
primary-expression
These represent two syntactic subcategories of function calls: quali-
fied function calls and unqualified function calls.
2 In qualified function calls, the name to be resolved is an id-expres-
sion and is preceded by an -> or . operator. Since the construct
A->B is generally equivalent to (*A).B, the rest of clause _over_
assumes, without loss of generality, that all member function calls
have been normalized to the form that uses an object and the . opera-
tor. Furthermore, clause _over_ assumes that the postfix-expression
that is the left operand of the . operator has type "cv T" where T
denotes a class4). Under this assumption, the id-expression in the
call is looked up as a member function of T following the rules for
looking up names in classes (_class.member.lookup_). If a member
_________________________
3) If F names a non-static member function, &F is a pointer-to-member,
which cannot be used with the function call syntax.
4) Note that cv-qualifiers on the type of objects are significant in
overload resolution for both lvalue and class rvalue objects.
function is found, that function and its overloaded declarations con-
stitute the set of candidate functions. The argument list is the
expression-list in the call augmented by the addition of the left
operand of the . operator in the normalized member function call as
the implied object argument (_over.match.funcs_).
3 In unqualified function calls, the name is not qualified by an -> or .
operator and has the more general form of a primary-expression. The
name is looked up in the context of the function call following the
normal rules for name lookup in function calls
(_basic.lookup.koenig_). If the name resolves to a non-member func-
tion declaration, that function and its overloaded declarations con-
stitute the set of candidate functions5). The argument list is the
same as the expression-list in the call. If the name resolves to a
nonstatic member function, then the function call is actually a member
function call. If the keyword this (_class.this_) is in scope and
refers to the class of that member function, or a derived class
thereof, then the function call is transformed into a normalized qual-
ified function call using (*this) as the postfix-expression to the
left of the . operator. The candidate functions and argument list
are as described for qualified function calls above. If the keyword
this is not in scope or refers to another class, then name resolution
found a static member of some class T. In this case, all overloaded
declarations of the function name in T become candidate functions and
a contrived object of type T becomes the implied object argument6).
The call is ill-formed, however, if overload resolution selects one of
the non-static member functions of T in this case.
13.3.1.1.2 Call to object of class type [over.call.object]
1 If the primary-expression E in the function call syntax evaluates to a
class object of type "cv T", then the set of candidate functions
includes at least the function call operators of T. The function call
operators of T are obtained by ordinary lookup of the name operator()
in the context of (E).operator().
2 In addition, for each conversion function declared in T of the form
operator conversion-type-id () cv-qualifier;
where cv-qualifier is the same cv-qualification as, or a greater cv-
qualification than, cv, and where conversion-type-id denotes the type
"pointer to function of (P1,...,Pn) returning R", or the type "refer-
ence to pointer to function of (P1,...,Pn) returning R", or the type
"reference to function of (P1,...,Pn) returning R", a surrogate call
_________________________
5) Because of the usual name hiding rules, these will be introduced by
declarations or by using-directives all found in the same block or all
found at namespace scope.
6) An implied object argument must be contrived to correspond to the
implicit object parameter attributed to member functions during over-
load resolution. It is not used in the call to the selected function.
Since the member functions all have the same implicit object parame-
ter, the contrived object will not be the cause to select or reject a
function.
function with the unique name call-function and having the form
R call-function (conversion-type-id F, P1 a1,...,Pn an) { return F (a1,...,an); }
is also considered as a candidate function. Similarly, surrogate call
functions are added to the set of candidate functions for each conver-
sion function declared in an accessible base class provided the func-
tion is not hidden within T by another intervening declaration7).
3 If such a surrogate call function is selected by overload resolution,
its body, as defined above, will be executed to convert E to the
appropriate function and then to invoke that function with the argu-
ments of the call.
4 The argument list submitted to overload resolution consists of the
argument expressions present in the function call syntax preceded by
the implied object argument (E). [Note: when comparing the call
against the function call operators, the implied object argument is
compared against the implicit object parameter of the function call
operator. When comparing the call against a surrogate call function,
the implied object argument is compared against the first parameter of
the surrogate call function. The conversion function from which the
surrogate call function was derived will be used in the conversion
sequence for that parameter since it converts the implied object argu-
ment to the appropriate function pointer or reference required by that
first parameter. ] [Example:
int f1(int);
int f2(float);
typedef int (*fp1)(int);
typedef int (*fp2)(float);
struct A {
operator fp1() { return f1; }
operator fp2() { return f2; }
} a;
int i = a(1); // Calls f1 via pointer returned from
// conversion function
--end example]
13.3.1.2 Operators in expressions [over.match.oper]
1 If no operand of an operator in an expression has a type that is a
class or an enumeration, the operator is assumed to be a built-in
operator and interpreted according to clause _expr_. [Note: because
., .*, and :: cannot be overloaded, these operators are always built-
in operators interpreted according to clause _expr_. ?: cannot be
overloaded, but the rules in this section are used to determine the
conversions to be applied to the second and third operands when they
have class or enumeration type (_expr.cond_). ] [Example:
_________________________
7) Note that this construction can yield candidate call functions that
cannot be differentiated one from the other by overload resolution be-
cause they have identical declarations or differ only in their return
type. The call will be ambiguous if overload resolution cannot select
a match to the call that is uniquely better than such undifferentiable
functions.
class String {
public:
String (const String&);
String (char*);
operator char* ();
};
String operator + (const String&, const String&);
void f(void)
{
char* p= "one" + "two"; // ill-formed because neither
// operand has user defined type
int I = 1 + 1; // Always evaluates to 2 even if
// user defined types exist which
// would perform the operation.
}
--end example]
2 If either operand has a type that is a class or an enumeration, a
user-defined operator function might be declared that implements this
operator or a user-defined conversion can be necessary to convert the
operand to a type that is appropriate for a built-in operator. In
this case, overload resolution is used to determine which operator
function or built-in operator is to be invoked to implement the opera-
tor. Therefore, the operator notation is first transformed to the
equivalent function-call notation as summarized in Table 1 (where @
denotes one of the operators covered in the specified subclause).
Table 1--relationship between operator and function call notation
+--------------+------------+--------------------+------------------------+
|Subclause | Expression | As member function | As non-member function |
+--------------+------------+--------------------+------------------------+
|_over.unary_ | @a | (a).operator@ () | operator@ (a) |
|_over.binary_ | a@b | (a).operator@ (b) | operator@ (a, b) |
|_over.ass_ | a=b | (a).operator= (b) | |
|_over.sub_ | a[b] | (a).operator[](b) | |
|_over.ref_ | a-> | (a).operator-> () | |
|_over.inc_ | a@ | (a).operator@ (0) | operator@ (a, 0) |
+--------------+------------+--------------------+------------------------+
3 For a unary operator @ with an operand of a type whose cv-unqualified
version is T1, and for a binary operator @ with a left operand of a
type whose cv-unqualified version is T1 and a right operand of a type
whose cv-unqualified version is T2, three sets of candidate functions,
designated member candidates, non-member candidates and built-in can-
didates, are constructed as follows:
--If T1 is a class type, the set of member candidates is the result of
the qualified lookup of T1::operator@ (_over.call.func_); otherwise,
the set of member candidates is empty.
--The set of non-member candidates is the result of the unqualified
lookup of operator@ in the context of the expression according to
the usual rules for name lookup in unqualified function calls
(_basic.lookup.koenig_) except that all member functions are
ignored. However, if no operand has a class type, only those non-
member functions in the lookup set that have a first parameter of
type T1 or "reference to (possibly cv-qualified) T1", when T1 is an
enumeration type, or (if there is a right operand) a second parame-
ter of type T2 or "reference to (possibly cv-qualified) T2", when T2
is an enumeration type, are candidate functions.
--For the operator ,, the unary operator &, or the operator ->, the
built-in candidates set is empty. For all other operators, the
built-in candidates include all of the candidate operator functions
defined in _over.built_ that, compared to the given operator,
--have the same operator name, and
--accept the same number of operands, and
--accept operand types to which the given operand or operands can be
converted according to _over.best.ics_, and
--do not have the same parameter type list as any non-template non-
member candidate.
4 For the built-in assignment operators, conversions of the left operand
are restricted as follows:
--no temporaries are introduced to hold the left operand, and
--no user-defined conversions are applied to the left operand to
achieve a type match with the left-most parameter of a built-in can-
didate.
5 For all other operators, no such restrictions apply.
6 The set of candidate functions for overload resolution is the union of
the member candidates, the non-member candidates, and the built-in
candidates. The argument list contains all of the operands of the
operator. The best function from the set of candidate functions is
selected according to _over.match.viable_ and _over.match.best_.8)
[Example:
_________________________
8) If the set of candidate functions is empty, overload resolution is
unsuccessful.
struct A {
operator int();
};
A operator+(const A&, const A&);
void m() {
A a, b;
a + b; // operator+(a,b) chosen over int(a) + int(b)
}
--end example]
7 If a built-in candidate is selected by overload resolution, the
operands are converted to the types of the corresponding parameters of
the selected operation function. Then the operator is treated as the
corresponding built-in operator and interpreted according to clause
_expr_.
8 The second operand of operator -> is ignored in selecting an opera-
tor-> function, and is not an argument when the operator-> function is
called. When operator-> returns, the operator -> is applied to the
value returned, with the original second operand.9)
9 If the operator is the operator ,, the unary operator &, or the opera-
tor ->, and there are no viable functions, then the operator is
assumed to be the built-in operator and interpreted according to
clause _expr_.
10[Note: the lookup rules for operators in expressions are different
than the lookup rules for operator function names in a function call,
as shown in the following example:
struct A { };
void operator + (A, A);
struct B {
void operator + (B);
void f ();
};
A a;
void B::f() {
operator+ (a,a); // ERROR - global operator hidden by member
a + a; // OK - calls global operator+
}
--end note]
_________________________
9) If the value returned by the operator-> function has class type,
this may result in selecting and calling another operator-> function.
The process repeats until an operator-> function returns a value of
non-class type.
13.3.1.3 Initialization by constructor [over.match.ctor]
1 When objects of class type are direct-initialized (_dcl.init_), over-
load resolution selects the constructor. The candidate functions are
all the constructors of the class of the object being initialized.
The argument list is the expression-list within the parentheses of the
initializer.
13.3.1.4 Copy-initialization of class by user- [over.match.copy]
defined conversion
1 Under the conditions specified in _dcl.init_, as part of a copy-ini-
tialization of an object of class type, a user-defined conversion can
be invoked to convert an initializer expression to the type of the
object being initialized. Overload resolution is used to select the
user-defined conversion to be invoked. Assuming that "cv1 T" is the
type of the object being initialized, with T a class type, the candi-
date functions are selected as follows:
--The converting constructors (_class.conv.ctor_) of T are candidate
functions.
--When the type of the initializer expression is a class type "cv S",
the conversion functions of S and its base classes are considered.
Those that are not hidden within S and yield a type whose cv-unqual-
ified version is the same type as T or is a derived class thereof
are candidate functions. Conversion functions that return "refer-
ence to T" return lvalues of type T and are therefore considered to
yield T for this process of selecting candidate functions.
2 In both cases, the argument list has one argument, which is the ini-
tializer expression. [Note: this argument will be compared against
the first parameter of the constructors and against the implicit
object parameter of the conversion functions. ]
13.3.1.5 Initialization by conversion function [over.match.conv]
1 Under the conditions specified in _dcl.init_, as part of an initial-
ization of an object of nonclass type, a conversion function can be
invoked to convert an initializer expression of class type to the type
of the object being initialized. Overload resolution is used to
select the conversion function to be invoked. Assuming that "cv1 T"
is the type of the object being initialized, and "cv S" is the type of
the initializer expression, with S a class type, the candidate func-
tions are selected as follows:
--The conversion functions of S and its base classes are considered.
Those that are not hidden within S and yield type T or a type that
can be converted to type T via a standard conversion sequence
(_over.ics.scs_) are candidate functions. Conversion functions that
return a cv-qualified type are considered to yield the cv-unquali-
fied version of that type for this process of selecting candidate
functions. Conversion functions that return "reference to T" return
lvalues of type T and are therefore considered to yield T for this
process of selecting candidate functions.
2 The argument list has one argument, which is the initializer expres-
sion. [Note: this argument will be compared against the implicit
object parameter of the conversion functions. ]
13.3.1.6 Initialization by conversion function [over.match.ref]
for direct reference binding
1 Under the conditions specified in _dcl.init.ref_, a reference can be
bound directly to an lvalue that is the result of applying a conver-
sion function to an initializer expression. Overload resolution is
used to select the conversion function to be invoked. Assuming that
"cv1 T" is the underlying type of the reference being initialized, and
"cv S" is the type of the initializer expression, with S a class type,
the candidate functions are selected as follows:
--The conversion functions of S and its base classes are considered.
Those that are not hidden within S and yield type "reference to cv2
T2", where "cv1 T" is reference-compatible (_dcl.init.ref_) with
"cv2 T2", are candidate functions.
2 The argument list has one argument, which is the initializer expres-
sion. [Note: this argument will be compared against the implicit
object parameter of the conversion functions. ]
13.3.2 Viable functions [over.match.viable]
1 From the set of candidate functions constructed for a given context
(_over.match.funcs_), a set of viable functions is chosen, from which
the best function will be selected by comparing argument conversion
sequences for the best fit (_over.match.best_). The selection of
viable functions considers relationships between arguments and func-
tion parameters other than the ranking of conversion sequences.
2 First, to be a viable function, a candidate function shall have enough
parameters to agree in number with the arguments in the list.
--If there are m arguments in the list, all candidate functions having
exactly m parameters are viable.
--A candidate function having fewer than m parameters is viable only
if it has an ellipsis in its parameter list (_dcl.fct_). For the
purposes of overload resolution, any argument for which there is no
corresponding parameter is considered to ``match the ellipsis''
(_over.ics.ellipsis_) .
--A candidate function having more than m parameters is viable only if
the (m+1)-st parameter has a default argument
(_dcl.fct.default_).10) For the purposes of overload resolution, the
_________________________
10) According to _dcl.fct.default_, parameters following the (m+1)-st
parameter must also have default arguments.
parameter list is truncated on the right, so that there are exactly
m parameters.
3 Second, for F to be a viable function, there shall exist for each
argument an implicit conversion sequence (_over.best.ics_) that con-
verts that argument to the corresponding parameter of F. If the
parameter has reference type, the implicit conversion sequence
includes the operation of binding the reference, and the fact that a
reference to non-const cannot be bound to an rvalue can affect the
viability of the function (see _over.ics.ref_).
13.3.3 Best Viable Function [over.match.best]
1 Define ICSi(F) as follows:
--if F is a static member function, ICS1(F) is defined such that
ICS1(F) is neither better nor worse than ICS1(G) for any function G,
and, symmetrically, ICS1(G) is neither better nor worse than
ICS1(F)11); otherwise,
--let ICSi(F) denote the implicit conversion sequence that converts
the i-th argument in the list to the type of the i-th parameter of
viable function F. _over.best.ics_ defines the implicit conversion
sequences and _over.ics.rank_ defines what it means for one implicit
conversion sequence to be a better conversion sequence or worse con-
version sequence than another.
Given these definitions, a viable function F1 is defined to be a bet-
ter function than another viable function F2 if for all arguments i,
ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then
--for some argument j, ICSj(F1) is a better conversion sequence than
ICSj(F2), or, if not that,
--F1 is a non-template function and F2 is a template function special-
ization, or, if not that,
--F1 and F2 are template functions, and the function template for F1
is more specialized than the template for F2 according to the par-
tial ordering rules described in _temp.func.order_, or, if not that,
--the context is an initialization by user-defined conversion (see
_dcl.init_, _over.match.conv_) and the standard conversion sequence
from the return type of F1 to the destination type (i.e., the type
of the entity being initialized) is a better conversion sequence
than the standard conversion sequence from the return type of F2 to
the destination type. [Example:
_________________________
11) If a function is a static member function, this definition means
that the first argument, the implied object parameter, has no effect
in the determination of whether the function is better or worse than
any other function.
struct A {
A();
operator int();
operator double();
} a;
int i = a; // a.operator int() followed by no conversion is better
// than a.operator double() followed by a conversion
// to int
float x = a; // ambiguous: both possibilities require conversions,
// and neither is better than the other
--end example]
2 If there is exactly one viable function that is a better function than
all other viable functions, then it is the one selected by overload
resolution; otherwise the call is ill-formed12).
3 [Example:
void Fcn(const int*, short);
void Fcn(int*, int);
int i;
short s = 0;
void f() {
Fcn(&i, s); // is ambiguous because
// &i -> int* is better than &i -> const int*
// but s -> short is also better than s -> int
Fcn(&i, 1L); // calls Fcn(int*, int), because
// &i -> int* is better than &i -> const int*
// and 1L -> short and 1L -> int are indistinguishable
Fcn(&i,'c'); // calls Fcn(int*, int), because
// &i -> int* is better than &i -> const int*
// and c -> int is better than c -> short
}
--end example]
13.3.3.1 Implicit conversion sequences [over.best.ics]
1 An implicit conversion sequence is a sequence of conversions used to
convert an argument in a function call to the type of the correspond-
ing parameter of the function being called. The sequence of conver-
sions is an implicit conversion as defined in clause _conv_, which
_________________________
12) The algorithm for selecting the best viable function is linear in
the number of viable functions. Run a simple tournament to find a
function W that is not worse than any opponent it faced. Although an-
other function F that W did not face might be at least as good as W, F
cannot be the best function because at some point in the tournament F
encountered another function G such that F was not better than G.
Hence, W is either the best function or there is no best function.
So, make a second pass over the viable functions to verify that W is
better than all other functions.
means it is governed by the rules for initialization of an object or
reference by a single expression (_dcl.init_, _dcl.init.ref_).
2 Implicit conversion sequences are concerned only with the type, cv-
qualification, and lvalue-ness of the argument and how these are con-
verted to match the corresponding properties of the parameter. Other
properties, such as the lifetime, storage class, alignment, or acces-
sibility of the argument and whether or not the argument is a bit-
field are ignored. So, although an implicit conversion sequence can
be defined for a given argument-parameter pair, the conversion from
the argument to the parameter might still be ill-formed in the final
analysis.
3 Except in the context of an initialization by user-defined conversion
(_over.match.copy_, _over.match.conv_), a well-formed implicit conver-
sion sequence is one of the following forms:
--a standard conversion sequence (_over.ics.scs_),
--a user-defined conversion sequence (_over.ics.user_), or
--an ellipsis conversion sequence (_over.ics.ellipsis_).
4 In the context of an initialization by user-defined conversion (i.e.,
when considering the argument of a user-defined conversion function;
see _over.match.copy_, _over.match.conv_), only standard conversion
sequences and ellipsis conversion sequences are allowed.
5 For the case where the parameter type is a reference, see
_over.ics.ref_.
6 When the parameter type is not a reference, the implicit conversion
sequence models a copy-initialization of the parameter from the argu-
ment expression. The implicit conversion sequence is the one required
to convert the argument expression to an rvalue of the type of the
parameter. [Note: when the parameter has a class type, this is a con-
ceptual conversion defined for the purposes of clause _over_; the
actual initialization is defined in terms of constructors and is not a
conversion. ] Any difference in top-level cv-qualification is sub-
sumed by the initialization itself and does not constitute a conver-
sion. [Example: a parameter of type A can be initialized from an
argument of type const A. The implicit conversion sequence for that
case is the identity sequence; it contains no "conversion" from const
A to A. ] When the parameter has a class type and the argument
expression has the same type, the implicit conversion sequence is an
identity conversion. When the parameter has a class type and the
argument expression has a derived class type, the implicit conversion
sequence is a derived-to-base Conversion from the derived class to the
base class. [Note: there is no such standard conversion; this
derived-to-base Conversion exists only in the description of implicit
conversion sequences. ] A derived-to-base Conversion has Conversion
rank (_over.ics.scs_).
7 In all contexts, when converting to the implicit object parameter or
when converting to the left operand of an assignment operation only
standard conversion sequences that create no temporary object for the
result are allowed.
8 If no conversions are required to match an argument to a parameter
type, the implicit conversion sequence is the standard conversion
sequence consisting of the identity conversion (_over.ics.scs_).
9 If no sequence of conversions can be found to convert an argument to a
parameter type or the conversion is otherwise ill-formed, an implicit
conversion sequence cannot be formed.
10If several different sequences of conversions exist that each convert
the argument to the parameter type, the implicit conversion sequence
associated with the parameter is defined to be the unique conversion
sequence designated the ambiguous conversion sequence. For the pur-
pose of ranking implicit conversion sequences as described in
_over.ics.rank_, the ambiguous conversion sequence is treated as a
user-defined sequence that is indistinguishable from any other user-
defined conversion sequence13). If a function that uses the ambiguous
conversion sequence is selected as the best viable function, the call
will be ill-formed because the conversion of one of the arguments in
the call is ambiguous.
_________________________
13) The ambiguous conversion sequence is ranked with user-defined con-
version sequences because multiple conversion sequences for an argu-
ment can exist only if they involve different user-defined conver-
sions. The ambiguous conversion sequence is indistinguishable from
any other user-defined conversion sequence because it represents at
least two user-defined conversion sequences, each with a different us-
er-defined conversion, and any other user-defined conversion sequence
must be indistinguishable from at least one of them.
This rule prevents a function from becoming non-viable because of an
ambiguous conversion sequence for one of its parameters. Consider
this example,
class B;
class A { A (B&); };
class B { operator A (); };
class C { C (B&); };
void f(A) { }
void f(C) { }
B b;
f(b); // ambiguous because b -> C via constructor and
// b -> A via constructor or conversion function.
If it were not for this rule, f(A) would be eliminated as a viable
function for the call f(b) causing overload resolution to select f(C)
as the function to call even though it is not clearly the best choice.
On the other hand, if an f(B) were to be declared then f(b) would re-
solve to that f(B) because the exact match with f(B) is better than
any of the sequences required to match f(A).
11The three forms of implicit conversion sequences mentioned above are
defined in the following subclauses.
13.3.3.1.1 Standard conversion sequences [over.ics.scs]
1 Table 2 summarizes the conversions defined in clause _conv_ and parti-
tions them into four disjoint categories: Lvalue Transformation, Qual-
ification Adjustment, Promotion, and Conversion. [Note: these cate-
gories are orthogonal with respect to lvalue-ness, cv-qualification,
and data representation: the Lvalue Transformations do not change the
cv-qualification or data representation of the type; the Qualification
Adjustments do not change the lvalue-ness or data representation of
the type; and the Promotions and Conversions do not change the lvalue-
ness or cv-qualification of the type. ]
2 [Note: As described in clause _conv_, a standard conversion sequence
is either the Identity conversion by itself (that is, no conversion)
or consists of one to three conversions from the other four cate-
gories. At most one conversion from each category is allowed in a
single standard conversion sequence. If there are two or more conver-
sions in the sequence, the conversions are applied in the canonical
order: Lvalue Transformation, Promotion or Conversion, Qualification
Adjustment. --end note]
3 Each conversion in Table 2 also has an associated rank (Exact Match,
Promotion, or Conversion). These are used to rank standard conversion
sequences (_over.ics.rank_). The rank of a conversion sequence is
determined by considering the rank of each conversion in the sequence
and the rank of any reference binding (_over.ics.ref_). If any of
those has Conversion rank, the sequence has Conversion rank; other-
wise, if any of those has Promotion rank, the sequence has Promotion
rank; otherwise, the sequence has Exact Match rank.
Table 2--conversions
+-------------------------------+--------------------------+-------------+-----------------+
|Conversion | Category | Rank | Subclause |
+-------------------------------+--------------------------+-------------+-----------------+
+-------------------------------+--------------------------+-------------+-----------------+
|No conversions required | Identity | | |
+-------------------------------+--------------------------+ +-----------------+
|Lvalue-to-rvalue conversion | | | _conv.lval_ |
+-------------------------------+ | +-----------------+
|Array-to-pointer conversion | Lvalue Transformation | Exact Match | _conv.array_ |
+-------------------------------+ | +-----------------+
|Function-to-pointer conversion | | | _conv.func_ |
+-------------------------------+--------------------------+ +-----------------+
|Qualification conversions | Qualification Adjustment | | _conv.qual_ |
+-------------------------------+--------------------------+-------------+-----------------+
|Integral promotions | | | _conv.prom_ |
+-------------------------------+ Promotion | Promotion +-----------------+
|Floating point promotion | | | _conv.fpprom_ |
+-------------------------------+--------------------------+-------------+-----------------+
|Integral conversions | | | _conv.integral_ |
+-------------------------------+ | +-----------------+
|Floating point conversions | | | _conv.double_ |
+-------------------------------+ | +-----------------+
|Floating-integral conversions | | | _conv.fpint_ |
+-------------------------------+ Conversion | Conversion +-----------------+
|Pointer conversions | | | _conv.ptr_ |
+-------------------------------+ | +-----------------+
|Pointer to member conversions | | | _conv.mem_ |
+-------------------------------+ | +-----------------+
|Boolean conversions | | | _conv.bool_ |
+-------------------------------+--------------------------+-------------+-----------------+
13.3.3.1.2 User-defined conversion sequences [over.ics.user]
1 A user-defined conversion sequence consists of an initial standard
conversion sequence followed by a user-defined conversion
(_class.conv_) followed by a second standard conversion sequence. If
the user-defined conversion is specified by a constructor
(_class.conv.ctor_), the initial standard conversion sequence converts
the source type to the type required by the argument of the construc-
tor. If the user-defined conversion is specified by a conversion
function (_class.conv.fct_), the initial standard conversion sequence
converts the source type to the implicit object parameter of the con-
version function.
2 The second standard conversion sequence converts the result of the
user-defined conversion to the target type for the sequence. Since an
implicit conversion sequence is an initialization, the special rules
for initialization by user-defined conversion apply when selecting the
best user-defined conversion for a user-defined conversion sequence
(see _over.match.best_ and _over.best.ics_).
3 If the user-defined conversion is specified by a template conversion
function, the second standard conversion sequence must have exact
match rank.
4 A conversion of an expression of class type to the same class type is
given Exact Match rank, and a conversion of an expression of class
type to a base class of that type is given Conversion rank, in spite
of the fact that a copy constructor (i.e., a user-defined conversion
function) is called for those cases.
13.3.3.1.3 Ellipsis conversion sequences [over.ics.ellipsis]
1 An ellipsis conversion sequence occurs when an argument in a function
call is matched with the ellipsis parameter specification of the func-
tion called.
13.3.3.1.4 Reference binding [over.ics.ref]
1 When a parameter of reference type binds directly (_dcl.init.ref_) to
an argument expression, the implicit conversion sequence is the iden-
tity conversion, unless the argument expression has a type that is a
derived class of the parameter type, in which case the implicit con-
version sequence is a derived-to-base Conversion (_over.best.ics_).
[Example:
struct A {};
struct B : public A {} b;
int f(A&);
int f(B&);
int i = f(b); // Calls f(B&), an exact match, rather than
// f(A&), a conversion
--end example] If the parameter binds directly to the result of
applying a conversion function to the argument expression, the
implicit conversion sequence is a user-defined conversion sequence
(_over.ics.user_), with the second standard conversion sequence either
an identity conversion or, if the conversion function returns an
entity of a type that is a derived class of the parameter type, a
derived-to-base Conversion.
2 When a parameter of reference type is not bound directly to an argu-
ment expression, the conversion sequence is the one required to con-
vert the argument expression to the underlying type of the reference
according to _over.best.ics_. Conceptually, this conversion sequence
corresponds to copy-initializing a temporary of the underlying type
with the argument expression. Any difference in top-level cv-qualifi-
cation is subsumed by the initialization itself and does not consti-
tute a conversion.
3 A standard conversion sequence cannot be formed if it requires binding
a reference to non-const to an rvalue (except when binding an implicit
object parameter; see the special rules for that case in
_over.match.funcs_). [Note: this means, for example, that a candidate
function cannot be a viable function if it has a non-const reference
parameter (other than the implicit object parameter) and the corre-
sponding argument is a temporary or would require one to be created to
initialize the reference (see _dcl.init.ref_). ]
4 Other restrictions on binding a reference to a particular argument do
not affect the formation of a standard conversion sequence, however.
[Example: a function with a "reference to int" parameter can be a
viable candidate even if the corresponding argument is an int bit-
field. The formation of implicit conversion sequences treats the int
bit-field as an int lvalue and finds an exact match with the parame-
ter. If the function is selected by overload resolution, the call
will nonetheless be ill-formed because of the prohibition on binding a
non-const reference to a bit-field (_dcl.init.ref_). ]
5 The binding of a reference to an expression that is reference-compati-
ble with added qualification influences the rank of a standard conver-
sion; see _over.ics.rank_ and _dcl.init.ref_.
13.3.3.2 Ranking implicit conversion sequences [over.ics.rank]
1 _over.ics.rank_ defines a partial ordering of implicit conversion
sequences based on the relationships better conversion sequence and
better conversion. If an implicit conversion sequence S1 is defined
by these rules to be a better conversion sequence than S2, then it is
also the case that S2 is a worse conversion sequence than S1. If con-
version sequence S1 is neither better than nor worse than conversion
sequence S2, S1 and S2 are said to be indistinguishable conversion
sequences.
2 When comparing the basic forms of implicit conversion sequences (as
defined in _over.best.ics_)
--a standard conversion sequence (_over.ics.scs_) is a better conver-
sion sequence than a user-defined conversion sequence or an ellipsis
conversion sequence, and
--a user-defined conversion sequence (_over.ics.user_) is a better
conversion sequence than an ellipsis conversion sequence
(_over.ics.ellipsis_).
3 Two implicit conversion sequences of the same form are indistinguish-
able conversion sequences unless one of the following rules apply:
--Standard conversion sequence S1 is a better conversion sequence than
standard conversion sequence S2 if
--S1 is a proper subsequence of S2 (comparing the conversion
sequences in the canonical form defined by _over.ics.scs_, exclud-
ing any Lvalue Transformation; the identity conversion sequence is
considered to be a subsequence of any non-identity conversion
sequence) or, if not that,
--the rank of S1 is better than the rank of S2 (by the rules defined
below), or, if not that,
--S1 and S2 differ only in their qualification conversion and yield
similar types T1 and T2 (_conv.qual_), respectively, and the cv-
qualification signature of type T1 is a proper subset of the cv-
qualification signature of type T2, [Example:
int f(const int *);
int f(int *);
int i;
int j = f(&i); // Calls f(int *)
--end example] or, if not that,
--S1 and S2 are reference bindings (_dcl.init.ref_), and the types
to which the references refer are the same type except for top-
level cv-qualifiers, and the type to which the reference initial-
ized by S2 refers is more cv-qualified than the type to which the
reference initialized by S1 refers. [Example:
int f(const int &);
int f(int &);
int g(const int &);
int g(int);
int i;
int j = f(i); // Calls f(int &)
int k = g(i); // ambiguous
class X {
public:
void f() const;
void f();
};
void g(const X& a, X b)
{
a.f(); // Calls X::f() const
b.f(); // Calls X::f()
}
--end example]
--User-defined conversion sequence U1 is a better conversion sequence
than another user-defined conversion sequence U2 if they contain the
same user-defined conversion function or constructor and if the sec-
ond standard conversion sequence of U1 is better than the second
standard conversion sequence of U2. [Example:
struct A {
operator short();
} a;
int f(int);
int f(float);
int i = f(a); // Calls f(int), because short -> int is
// better than short -> float.
--end example]
4 Standard conversion sequences are ordered by their ranks: an Exact
Match is a better conversion than a Promotion, which is a better con-
version than a Conversion. Two conversion sequences with the same
rank are indistinguishable unless one of the following rules applies:
--A conversion that is not a conversion of a pointer, or pointer to
member, to bool is better than another conversion that is such a
conversion.
--If class B is derived directly or indirectly from class A, conver-
sion of B* to A* is better than conversion of B* to void*, and con-
version of A* to void* is better than conversion of B* to void*.
--If class B is derived directly or indirectly from class A and class
C is derived directly or indirectly from B,
--conversion of C* to B* is better than conversion of C* to A*,
[Example:
struct A {};
struct B : public A {};
struct C : public B {};
C *pc;
int f(A *);
int f(B *);
int i = f(pc); // Calls f(B *)
--end example]
--binding of an expression of type C to a reference of type B& is
better than binding an expression of type C to a reference of type
A&,
--conversion of A::* to B::* is better than conversion of A::* to
C::*,
--conversion of C to B is better than conversion of C to A,
--conversion of B* to A* is better than conversion of C* to A*,
--binding of an expression of type B to a reference of type A& is
better than binding an expression of type C to a reference of type
A&,
--conversion of B::* to C::* is better than conversion of A::* to
C::*, and
--conversion of B to A is better than conversion of C to A.
[Note: compared conversion sequences will have different source
types only in the context of comparing the second standard conver-
sion sequence of an initialization by user-defined conversion (see
_over.match.best_); in all other contexts, the source types will be
the same and the target types will be different. ]
13.4 Address of overloaded function [over.over]
1 A use of an overloaded function name without arguments is resolved in
certain contexts to a function, a pointer to function or a pointer to
member function for a specific function from the overload set. A
function template name is considered to name a set of overloaded func-
tions in such contexts. The function selected is the one whose type
matches the target type required in the context. The target can be
--an object or reference being initialized (_dcl.init_,
_dcl.init.ref_),
--the left side of an assignment (_expr.ass_),
--a parameter of a function (_expr.call_),
--a parameter of a user-defined operator (_over.oper_),
--the return value of a function, operator function, or conversion
(_stmt.return_), or
--an explicit type conversion (_expr.type.conv_, _expr.static.cast_,
_expr.cast_).
The overloaded function name can be preceded by the & operator. An
overloaded function name shall not be used without arguments in con-
texts other than those listed. [Note: any redundant set of parenthe-
ses surrounding the overloaded function name is ignored (_expr.prim_).
]
2 If the name is a function template, template argument deduction is
done (_temp.deduct.funcaddr_), and if the argument deduction succeeds,
the deduced template arguments are used to generate a single template
function, which is added to the set of overloaded functions consid-
ered.
3 Non-member functions and static member functions match targets of type
"pointer-to-function" or "reference-to-function." Nonstatic member
functions match targets of type "pointer-to-member-function;" the
function type of the pointer to member is used to select the member
function from the set of overloaded member functions. If a nonstatic
member function is selected, the reference to the overloaded function
name is required to have the form of a pointer to member as described
in _expr.unary.op_.
4 If more than one function is selected, any template functions in the
set are eliminated if the set also contains a non-template function,
and any given template function is eliminated if the set contains a
second template function that is more specialized than the first
according to the partial ordering rules of _temp.func.order_. After
such eliminations, if any, there shall remain exactly one selected
function.
5 [Example:
int f(double);
int f(int);
int (*pfd)(double) = &f; // selects f(double)
int (*pfi)(int) = &f; // selects f(int)
int (*pfe)(...) = &f; // error: type mismatch
int (&rfi)(int) = f; // selects f(int)
int (&rfd)(double) = f; // selects f(double)
void g() {
(int (*)(int))&f; // cast expression as selector
}
The initialization of pfe is ill-formed because no f() with type
int(...) has been defined, and not because of any ambiguity. For
another example,
struct X {
int f(int);
static int f(long);
};
int (X::*p1)(int) = &X::f; // OK
int (*p2)(int) = &X::f; // error: mismatch
int (*p3)(long) = &X::f; // OK
int (X::*p4)(long) = &X::f; // error: mismatch
int (X::*p5)(int) = &(X::f); // error: wrong syntax for
// pointer to member
int (*p6)(long) = &(X::f); // OK
--end example]
6 [Note: if f() and g() are both overloaded functions, the cross product
of possibilities must be considered to resolve f(&g), or the equiva-
lent expression f(g). ]
7 [Note: there are no standard conversions (clause _conv_) of one
pointer-to-function type into another. In particular, even if B is a
public base of D, we have
D* f();
B* (*p1)() = &f; // error
void g(D*);
void (*p2)(B*) = &g; // error
--end note]
13.5 Overloaded operators [over.oper]
1 A function declaration having one of the following operator-function-
ids as its name declares an operator function. An operator function
is said to implement the operator named in its operator-function-id.
operator-function-id:
operator operator
operator: one of
new delete new[] delete[]
+ - * / % ^ & | ~
! = < > += -= *= /= %=
^= &= |= << >> >>= <<= == !=
<= >= && || ++ -- , ->* ->
() []
[Note: the last two operators are function call (_expr.call_) and
subscripting (_expr.sub_). The operators new[], delete[], (), and []
are formed from more than one token. ]
2 Both the unary and binary forms of
+ - * &
can be overloaded.
3 The following operators cannot be overloaded:
. .* :: ?:
nor can the preprocessing symbols # and ## (clause _cpp_).
4 Operator functions are usually not called directly; instead they are
invoked to evaluate the operators they implement (_over.unary_ -
_over.inc_). They can be explicitly called, however, using the opera-
tor-function-id as the name of the function in the function call syn-
tax (_expr.call_). [Example:
complex z = a.operator+(b); // complex z = a+b;
void* p = operator new(sizeof(int)*n);
--end example]
5 The allocation and deallocation functions, operator new, operator
new[], operator delete and operator delete[], are described completely
in _basic.stc.dynamic_. The attributes and restrictions found in the
rest of this section do not apply to them unless explicitly stated in
_basic.stc.dynamic_.
6 An operator function shall either be a non-static member function or
be a non-member function and have at least one parameter whose type is
a class, a reference to a class, an enumeration, or a reference to an
enumeration. It is not possible to change the precedence, grouping,
or number of operands of operators. The meaning of the operators =,
(unary) &, and , (comma), predefined for each type, can be changed for
specific class and enumeration types by defining operator functions
that implement these operators. Operator functions are inherited in
the same manner as other base class functions.
7 The identities among certain predefined operators applied to basic
types (for example, ++a == a+=1) need not hold for operator functions.
Some predefined operators, such as +=, require an operand to be an
lvalue when applied to basic types; this is not required by operator
functions.
8 An operator function cannot have default arguments
(_dcl.fct.default_), except where explicitly stated below. Operator
functions cannot have more or fewer parameters than the number
required for the corresponding operator, as described in the rest of
this subclause.
9 Operators not mentioned explicitly below in _over.ass_ to _over.inc_
act as ordinary unary and binary operators obeying the rules of sec-
tion _over.unary_ or _over.binary_.
13.5.1 Unary operators [over.unary]
1 A prefix unary operator shall be implemented by a non-static member
function (_class.mfct_) with no parameters or a non-member function
with one parameter. Thus, for any prefix unary operator @, @x can be
interpreted as either x.operator@() or operator@(x). If both forms of
the operator function have been declared, the rules in
_over.match.oper_ determine which, if any, interpretation is used.
See _over.inc_ for an explanation of the postfix unary operators ++
and --.
2 The unary and binary forms of the same operator are considered to have
the same name. [Note: consequently, a unary operator can hide a
binary operator from an enclosing scope, and vice versa. ]
13.5.2 Binary operators [over.binary]
1 A binary operator shall be implemented either by a non-static member
function (_class.mfct_) with one parameter or by a non-member function
with two parameters. Thus, for any binary operator @, x@y can be
interpreted as either x.operator@(y) or operator@(x,y). If both forms
of the operator function have been declared, the rules in
_over.match.oper_ determines which, if any, interpretation is used.
13.5.3 Assignment [over.ass]
1 An assignment operator shall be implemented by a non-static member
function with exactly one parameter. Because a copy assignment opera-
tor operator= is implicitly declared for a class if not declared by
the user (_class.copy_), a base class assignment operator is always
hidden by the copy assignment operator of the derived class.
2 Any assignment operator, even the copy assignment operator, can be
virtual. [Note: for a derived class D with a base class B for which a
virtual copy assignment has been declared, the copy assignment opera-
tor in D does not override B's virtual copy assignment operator.
[Example:
struct B {
virtual int operator= (int);
virtual B& operator= (const B&);
};
struct D : B {
virtual int operator= (int);
virtual D& operator= (const B&);
};
D dobj1;
D dobj2;
B* bptr = &dobj1;
void f() {
bptr->operator=(99); // calls D::operator=(int)
*bptr = 99; // ditto
bptr->operator=(dobj2); // calls D::operator=(const B&)
*bptr = dobj2; // ditto
dobj1 = dobj2; // calls implicitly-declared
// D::operator=(const D&)
}
--end example] --end note]
13.5.4 Function call [over.call]
1 operator() shall be a non-static member function with an arbitrary
number of parameters. It can have default arguments. It implements
the function call syntax
postfix-expression ( expression-listopt )
where the postfix-expression evaluates to a class object and the pos-
sibly empty expression-list matches the parameter list of an opera-
tor() member function of the class. Thus, a call x(arg1,...) is
interpreted as x.operator()(arg1,...) for a class object x of type T
if T::operator()(T1, T2, T3) exists and if the operator is selected as
the best match function by the overload resolution mechanism
(_over.match.best_).
13.5.5 Subscripting [over.sub]
1 operator[] shall be a non-static member function with exactly one
parameter. It implements the subscripting syntax
postfix-expression [ expression ]
Thus, a subscripting expression x[y] is interpreted as x.operator[](y)
for a class object x of type T if T::operator[](T1) exists and if the
operator is selected as the best match function by the overload reso-
lution mechanism (_over.match.best_).
13.5.6 Class member access [over.ref]
1 operator-> shall be a non-static member function taking no parameters.
It implements class member access using ->
postfix-expression -> id-expression
An expression x->m is interpreted as (x.operator->())->m for a class
object x of type T if T::operator->() exists and if the operator is
selected as the best match function by the overload resolution mecha-
nism (_over.match_).
13.5.7 Increment and decrement [over.inc]
1 The user-defined function called operator++ implements the prefix and
postfix ++ operator. If this function is a member function with no
parameters, or a non-member function with one parameter of class or
enumeration type, it defines the prefix increment operator ++ for
objects of that type. If the function is a member function with one
parameter (which shall be of type int) or a non-member function with
two parameters (the second of which shall be of type int), it defines
the postfix increment operator ++ for objects of that type. When the
postfix increment is called as a result of using the ++ operator, the
int argument will have value zero.14) [Example:
class X {
public:
X& operator++(); // prefix ++a
X operator++(int); // postfix a++
};
class Y { };
Y& operator++(Y&); // prefix ++b
Y operator++(Y&, int); // postfix b++
void f(X a, Y b)
{
++a; // a.operator++();
a++; // a.operator++(0);
++b; // operator++(b);
b++; // operator++(b, 0);
a.operator++(); // explicit call: like ++a;
a.operator++(0); // explicit call: like a++;
operator++(b); // explicit call: like ++b;
operator++(b, 0); // explicit call: like b++;
}
--end example]
2 The prefix and postfix decrement operators -- are handled analogously.
13.6 Built-in operators [over.built]
1 The candidate operator functions that represent the built-in operators
defined in clause _expr_ are specified in this subclause. These can-
didate functions participate in the operator overload resolution pro-
cess as described in _over.match.oper_ and are used for no other pur-
pose.
2 [Note: since built-in operators take only operands with non-class
type, and operator overload resolution occurs only when an operand
expression originally has class or enumeration type, operator overload
resolution can resolve to a built-in operator only when an operand has
a class type that has a user-defined conversion to a non-class type
appropriate for the operator, or when an operand has an enumeration
type that can be converted to a type appropriate for the operator.
Also note that the candidate operator functions given in this section
are in some cases more permissive than the built-in operators them-
selves. As described in _over.match.oper_, after a built-in operator
is selected by overload resolution the expression is subject to the
requirements for the built-in operator given in clause _expr_, and
therefore to any additional semantic constraints given there. If
there is a user-written candidate with the same name and parameter
_________________________
14) Calling operator++ explicitly, as in expressions like a.opera-
tor++(2), has no special properties: The argument to operator++ is 2.
types as a built-in candidate operator function, the built-in operator
function is hidden and is not included in the set of candidate func-
tions. ]
3 In this section, the term promoted integral type is used to refer to
those integral types which are preserved by integral promotion
(including e.g. int and long but excluding e.g. char). Similarly,
the term promoted arithmetic type refers to promoted integral types
plus floating types. [Note: in all cases where a promoted integral
type or promoted arithmetic type is required, an operand of enumera-
tion type will be acceptable by way of the integral promotions. ]
4 For every pair T, VQ), where T is an arithmetic type, and VQ is either
volatile or empty, there exist candidate operator functions of the
form
VQ T& operator++(VQ T&);
T operator++(VQ T&, int);
5 For every pair T, VQ), where T is an arithmetic type other than bool,
and VQ is either volatile or empty, there exist candidate operator
functions of the form
VQ T& operator--(VQ T&);
T operator--(VQ T&, int);
6 For every pair T, VQ), where T is a cv-qualified or cv-unqualified
object type, and VQ is either volatile or empty, there exist candidate
operator functions of the form
T*VQ& operator++(T*VQ&);
T*VQ& operator--(T*VQ&);
T* operator++(T*VQ&, int);
T* operator--(T*VQ&, int);
7 For every cv-qualified or cv-unqualified object type T, there exist
candidate operator functions of the form
T& operator*(T*);
8 For every function type T, there exist candidate operator functions of
the form
T& operator*(T*);
9 For every type T, there exist candidate operator functions of the form
T* operator+(T*);
10For every promoted arithmetic type T, there exist candidate operator
functions of the form
T operator+(T);
T operator-(T);
11For every promoted integral type T, there exist candidate operator
functions of the form
T operator~(T);
12For every quintuple C1, C2, T, CV1, CV2), where C2 is a class type, C1
is the same type as C2 or is a derived class of C2, T is an object
type or a function type, and CV1 and CV2 are cv-qualifier-seqs, there
exist candidate operator functions of the form
CV12 T& operator->*(CV1 C1*, CV2 T C2::*);
where CV12 is the union of CV1 and CV2.
13For every pair of promoted arithmetic types L and R, there exist can-
didate operator functions of the form
LR operator*(L, R);
LR operator/(L, R);
LR operator+(L, R);
LR operator-(L, R);
bool operator<(L, R);
bool operator>(L, R);
bool operator<=(L, R);
bool operator>=(L, R);
bool operator==(L, R);
bool operator!=(L, R);
where LR is the result of the usual arithmetic conversions between
types L and R.
14For every cv-qualified or cv-unqualified object type T there exist
candidate operator functions of the form
T* operator+(T*, ptrdiff_t);
T& operator[](T*, ptrdiff_t);
T* operator-(T*, ptrdiff_t);
T* operator+(ptrdiff_t, T*);
T& operator[](ptrdiff_t, T*);
15For every T, where T is a pointer to object type, there exist candi-
date operator functions of the form
ptrdiff_t operator-(T, T);
16For every pointer or enumeration type T, there exist candidate opera-
tor functions of the form
bool operator<(T, T);
bool operator>(T, T);
bool operator<=(T, T);
bool operator>=(T, T);
bool operator==(T, T);
bool operator!=(T, T);
17For every pointer to member type T, there exist candidate operator
functions of the form
bool operator==(T, T);
bool operator!=(T, T);
18For every pair of promoted integral types L and R, there exist candi-
date operator functions of the form
LR operator%(L, R);
LR operator&(L, R);
LR operator^(L, R);
LR operator|(L, R);
L operator<<(L, R);
L operator>>(L, R);
where LR is the result of the usual arithmetic conversions between
types L and R.
19For every triple L, VQ, R), where L is an arithmetic type, VQ is
either volatile or empty, and R is a promoted arithmetic type, there
exist candidate operator functions of the form
VQ L& operator=(VQ L&, R);
VQ L& operator*=(VQ L&, R);
VQ L& operator/=(VQ L&, R);
VQ L& operator+=(VQ L&, R);
VQ L& operator-=(VQ L&, R);
20For every pair T, VQ), where T is any type and VQ is either volatile
or empty, there exist candidate operator functions of the form
T*VQ& operator=(T*VQ&, T*);
21For every pair T, VQ), where T is an enumeration or pointer to member
type and VQ is either volatile or empty, there exist candidate opera-
tor functions of the form
VQ T& operator=(VQ T&, T);
22For every pair T, VQ), where T is a cv-qualified or cv-unqualified
object type and VQ is either volatile or empty, there exist candidate
operator functions of the form
T*VQ& operator+=(T*VQ&, ptrdiff_t);
T*VQ& operator-=(T*VQ&, ptrdiff_t);
23For every triple L, VQ, R), where L is an integral type, VQ is either
volatile or empty, and R is a promoted integral type, there exist can-
didate operator functions of the form
VQ L& operator%=(VQ L&, R);
VQ L& operator<<=(VQ L&, R);
VQ L& operator>>=(VQ L&, R);
VQ L& operator&=(VQ L&, R);
VQ L& operator^=(VQ L&, R);
VQ L& operator|=(VQ L&, R);
24There also exist candidate operator functions of the form
bool operator!(bool);
bool operator&&(bool, bool);
bool operator||(bool, bool);
25For every pair of promoted arithmetic types L and R, there exist can-
didate operator functions of the form
LR operator?(bool, L, R);
where LR is the result of the usual arithmetic conversions between
types L and R. [Note: as with all these descriptions of candidate
functions, this declaration serves only to describe the built-in oper-
ator for purposes of overload resolution. The operator ?" cannot be
overloaded. ]
26For every type T, where T is a pointer or pointer-to-member type,
there exist candidate operator functions of the form
T operator?(bool, T, T);