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1 General [intro]
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1.1 Scope [intro.scope]
1 This International Standard specifies requirements for implementations
of the C++ programming language. The first such requirement is that
they implement the language, and so this International Standard also
defines C++. Other requirements and relaxations of the first require
ment appear at various places within the Standard.
2 C++ is a general purpose programming language based on the C program
ming language as described in ISO/IEC 9899:1990 Programming Languages
C (_intro.refs_). In addition to the facilities provided by C, C++
provides additional data types, classes, templates, exceptions, inline
functions, operator overloading, function name overloading, refer
ences, free store management operators, function argument checking and
type conversion, and additional library facilities. These extensions
to C are summarized in _diff.c_. The differences between C++ and ISO
C1) are summarized in _diff.iso_. The extensions to C++ since 1985
are summarized in _diff.c++_.
3 Clauses _lib.library_ through _lib.input.output_ (the library clauses)
describe the Standard C++ library, which provides definitions for the
following kinds of entities: macros (_cpp.replace_), values (_basic_),
types (_dcl.name_, _dcl.meaning_), templates (_temp_), classes
(_class_), functions (_dcl.fct_), and objects (_dcl.dcl_).
4 For classes and class templates, the library clauses specify partial
definitions. Private members (_class.access_) are not specified, but
each implementation shall supply them to complete the definitions
according to the description in the library clauses.
5 For functions, function templates, objects, and values, the library
clauses specify declarations. Implementations shall supply defini
tions consistent with the descriptions in the library clauses.
6 The names defined in the library have namespace scope
(_basic.namespace_). A C++ translation unit (_lex.phases_) obtains
access to these names by including the appropriate standard library
header (_cpp.include_).
7 The templates, classes, functions, and objects in the library have
external linkage (_basic.link_). An implementation provides defini
tions for standard library entities, as necessary, while combining
translation units to form a complete C++ program (_lex.phases_).
1.2 Normative references [intro.refs]
1 The following standards contain provisions which, through reference in
this text, constitute provisions of this International Standard. At
the time of publication, the editions indicated were valid. All stan
dards are subject to revision, and parties to agreements based on this
International Standard are encouraged to investigate the possibility
of applying the most recent editions of the standards indicated below.
Members of IEC and ISO maintain registers of currently valid Interna
tional Standards.
--ISO/IEC 2382 Dictionary for Information Processing Systems.
--ISO/IEC 9899:1990, C Standard
--ISO/IEC 9899:1990/DAM 1, Amendment 1 to C Standard
2 The library described in Clause 7 of the C Standard and Clause 7 of
Amendment 1 to the C Standard is hereinafter called the Standard C
Library.1)
1.3 Definitions [intro.defs]
1 For the purposes of this International Standard, the definitions given
in ISO/IEC 2382 and the following definitions apply.
--argument: An expression in the comma-separated list bounded by the
parentheses in a function call expression, a sequence of preprocess
ing tokens in the comma-separated list bounded by the parentheses in
a function-like macro invocation, the operand of throw, or an
expression in the comma-separated list bounded by the angle brackets
in a template instantiation. Also known as an "actual argument" or
"actual parameter."
--diagnostic message: A message belonging to an implementation-defined
subset of the implementation's message output.
--dynamic type: The dynamic type of an expression is determined by its
current value and can change during the execution of a program. If
a pointer (_dcl.ptr_) whose static type is "pointer to class B" is
pointing to an object of class D, derived from B (_class.derived_),
the dynamic type of the pointer is "pointer to D." References
(_dcl.ref_) are treated similarly.
--ill-formed program: input to a C++ implementation that is not a
well-formed program (q. v.).
--implementation-defined behavior: Behavior, for a correct program
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1) With the qualifications noted in clauses _lib.library_ through
_lib.input.output_, and in _diff.library_, the Standard C library is a
subset of the Standard C++ library.
construct and correct data, that depends on the implementation and
that each implementation shall document. The range of possible
behaviors is delineated by the standard.
--implementation limits: Restrictions imposed upon programs by the
implementation.
--locale-specific behavior: Behavior that depends on local conventions
of nationality, culture, and language that each implementation shall
document.
--multibyte character: A sequence of one or more bytes representing a
member of the extended character set of either the source or the
execution environment. The extended character set is a superset of
the basic character set.
--parameter: an object or reference declared as part of a function
declaration or definition in the catch clause of an exception han
dler that acquires a value on entry to the function or handler; an
identifier from the comma-separated list bounded by the parentheses
immediately following the macro name in a function-like macro defi
nition; or a template-parameter. A function can be said to "take
arguments" or to "have parameters." Parameters are also known as
"formal arguments" or "formal parameters."
--signature: The signature of a function is the information about that
function that participates in overload resolution (_over.match_):
the types of its parameters and, if the function is a non-static
member of a class, the CV-qualifiers (if any) on the function itself
and whether the function is a direct member of its class or inher
ited from a base class.2) The signature of a template function spe
cialization includes the types of its template arguments
(_temp.over.link_).
--static type: The static type of an expression is the type
(_basic.types_) resulting from analysis of the program without con
sideration of execution semantics. It depends only on the form of
the program and does not change while the program is executing.
--undefined behavior: Behavior, such as might arise upon use of an
erroneous program construct or of erroneous data, for which the
standard imposes no requirements. Permissible undefined behavior
ranges from ignoring the situation completely with unpredictable
results, to behaving during translation or program execution in a
documented manner characteristic of the environment (with or without
the issuance of a diagnostic message), to terminating a translation
or execution (with the issuance of a diagnostic message). Note that
many erroneous program constructs do not engender undefined behav
ior; they are required to be diagnosed.
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2) Function signatures do not include return type, because that does
not participate in overload resolution.
--unspecified behavior: Behavior, for a correct program construct and
correct data, that depends on the implementation. The implementa
tion is not required to document which behavior occurs. [Note: usu
ally, the range of possible behaviors is delineated by the standard.
--end note]
--well-formed program: a C++ program constructed according to the syn
tax rules, diagnosable semantic rules, and the One Definition Rule
(_basic.def.odr_).
2 Clause _lib.definitions_ defines additional terms that are used only
in the library clauses (_lib.library_-_lib.input.output_).
1.4 Syntax notation [syntax]
1 In the syntax notation used in this International Standard, syntactic
categories are indicated by italic type, and literal words and charac
ters in constant width type. Alternatives are listed on separate
lines except in a few cases where a long set of alternatives is pre
sented on one line, marked by the phrase "one of." An optional termi
nal or nonterminal symbol is indicated by the subscript "opt," so
{ expressionopt }
indicates an optional expression enclosed in braces.
2 Names for syntactic categories have generally been chosen according to
the following rules:
--X-name is a use of an identifier in a context that determines its
meaning (e.g. class-name, typedef-name).
--X-id is an identifier with no context-dependent meaning (e.g. qual
ified-id).
--X-seq is one or more X's without intervening delimiters (e.g. dec
laration-seq is a sequence of declarations).
--X-list is one or more X's separated by intervening commas (e.g.
expression-list is a sequence of expressions separated by commas).
1.5 The C++ memory model [intro.memory]
1 The fundamental storage unit in the C++ memory model is the byte. A
byte is at least large enough to contain any member of the basic exe
cution character set and is composed of a contiguous sequence of bits,
the number of which is implementation-defined. The least significant
bit is called the low-order bit; the most significant bit is called
the high-order bit. The memory accessible to a C++ program is one or
more contiguous sequences of bytes. Every byte has a unique
address.3)
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3) An implementation is free to disregard this requirement as long as
doing so has no perceptible effect on the execution of the program.
Thus, for example, an implementations is free to place any variable in
an internal register that does not have an address as long as the pro
2 [Note: the representation of types is described in _basic.types_. ]
1.6 The C++ object model [intro.object]
1 The constructs in a C++ program create, refer to, access, and manipu
late objects. An object is a region of storage and, unless it is a
bit-field (_class.bit_), or is part of another object and has a vir
tual base class (_class.mi_), occupies one or more contiguous bytes of
storage. An object is created by a definition (_basic.def_), by a
new-expression (_expr.new_) or by the implementation
(_class.temporary_) when needed. The properties of an object are
determined when the object is created. An object can have a name
(_basic_). An object has a storage duration (_basic.stc_) which influ
ences its lifetime (_basic.life_). An object has a type
(_basic.types_). The term object type refers to the type with which
the object is created. The object's type determines the number of
bytes that the object occupies and the interpretation of its content.
Some objects are polymorphic (_class.virtual_); the implementation
generates information carried in each such object that makes it possi
ble to determine that object's type during program execution. For
other objects, the interpretation of the values found therein is
determined by the type of the expressions (_expr_) used to access
them.
2 Objects can contain other objects, called sub-objects. A sub-object
can be a member sub-object (_class.mem_) or a base class sub-object
(_class.derived_). An object that is not a sub-object of any other
object is called a complete object. For every object x, there is some
object called the complete object of x, determined as follows:
--If x is a complete object, then x is the complete object of x.
--Otherwise, the complete object of x is the complete object of the
(unique) object that contains x.
3 C++ provides a variety of built-in types and several ways of composing
new types from existing types (_basic.types_).
1.7 Processor compliance [intro.compliance]
1 Every conforming C++ implementation shall, within its resource limits,
accept and correctly execute well-formed C++ programs, and shall issue
at least one diagnostic message when presented with any ill-formed
program that contains a violation of any diagnosable semantic rule or
of any syntax rule, except as noted in this International Standard.
2 If an ill-formed program contains no diagnosable errors, this Interna
tional Standard places no requirement on implementations with respect
to that program.
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gram does not do anything that depends on the address of the variable.
3 The set of "diagnosable semantic rules" consists of all semantic rules
in this International Standard except for those rules containing an
explicit notation that "no diagnostic is required."
4 Two kinds of implementations are defined: hosted and freestanding.
For a hosted implementation, this International Standard defines the
set of available libraries. A freestanding implementation is one in
which execution may take place without the benefit of an operating
system, and has an implementation-defined set of libraries that
includes certain language-support libraries (_lib.compliance_).
5 Although this International Standard states only requirements on C++
implementations, those requirements are often easier to understand if
they are phrased as requirements on programs, parts of programs, or
execution of programs. Such requirements have the following meaning:
--Whenever this International Standard places a requirement on the
form of a program (that is, the characters, tokens, syntactic ele
ments, and types that make up the program), and a program does not
meet that requirement, the program is ill-formed and an implementa
tion shall issue a diagnostic message when processing that program.
--Whenever this International Standard places a requirement on the
execution of a program (that is, the values of data that are used as
part of program execution) and the data encountered during execution
do not meet that requirement, the behavior of the program is unde
fined and this International Standard places no requirements at all
on the behavior of the program.
6 In this International Standard, the examples, the notes, the foot
notes, and the non-normative annexes are not part of the normative
Standard. Each example is introduced by "[Example:" and terminated by
"]". Each note is introduced by "[Note:" and terminated by "]".
7 A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any
well-formed program. One example of such an extension is allowing
identifiers to contain characters outside the basic source character
set. Implementations are required to diagnose programs that use such
extensions that are ill-formed according to this Standard. Having
done so, however, they can compile and execute such programs.
1.8 Program execution [intro.execution]
1 The semantic descriptions in this International Standard define a
parameterized nondeterministic abstract machine. This International
Standard places no requirement on the structure of conforming imple
mentations. In particular, they need not copy or emulate the struc
ture of the abstract machine. Rather, conforming implementations are
required to emulate (only) the observable behavior of the abstract
machine as explained below.
2 Certain aspects and operations of the abstract machine are described
in this International Standard as implementation-defined (for example,
sizeof(int)). These constitute the parameters of the abstract
machine. Each implementation shall include documentation describing
its characteristics and behavior in these respects. Such documenta
tion shall define the instance of the abstract machine that corre
sponds to that implementation (referred to as the ``corresponding
instance'' below).
3 Certain other aspects and operations of the abstract machine are
described in this International Standard as unspecified (for example,
order of evaluation of arguments to a function). Where possible, the
Standard defines a set of allowable behaviors.
+------- BEGIN BOX 1 -------+
Change: This used to say ``In each case.'' It seems unwise to claim
that the standard does something impossible.
+------- END BOX 1 -------+
These define the nondeterministic aspects of the abstract machine. An
instance of the abstract machine can thus have more than one possible
execution sequence for a given program and a given input.
4 Certain other operations are described in this International Standard
as undefined (for example, the effect of dereferencing the null
pointer).
5 A conforming implementation executing a well-formed program shall pro
duce the same observable behavior as one of the possible execution
sequences of the corresponding instance of the abstract machine with
the same program and the same input. However, if any such execution
sequence contains an undefined operation, this International Standard
places no requirement on the implementation executing that program
with that input (not even with regard to operations previous to the
first undefined operation).
6 The observable behavior of the abstract machine is its sequence of
reads and writes to volatile data and calls to library I/O
functions.4)
7 Accessing an object designated by a volatile lvalue (_basic.lval_),
modifying an object, modifying a file, or calling a function that does
any of those operations are all side effects, which are changes in the
state of the execution environment. Evaluation of an expression might
produce side effects. At certain specified points in the execution
sequence called sequence points, all side effects of previous evalua
tions shall be complete and no side effects of subsequent evaluations
shall have taken place.5)
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4) An implementation can offer additional library I/O functions as an
extension. Implementations that do so should treat calls to those
functions as ``observable behavior'' as well.
5) Note that some aspects of sequencing in the abstract machine are
8 Once the execution of a function begins, no expressions from the call
ing function are evaluated until execution of the called function is
completed.6)
9 In the abstract machine, all expressions are evaluated as specified by
the semantics. An actual implementation need not evaluate part of an
expression if it can deduce that its value is not used and that no
needed side effects are produced (including any caused by calling a
function or accessing a volatile object).
10When the processing of the abstract machine is interrupted by receipt
of a signal, only the values of objects as of the previous sequence
point may be relied on. Objects that may be modified between the pre
vious sequence point and the next sequence point need not have
received their correct values yet.
11An instance of each object with automatic storage duration
(_basic.stc.auto_) is associated with each entry into its block. Such
an object exists and retains its last-stored value during the execu
tion of the block and while the block is suspended (by a call of a
function or receipt of a signal).
12The least requirements on a conforming implementation are:
--At sequence points, volatile objects are stable in the sense that
previous evaluations are complete and subsequent evaluations have
not yet occurred.
--At program termination, all data written into files shall be identi
cal to one of the possible results that execution of the program
according to the abstract semantics would have produced.
--The input and output dynamics of interactive devices shall take
place in such a fashion that prompting messages actually appear
prior to a program waiting for input. What constitutes an interac
tive device is implementation-defined.
--More stringent correspondences between abstract and actual semantics
may be defined by each implementation.
13A full-expression is an expression that is not a subexpression of
another expression.
14[Note: certain contexts in C++ cause the evaluation of a full-
expression that results from a syntactic construct other than expres
sion (_expr.comma_). For example, in _dcl.init_ one syntax for
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unspecified; the preceding restriction upon side effects applies to
that particular execution sequence in which the actual code is gener
ated.
6) In other words, function executions do not interleave with each
other.
initializer is
( expression-list )
but the resulting construct is a function call upon a constructor
function with expression-list as an argument list; such a function
call is a full-expression. For another example in _dcl.init_, another
syntax for initializer is
= initializer-clause
but again the resulting construct might be a function call upon a con
structor function with one assignment-expression as an argument;
again, the function call is a full-expression. ]
15[Note: that the evaluation of a full-expression can include the evalu
ation of subexpressions that are not lexically part of the full-
expression. For example, subexpressions involved in evaluating
default argument expressions (_dcl.fct.default_) are considered to be
created in the expression that calls the function, not the expression
that defines the default argument. ]
16[Note: operators can be regrouped according to the usual mathematical
rules only where the operators really are associative or
commutative.7) For example, in the following fragment
int a, b;
/*...*/
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the
result of the sum (a + 32760) is next added to b, and that result is
then added to 5 which results in the value assigned to a. On a
machine in which overflows produce an exception and in which the range
of values representable by an int is [-32768,+32767], the implementa
tion cannot rewrite this expression as
a = ((a + b) + 32765);
since if the values for a and b were, respectively, -32754 and -15,
the sum a + b would produce an exception while the original expression
would not; nor can the expression be rewritten either as
a = ((a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been, respectively, 4 and -8
or -17 and 12. However on a machine in which overflows do not produce
an exception and in which the results of overflows are reversible, the
above expression statement can be rewritten by the implementation in
any of the above ways because the same result will occur. ]
17There is a sequence point at the completion of evaluation of each
full-expression8).
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7) Overloaded operators are never assumed to be associative or commu
tative.
8) As specified in _class.temporary_, after the "end-of-full-
expression" sequence point, a sequence of zero or more invocations of
destructor functions for temporary objects takes place, in reverse or
18When calling a function (whether or not the function is inline), there
is a sequence point after the evaluation of all function arguments (if
any) which takes place before execution of any expressions or state
ments in the function body. There is also a sequence point after the
copying of a returned value and before the execution of any expres
sions outside the function9). Several contexts in C++ cause evalua
tion of a function call, even though no corresponding function call
syntax appears in the translation unit. [Example: evaluation of a new
expression invokes one or more allocation and constructor functions;
see _expr.new_. For another example, invocation of a conversion func
tion (_class.conv.fct_) can arise in contexts in which no function
call syntax appears. ] The sequence points at function-entry and
function-exit (as described above) are features of the function calls
as evaluated, whatever the syntax of the expression that calls the
function might be.
19In the evaluation of each of the expressions
a && b
a || b
a ? b : c
a , b
using the builtin meaning of the operators in these expressions
(_expr.log.and_, _expr.log.or_, _expr.cond_, _expr.comma_) there is a
sequence point after the evaluation of the first expression10).
+------- BEGIN BOX 2 -------+
The Working Group is still discussing whether there is a sequence
point after the operand of dynamic-cast is evaluated; this is a con
text from which an exception might be thrown, even though no function
call is performed. This has not yet been voted upon by the Working
Group, and it may be redundant with the sequence point at function-
exit.
+------- END BOX 2 -------+
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der of the construction of each temporary object.
9) The sequence point at the function return is not explicitly speci
fied in ISO C, and can be considered redundant with sequence points at
full-expressions, but the extra clarity is important in C++. In C++,
there are more ways in which a called function can terminate its exe
cution, such as the throw of an exception.
10) The operators indicated in this paragraph are the builtin opera
tors, as described in Clause _expr_. When one of these operators is
overloaded (_over_) in a valid context, thus designating a user-
defined operator function, the expression designates a function invo
cation, and the operands form an argument list, without an implied se
quence point between them.