\rSec0[intro]{General}

\indextext{diagnostic message|see{message, diagnostic}}%

\indexdefn{conditionally-supported behavior!see{behavior, conditionally-supported}}%

\indextext{dynamic type|see{type, dynamic}}%

\indextext{static type|see{type, static}}%

\indextext{ill-formed program|see{program, ill-formed}}%

\indextext{well-formed program|see{program, well-formed}}%

\indextext{implementation-defined behavior|see{behavior, implemen\-tation-defined}}%

\indextext{undefined behavior|see{behavior, undefined}}%

\indextext{unspecified behavior|see{behavior, unspecified}}%

\indextext{implementation limits|see{limits, implementation}}%

\indextext{locale-specific behavior|see{behavior, locale-specific}}%

\indextext{multibyte character|see{character, multibyte}}%

\indextext{object|seealso{object model}}%

\indextext{subobject|seealso{object model}}%

\indextext{derived class!most|see{most derived class}}%

\indextext{derived object!most|see{most derived object}}%

\indextext{program execution!as-if rule|see{as-if~rule}}%

\indextext{observable behavior|see{behavior, observable}}%

\indextext{precedence of operator|see{operator, precedence of}}%

\indextext{order of evaluation in expression|see{expression, order of evaluation of}}%

\indextext{atomic operations|see{operation, atomic}}%

\indextext{multiple threads|see{threads, multiple}}%

\indextext{normative references|see{references, normative}}

\rSec1[intro.scope]{Scope}

\pnum

\indextext{scope|(}%

This International Standard specifies requirements for implementations

of the \Cpp programming language. The first such requirement is that

they implement the language, and so this International Standard also

defines \Cpp. Other requirements and relaxations of the first

requirement appear at various places within this International Standard.

\pnum

\Cpp is a general purpose programming language based on the C

programming language as described in ISO/IEC 9899:1999

\doccite{Programming languages --- C} (hereinafter referred to as the

\indexdefn{C!standard}\term{C standard}). In addition to

the facilities provided by C, \Cpp provides additional data types,

classes, templates, exceptions, namespaces, operator

overloading, function name overloading, references, free store

management operators, and additional library facilities.%

\indextext{scope|)}

\rSec1[intro.refs]{Normative references}

\pnum

\indextext{references!normative|(}%

The following referenced documents are indispensable for the application

of this document. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document

(including any amendments) applies.

\begin{itemize}

\item Ecma International, \doccite{ECMAScript Language Specification},

Standard Ecma-262, third edition, 1999.

\item ISO/IEC 2382 (all parts), \doccite{Information technology ---

Vocabulary}

\item ISO/IEC 9899:1999, \doccite{Programming languages --- C}

\item ISO/IEC 9899:1999/Cor.1:2001(E), \doccite{Programming languages --- C,

Technical Corrigendum 1}

\item ISO/IEC 9899:1999/Cor.2:2004(E), \doccite{Programming languages --- C,

Technical Corrigendum 2}

\item ISO/IEC 9899:1999/Cor.3:2007(E), \doccite{Programming languages --- C,

Technical Corrigendum 3}

\item ISO/IEC 9945:2003, \doccite{Information Technology --- Portable

Operating System Interface (POSIX)}

\item ISO/IEC 10646-1:1993, \doccite{Information technology ---

Universal Multiple-Octet Coded Character Set (UCS) --- Part 1:

Architecture and Basic Multilingual Plane}

\item ISO/IEC TR 19769:2004, \doccite{Information technology ---

Programming languages, their environments and system software interfaces

--- Extensions for the programming language C to support new character

data types}

\end{itemize}

\pnum

The library described in Clause 7 of ISO/IEC 9899:1999 and Clause 7 of

ISO/IEC 9899:1999/Cor.1:2001 and Clause 7 of ISO/IEC

9899:1999/Cor.2:2003 is hereinafter called the

\indexdefn{C!standard library}\term{C standard

library}.\footnote{With the qualifications noted in Clauses~\ref{\firstlibchapter}

through~\ref{\lastlibchapter} and in~\ref{diff.library}, the C standard

library is a subset of the \Cpp standard library.}

\pnum

The library described in ISO/IEC TR 19769:2004 is hereinafter called the

\indexdefn{C!Unicode TR}\term{C Unicode TR}.

\pnum

The operating system interface described in ISO/IEC 9945:2003 is

hereinafter called \defn{POSIX}.

\pnum

The ECMAScript Language Specification described in Standard Ecma-262 is

hereinafter called \defn{ECMA-262}.

\indextext{references!normative|)}

\rSec1[intro.defs]{Terms and definitions}

\pnum

\indextext{definitions|(}%

For the purposes of this document, the following definitions apply.

\pnum

\ref{definitions}

defines additional terms that are used only in Clauses~\ref{library}

through~\ref{\lastlibchapter} and Annex~\ref{depr}.

\pnum

Terms that are used only in a small portion of this International

Standard are defined where they are used and italicized where they are

defined.

\indexdefn{argument}%

\indexdefn{argument!function call expression}

\definition{argument}{defns.argument}

actual argument\\

actual parameter\\

<function call expression> expression in the comma-separated list bounded by the parentheses

\indexdefn{argument}%

\indexdefn{argument!function-like macro}%

\definition{argument}{defns.argument.macro}

actual argument\\

actual parameter\\

<function-like macro> sequence of preprocessing tokens in the

comma-separated list bounded by the parentheses

\indexdefn{argument}%

\indexdefn{argument!throw expression}%

\definition{argument}{defns.argument.throw}

actual argument\\

actual parameter\\

<throw expression> the operand of \tcode{throw}

\indexdefn{argument}%

\indexdefn{argument!template instantiation}%

\definition{argument}{defns.argument.templ}

actual argument\\

actual parameter\\

<template instantiation>

expression,

\grammarterm{type-id} or \grammarterm{template-name} in the comma-separated

list bounded by the angle brackets

\indexdefn{behavior!conditionally-supported}%

\definition{conditionally-supported}{defns.cond.supp}

program construct that an implementation is not required to support\\

\enternote Each implementation documents all conditionally-supported

constructs that it does not support.\exitnote

\indexdefn{message!diagnostic}%

\definition{diagnostic message}{defns.diagnostic}

message belonging to an \impldef{diagnostic message} subset of the

implementation's output messages

\indexdefn{type!dynamic}%

\definition{dynamic type}{defns.dynamic.type}

<glvalue> type of the most derived object~(\ref{intro.object}) to which the

glvalue denoted by a glvalue expression refers\\

\enterexample

if a pointer~(\ref{dcl.ptr}) \tcode{p} whose static type is ``pointer to

class \tcode{B}'' is pointing to an object of class \tcode{D}, derived

from \tcode{B} (Clause~\ref{class.derived}), the dynamic type of the

expression \tcode{*p} is ``\tcode{D}.'' References~(\ref{dcl.ref}) are

treated similarly.

\exitexample

\indexdefn{type!dynamic}%

\definition{dynamic type}{defns.dynamic.type.prvalue}

<prvalue> static type of the prvalue expression

\indexdefn{program!ill-formed}%

\definition{ill-formed program}{defns.ill.formed}

program that is not well formed

\indexdefn{behavior!implementation-defined}%

\definition{implementation-defined behavior}{defns.impl.defined}

behavior, for a well-formed program construct and correct data, that

depends on the implementation and that each implementation documents

\indexdefn{limits!implementation}%

\definition{implementation limits}{defns.impl.limits}

restrictions imposed upon programs by the implementation

\indexdefn{behavior!locale-specific}%

\definition{locale-specific behavior}{defns.locale.specific}

behavior that depends on local conventions of nationality, culture, and

language that each implementation documents

\indexdefn{character!multibyte}%

\definition{multibyte character}{defns.multibyte}

sequence of one or more bytes representing a member of the extended

character set of either the source or the execution environment\\

\enternote The

extended character set is a superset of the basic character

set~(\ref{lex.charset}).\exitnote

\indexdefn{parameter}%

\indexdefn{parameter!function}%

\indexdefn{parameter!catch clause}%

\definition{parameter}{defns.parameter}

formal argument\\

formal parameter\\

<function or catch clause> object or reference declared as part of a function declaration or

definition or in the catch clause of an exception handler that

acquires a value on entry to the function or handler

\indexdefn{parameter}%

\indexdefn{parameter!function-like macro}%

\definition{parameter}{defns.parameter.macro}

formal argument\\

formal parameter\\

<function-like macro> identifier from

the comma-separated list bounded by the parentheses immediately

following the macro name

\indexdefn{parameter}%

\indexdefn{parameter!template}%

\definition{parameter}{defns.parameter.templ}

formal argument\\

formal parameter\\

<template> \grammarterm{template-parameter}

\indexdefn{signature}%

\definition{signature}{defns.signature}

<function> name, parameter type list~(\ref{dcl.fct}), and enclosing namespace (if any)\\

\enternote Signatures are used as a basis for

name mangling and linking.\exitnote

\indexdefn{signature}%

\definition{signature}{defns.signature.templ}

<function template> name, parameter type list~(\ref{dcl.fct}), enclosing namespace (if any),

return type, and template parameter list

\indexdefn{signature}%

\definition{signature}{defns.signature.spec}

<function template specialization> signature of the template of which it is a specialization

and its template arguments (whether explicitly specified or deduced)

\indexdefn{signature}%

\definition{signature}{defns.signature.member}

<class member function> name, parameter type list~(\ref{dcl.fct}), class of which the

function is a member, \cv-qualifiers (if any),

and \grammarterm{ref-qualifier} (if any)

\indexdefn{signature}%

\definition{signature}{defns.signature.member.templ}

<class member function template> name, parameter type list~(\ref{dcl.fct}), class of which the

function is a member, \cv-qualifiers (if any),

\grammarterm{ref-qualifier} (if any), return type, and template parameter list

\indexdefn{signature}%

\definition{signature}{defns.signature.member.spec}

<class member function template specialization> signature of the member function template

of which it is a specialization and its template arguments (whether explicitly specified or deduced)

\indexdefn{type!static}%

\definition{static type}{defns.static.type}

type of an expression~(\ref{basic.types}) resulting from

analysis of the program without considering execution semantics\\

\enternote The

static type of an expression depends only on the form of the program in

which the expression appears, and does not change while the program is

executing. \exitnote

\indexdefn{behavior!undefined}%

\definition{undefined behavior}{defns.undefined}

behavior for which this International Standard

imposes no requirements\\

\enternote Undefined behavior may be expected when

this International Standard omits any explicit

definition of behavior or when a program uses an erroneous construct or erroneous data.

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). Many erroneous program constructs do

not engender undefined behavior; they are required to be diagnosed.

\exitnote

\indexdefn{behavior!unspecified}%

\definition{unspecified behavior}{defns.unspecified}

behavior, for a well-formed program construct and correct data, that

depends on the implementation\\

\enternote The implementation is not required to

document which behavior occurs. The range of

possible behaviors is usually delineated by this International Standard.

\exitnote

\indexdefn{program!well-formed}%

\definition{well-formed program}{defns.well.formed}

\Cpp program constructed according to the syntax rules, diagnosable

semantic rules, and the One Definition Rule~(\ref{basic.def.odr}).%

\indextext{definitions|)}

\rSec1[intro.compliance]{Implementation compliance}

\pnum

\indextext{conformance requirements|(}%

\indextext{conformance requirements!general|(}%

The set of

\defn{diagnosable rules}

consists of all syntactic and semantic rules in this International

Standard except for those rules containing an explicit notation that

``no diagnostic is required'' or which are described as resulting in

``undefined behavior.''

\pnum

\indextext{conformance requirements!method of description}%

Although this International Standard states only requirements on \Cpp

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:

\begin{itemize}

\item

If a program contains no violations of the rules in this

International Standard, a conforming implementation shall,

within its resource limits, accept and correctly execute\footnote{``Correct execution'' can include undefined behavior, depending on

the data being processed; see~\ref{intro.defs} and~\ref{intro.execution}.}

that program.

\item

\indextext{message!diagnostic}%

If a program contains a violation of any diagnosable rule or an occurrence

of a construct described in this Standard as ``conditionally-supported'' when

the implementation does not support that construct, a conforming implementation

shall issue at least one diagnostic message.

\item

\indextext{behavior!undefined}%

If a program contains a violation of a rule for which no diagnostic

is required, this International Standard places no requirement on

implementations with respect to that program.

\end{itemize}

\pnum

\indextext{conformance requirements!library|(}%

\indextext{conformance requirements!classes}%

\indextext{conformance requirements!class templates}%

For classes and class templates, the library Clauses specify partial

definitions. Private members~(Clause~\ref{class.access}) are not

specified, but each implementation shall supply them to complete the

definitions according to the description in the library Clauses.

\pnum

For functions, function templates, objects, and values, the library

Clauses specify declarations. Implementations shall supply definitions

consistent with the descriptions in the library Clauses.

\pnum

The names defined in the library have namespace

scope~(\ref{basic.namespace}). A \Cpp translation

unit~(\ref{lex.phases}) obtains access to these names by including the

appropriate standard library header~(\ref{cpp.include}).

\pnum

The templates, classes, functions, and objects in the library have

external linkage~(\ref{basic.link}). The implementation provides

definitions for standard library entities, as necessary, while combining

translation units to form a complete \Cpp program~(\ref{lex.phases}).%

\indextext{conformance requirements!library|)}

\pnum

Two kinds of implementations are defined: a \defn{hosted implementation} and a

\defn{freestanding implementation}. 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 \impldef{required libraries for freestanding

implementation} set of libraries that includes certain language-support

libraries~(\ref{compliance}).

\pnum

A conforming implementation may have extensions (including

additional library functions), provided they do not alter the

behavior of any well-formed program.

Implementations are required to diagnose programs that use such

extensions that are ill-formed according to this International Standard.

Having done so, however, they can compile and execute such programs.

\pnum

Each implementation shall include documentation that identifies all

conditionally-supported constructs\indextext{behavior!conditionally-supported}

that it does not support and defines all locale-specific characteristics.\footnote{This documentation also defines implementation-defined behavior;

see~\ref{intro.execution}.}%

\indextext{conformance requirements!general|)}%

\indextext{conformance requirements|)}%

\rSec1[intro.structure]{Structure of this International Standard}

\pnum

\indextext{standard!structure of|(}%

\indextext{standard!structure of}%

Clauses~\ref{lex} through~\ref{cpp} describe the \Cpp programming

language. That description includes detailed syntactic specifications in

a form described in~\ref{syntax}. For convenience, Annex~\ref{gram}

repeats all such syntactic specifications.

\pnum

Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter} and Annex~\ref{depr}

(the \defn{library clauses}) describe the Standard \Cpp library.

That description includes detailed descriptions of the templates, classes, functions, constants, and macros that constitute the library, in a form described in Clause~\ref{library}.

\pnum

Annex~\ref{implimits} recommends lower bounds on the capacity of conforming

implementations.

\pnum

Annex~\ref{diff} summarizes the evolution of \Cpp since its first

published description, and explains in detail the differences between

\Cpp and C. Certain features of \Cpp exist solely for compatibility

purposes; Annex~\ref{depr} describes those features.

\pnum

Throughout this International Standard, each example is introduced by

``\enterexample'' and terminated by ``\exitexample''. Each note is

introduced by ``\enternote'' and terminated by ``\exitnote''. Examples

and notes may be nested.%

\indextext{standard!structure of|)}

\rSec1[syntax]{Syntax notation}

\pnum

\indextext{notation!syntax|(}%

In the syntax notation used in this International Standard, syntactic

categories are indicated by \grammarterm{italic} type, and literal words

and characters in \tcode{constant} \tcode{width} type. Alternatives are

listed on separate lines except in a few cases where a long set of

alternatives is marked by the phrase ``one of.'' If the text of an alternative is too long to fit on a line, the text is continued on subsequent lines indented from the first one.

An optional terminal or non-terminal symbol is indicated by the subscript

``\opt'', so

\begin{ncbnf}

\terminal{\{} expression\opt \terminal{\}}

\end{ncbnf}

indicates an optional expression enclosed in braces.%

\pnum

Names for syntactic categories have generally been chosen according to

the following rules:

\begin{itemize}

\item \grammarterm{X-name} is a use of an identifier in a context that

determines its meaning (e.g., \grammarterm{class-name},

\grammarterm{typedef-name}).

\item \grammarterm{X-id} is an identifier with no context-dependent meaning

(e.g., \grammarterm{qualified-id}).

\item \grammarterm{X-seq} is one or more \grammarterm{X}'s without intervening

delimiters (e.g., \grammarterm{declaration-seq} is a sequence of

declarations).

\item \grammarterm{X-list} is one or more \grammarterm{X}'s separated by

intervening commas (e.g., \grammarterm{expression-list} is a sequence of

expressions separated by commas).

\end{itemize}%

\indextext{notation!syntax|)}

\rSec1[intro.memory]{The \Cpp memory model}

\pnum

\indextext{memory model|(}%

The fundamental storage unit in the \Cpp memory model is the

\defn{byte}.

A byte is at least large enough to contain any member of the basic

\indextext{character set!basic execution}%

execution character set~(\ref{lex.charset})

and the eight-bit code units of the Unicode UTF-8 encoding form

and is composed of a contiguous sequence of

bits, the number of which is \impldef{bits in a byte}. The least

significant bit is called the \defn{low-order bit}; the most

significant bit is called the \defn{high-order bit}. The memory

available to a \Cpp program consists of one or more sequences of

contiguous bytes. Every byte has a unique address.

\pnum

\enternote The representation of types is described

in~\ref{basic.types}. \exitnote

\pnum

A \defn{memory location} is either an object of scalar type or a maximal

sequence of adjacent bit-fields all having non-zero width. \enternote Various

features of the language, such as references and virtual functions, might

involve additional memory locations that are not accessible to programs but are

managed by the implementation. \exitnote Two \DIFaddbegin \DIFadd{or more }\DIFaddend threads of

execution~(\ref{intro.multithread}) can update and access separate memory

locations without interfering with each other.

\pnum

\enternote Thus a bit-field and an adjacent non-bit-field are in separate memory

locations, and therefore can be concurrently updated by two threads of execution

without interference. The same applies to two bit-fields, if one is declared

inside a nested struct declaration and the other is not, or if the two are

separated by a zero-length bit-field declaration, or if they are separated by a

non-bit-field declaration. It is not safe to concurrently update two bit-fields

in the same struct if all fields between them are also bit-fields of non-zero

width. \exitnote

\pnum

\enterexample A structure declared as

\begin{codeblock}

struct {

char a;

int b:5,

c:11,

:0,

d:8;

struct {int ee:8;} e;

}

\end{codeblock}

contains four separate memory locations: The field \tcode{a} and bit-fields

\tcode{d} and \tcode{e.ee} are each separate memory locations, and can be

modified concurrently without interfering with each other. The bit-fields

\tcode{b} and \tcode{c} together constitute the fourth memory location. The

bit-fields \tcode{b} and \tcode{c} cannot be concurrently modified, but

\tcode{b} and \tcode{a}, for example, can be. \exitexample%

\indextext{memory model|)}

\rSec1[intro.object]{The \Cpp object model}

\pnum

\indextext{object model|(}%

\indextext{object}%

The constructs in a \Cpp program create, destroy, refer to, access, and

manipulate objects. An \defn{object} is a region of storage.

\enternote A function is not an object, regardless of whether or not it

occupies storage in the way that objects do. \exitnote An object is

created by a \grammarterm{definition}~(\ref{basic.def}), by a

\grammarterm{new-expression}~(\ref{expr.new}) or by the

implementation~(\ref{class.temporary}) when needed. The properties of an

object are determined when the object is created. An object can have a

\grammarterm{name}~(Clause~\ref{basic}). An object has a \term{storage

duration}~(\ref{basic.stc}) which influences its

\term{lifetime}~(\ref{basic.life}). An object has a

\term{type}~(\ref{basic.types}). The term \defn{object type} refers to

the type with which the object is created.

Some objects are

\term{polymorphic}~(\ref{class.virtual}); the implementation

generates information associated with each such object that makes it

possible 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 \grammarterm{expression}{s} (Clause~\ref{expr})

used to access them.

\pnum

\indextext{subobject}%

Objects can contain other objects, called \defnx{subobjects}{subobject}.

A subobject can be

a \defn{member subobject}~(\ref{class.mem}), a \defn{base class subobject}

(Clause~\ref{class.derived}), or an array element.

\indextext{object!complete}%

An object that is not a subobject of any other object is called a \defn{complete

object}.

\pnum

For every object \tcode{x}, there is some object called the

\defn{complete object of} \tcode{x}, determined as follows:

\begin{itemize}

\item

If \tcode{x} is a complete object, then \tcode{x} is the complete

object of \tcode{x}.

\item

Otherwise, the complete object of \tcode{x} is the complete object

of the (unique) object that contains \tcode{x}.

\end{itemize}

\pnum

If a complete object, a data member~(\ref{class.mem}), or an array element is of

class type, its type is considered the \defn{most derived

class}, to distinguish it from the class type of any base class subobject;

an object of a most derived class type or of a non-class type is called a

\defn{most derived object}.

\pnum

\indextext{most derived object!bit-field}%

Unless it is a bit-field~(\ref{class.bit}), a most derived object shall have a

non-zero size and shall occupy one or more bytes of storage. Base class

subobjects may have zero size. An object of trivially copyable or

standard-layout type~(\ref{basic.types}) shall occupy contiguous bytes of

storage.

\pnum

\indextext{most derived object!bit-field}%

\indextext{most derived object!zero size subobject}%

Unless an object is a bit-field or a base class subobject of zero size, the

address of that object is the address of the first byte it occupies. Two

objects that are not bit-fields

may have the same address if one is a subobject of the other, or if at least

one is a base class subobject of zero size and they are of different types;

otherwise, they shall have distinct addresses.\footnote{Under the ``as-if'' rule an

implementation is allowed to store two objects at the same machine address or

not store an object at all if the program cannot observe the

difference~(\ref{intro.execution}).}

\enterexample

\begin{codeblock}

static const char test1 = 'x';

static const char test2 = 'x';

const bool b = &test1 != &test2; // always true

\end{codeblock}

\exitexample

\pnum

\enternote

\Cpp provides a variety of fundamental types and several ways of composing

new types from existing types~(\ref{basic.types}).

\exitnote%

\indextext{object model|)}

\rSec1[intro.execution]{Program execution}

\pnum

\indextext{program execution|(}%

\indextext{program execution!abstract machine|(}%

The semantic descriptions in this International Standard define a

parameterized nondeterministic abstract machine. This International

Standard places no requirement on the structure of conforming

implementations. In particular, they need not copy or emulate the

structure of the abstract machine.

\indextext{as-if~rule}%

\indextext{behavior!observable}%

Rather, conforming implementations are required to emulate (only) the observable

behavior of the abstract machine as explained below.\footnote{This provision is

sometimes called the ``as-if'' rule, because an implementation is free to

disregard any requirement of this International Standard as long as the result

is \emph{as if} the requirement had been obeyed, as far as can be determined

from the observable behavior of the program. For instance, an actual

implementation need not evaluate part of an expression if it can deduce that its

value is not used and that no

\indextext{side effects}%

side effects affecting the

observable behavior of the program are produced.}

\indextext{behavior!implementation-defined}%

\pnum

Certain aspects and operations of the abstract machine are described in this

International Standard as implementation-defined (for example,

\tcode{sizeof(int)}). These constitute the parameters of the abstract machine.

Each implementation shall include documentation describing its characteristics

and behavior in these respects.\footnote{This documentation also includes

\DIFdelbegin \DIFdel{conditonally-supported }\DIFdelend \DIFaddbegin \DIFadd{conditionally-supported }\DIFaddend constructs and locale-specific behavior.

See~\ref{intro.compliance}.} Such documentation shall define the instance of the

abstract machine that corresponds to that implementation (referred to as the

``corresponding instance'' below).

\indextext{behavior!unspecified}%

\pnum

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, this

International Standard defines a set of allowable behaviors. These

define the nondeterministic aspects of the abstract machine. An instance

of the abstract machine can thus have more than one possible execution

for a given program and a given input.

\indextext{behavior!undefined}%

\pnum

Certain other operations are described in this International Standard as

undefined (for example, the effect of

attempting to modify a \tcode{const} object).

\enternote This International Standard imposes no requirements on the

behavior of programs that contain undefined behavior. \exitnote

\indextext{program!well-formed}%

\indextext{behavior!observable}%

\pnum

A conforming implementation executing a well-formed program shall

produce the same observable behavior as one of the possible executions

of the corresponding instance of the abstract machine with the

same program and the same input. However, if any such execution 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 preceding the first undefined

operation).

\indextext{behavior!unspecified}%

\indextext{behavior!undefined}%

\indextext{behavior!on receipt of signal}%

\indextext{signal}%

\pnum

When the processing of the abstract machine is interrupted by receipt of

a signal, the values of objects which are neither

\begin{itemize}

\item of type \tcode{volatile std::sig_atomic_t} nor

\item lock-free atomic objects~(\ref{atomics.lockfree})

\end{itemize}

are unspecified during the execution of the signal handler, and the value of any

object not in either of these two categories that is modified by the handler

becomes undefined.%

\indextext{program execution!abstract machine|)}

\pnum

An instance of each object with automatic storage

duration~(\ref{basic.stc.auto}) is associated with each entry into its

block. Such an object exists and retains its last-stored value during

the execution of the block and while the block is suspended (by a call

of a function or receipt of a signal).

\indextext{conformance requirements}

\pnum

The least requirements on a conforming implementation are:

\begin{itemize}

\item

Access to volatile objects are evaluated strictly according to the

rules of the abstract machine.

\item

At program termination, all data written into files shall be

identical to one of the possible results that execution of the program

according to the abstract semantics would have produced.

\item

The input and output dynamics of interactive devices shall take

place in such a fashion that prompting output is actually delivered before a program waits for input. What constitutes an interactive device is

\impldef{interactive device}.

\end{itemize}

These collectively are referred to as the

\defnx{observable behavior}{behavior!observable} of the program.

\enternote More stringent correspondences between abstract and actual

semantics may be defined by each implementation. \exitnote

\pnum

\indextext{operator!precedence of}%

\indextext{expression!order of evaluation of}%

\enternote Operators can be regrouped according to the usual

mathematical rules only where the operators really are associative or

commutative.\footnote{Overloaded operators are never assumed to be associative or

commutative. }

For example, in the following fragment

\begin{codeblock}

int a, b;

/*...*/

a = a + 32760 + b + 5;

\end{codeblock}

the expression statement behaves exactly the same as

\begin{codeblock}

a = (((a + 32760) + b) + 5);

\end{codeblock}

due to the associativity and precedence of these operators. Thus, the

result of the sum \tcode{(a + 32760)} is next added to \tcode{b}, and

that result is then added to 5 which results in the value assigned to

\tcode{a}. On a machine in which overflows produce an exception and in

which the range of values representable by an \tcode{int} is

\crange{-32768}{+32767}, the implementation cannot rewrite this

expression as

\begin{codeblock}

a = ((a + b) + 32765);

\end{codeblock}

since if the values for \tcode{a} and \tcode{b} were, respectively,

-32754 and -15, the sum \tcode{a + b} would produce an exception while

the original expression would not; nor can the expression be rewritten

either as

\begin{codeblock}

a = ((a + 32765) + b);

\end{codeblock}

or

\begin{codeblock}

a = (a + (b + 32765));

\end{codeblock}

since the values for \tcode{a} and \tcode{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. \exitnote

\pnum

\indextext{full-expression}%

A \defn{full-expression} is an expression that is not a

subexpression of another expression. If a language construct is defined

to produce an implicit call of a function, a use of the language

construct is considered to be an expression for the purposes of this

definition. A call to a destructor generated at the end of the lifetime

of an object other than a temporary object is an implicit

full-expression. Conversions applied to the result of an expression in

order to satisfy the requirements of the language construct in which the

expression appears are also considered to be part of the

full-expression.

\enterexample

\begin{codeblock}

struct S {

S(int i): I(i) { }

int& v() { return I; }

private:

int I;

};

S s1(1); // full-expression is call of \tcode{S::S(int)}

S s2 = 2; // full-expression is call of \tcode{S::S(int)}

void f() {

if (S(3).v()) // full-expression includes lvalue-to-rvalue and

// \tcode{int} to \tcode{bool} conversions, performed before

// temporary is deleted at end of full-expression

{ }

}

\end{codeblock}

\exitexample

\pnum

\enternote The evaluation of a full-expression can include the

evaluation of subexpressions that are not lexically part of the

full-expression. For example, subexpressions involved in evaluating

default arguments~(\ref{dcl.fct.default}) are considered to

be created in the expression that calls the function, not the expression

that defines the default argument. \exitnote

\pnum

\indextext{value computation|(}%

Accessing an object designated by a \tcode{volatile}

glvalue~(\ref{basic.lval}), modifying an object, calling a library I/O

function, or calling a function that does any of those operations are

all

\defn{side effects}, which are changes in the state of the execution

environment. \defn{Evaluation} of an expression (or a

sub-expression) in general includes both value computations (including

determining the identity of an object for glvalue evaluation and fetching

a value previously assigned to an object for prvalue evaluation) and

initiation of side effects. When a call to a library I/O function

returns or an access to a \tcode{volatile} object is evaluated the side

effect is considered complete, even though some external actions implied

by the call (such as the I/O itself) or by the \tcode{volatile} access

may not have completed yet.

\pnum

\defn{Sequenced before} is an asymmetric, transitive, pair-wise relation between

evaluations executed by a single thread~(\ref{intro.multithread}), which induces

a partial order among those evaluations. Given any two evaluations \term{A} and

\term{B}, if \term{A} is sequenced before \term{B}, then the execution of

\term{A} shall precede the execution of \term{B}. If \term{A} is not sequenced

before \term{B} and \term{B} is not sequenced before \term{A}, then \term{A} and

\term{B} are \defn{unsequenced}. \enternote The execution of unsequenced

evaluations can overlap. \exitnote Evaluations \term{A} and \term{B} are

\defn{indeterminately sequenced} when either \term{A} is sequenced before

\term{B} or \term{B} is sequenced before \term{A}, but it is unspecified which.

\enternote Indeterminately sequenced evaluations cannot overlap, but either

could be executed first. \exitnote

\pnum

Every

\indextext{value computation}%

value computation and

\indextext{side effects}%

side effect associated with a full-expression is

sequenced before every value computation and side effect associated with the

next full-expression to be evaluated.\footnote{As specified

in~\ref{class.temporary}, after a full-expression is evaluated, a sequence of

zero or more invocations of destructor functions for temporary objects takes

place, usually in reverse order of the construction of each temporary object.}.

\pnum

\indextext{evaluation!unspecified order~of}%

Except where noted, evaluations of operands of individual operators and

of subexpressions of individual expressions are unsequenced. \enternote

In an expression that is evaluated more than once during the execution

of a program, unsequenced and indeterminately sequenced evaluations of

its subexpressions need not be performed consistently in different

evaluations. \exitnote The value computations of the operands of an

operator are sequenced before the value computation of the result of the

operator. If a

\indextext{side effects}%

side effect on a scalar object is unsequenced relative to

either another side effect on the same scalar object or a value

computation using the value of the same scalar object, the behavior is

undefined.

\enterexample

\begin{codeblock}

void f(int, int);

void g(int i, int* v) {

i = v[i++]; // the behavior is undefined

i = 7, i++, i++; // \tcode{i} becomes \tcode{9}

i = i++ + 1; // the behavior is undefined

i = i + 1; // the value of \tcode{i} is incremented

f(i = -1, i = -1); // the behavior is undefined

}

\end{codeblock}

\exitexample

When calling a function (whether or not the function is inline), every

\indextext{value computation}%

value computation and

\indextext{side effects}%

side effect associated with any argument

expression, or with the postfix expression designating the called

function, is sequenced before execution of every expression or statement

in the body of the called function. \enternote

Value computations and

side effects associated with different argument expressions are

unsequenced. \exitnote Every evaluation in the calling function

(including other function calls) that is not otherwise specifically

sequenced before or after the execution of the body of the called

function is indeterminately sequenced with respect to the execution of

the called function.\footnote{In other words, function executions do not interleave with

each other.}

Several contexts in \Cpp cause evaluation of a function call, even

though no corresponding function call syntax appears in the translation

unit.

\enterexample

Evaluation of a \tcode{new} expression invokes one or more allocation

and constructor functions; see~\ref{expr.new}. For another example,

invocation of a conversion function~(\ref{class.conv.fct}) can arise in

contexts in which no function call syntax appears.

\exitexample

The sequencing constraints on the execution of the called function (as

described above) are features of the function calls as evaluated,

whatever the syntax of the expression that calls the function might be.%

\indextext{value computation|)}%

\indextext{program execution|)}

\rSec1[intro.multithread]{Multi-threaded executions and data races}

\pnum

\indextext{threads!multiple|(}%

\indextext{operation!atomic|(}%

A \defn{thread of execution} (also known as a \defn{thread}) is a single flow of

control within a program, including the initial invocation of a specific

top-level function, and recursively including every function invocation

subsequently executed by the thread. \enternote When one thread creates another,

the initial call to the top-level function of the new thread is executed by the

new thread, not by the creating thread. \exitnote Every thread in a program can

potentially access every object and function in a program.\footnote{An object

with automatic or thread storage duration~(\ref{basic.stc}) is associated with

one specific thread, and can be accessed by a different thread only indirectly

through a pointer or reference~(\ref{basic.compound}).} Under a hosted

implementation, a \Cpp program can have more than one thread running

concurrently. The execution of each thread proceeds as defined by the remainder

of this standard. The execution of the entire program consists of an execution

of all of its threads. \enternote Usually the execution can be viewed as an

interleaving of all its threads. However, some kinds of atomic operations, for

example, allow executions inconsistent with a simple interleaving, as described

below. \exitnote Under a freestanding implementation, it is \impldef{number of

threads in a program under a freestanding implementation} whether a program can

have more than one thread of execution.

\pnum

Implementations should ensure that all unblocked threads eventually make

progress. \enternote Standard library functions may silently block on I/O or

locks. Factors in the execution environment, including externally-imposed thread

priorities, may prevent an implementation from making certain guarantees of

forward progress. \exitnote

\pnum

The value of an object visible to a thread \term{T} at a particular point is the

initial value of the object, a value assigned to the object by \term{T}, or a

value assigned to the object by another thread, according to the rules below.

\enternote In some cases, there may instead be undefined behavior. Much of this

section is motivated by the desire to support atomic operations with explicit

and detailed visibility constraints. However, it also implicitly supports a

simpler view for more restricted programs. \exitnote

\pnum

Two expression evaluations \defn{conflict} if one of them modifies a memory

location~(\ref{intro.memory}) and the other one accesses or modifies the same

memory location.

\pnum

The library defines a number of atomic operations (Clause~\ref{atomics}) and

operations on mutexes (Clause~\ref{thread}) that are specially identified as

synchronization operations. These operations play a special role in making

assignments in one thread visible to another. A synchronization operation on one

or more memory locations is either a consume operation, an acquire operation, a

release operation, or both an acquire and release operation. A synchronization

operation without an associated memory location is a fence and can be either an

acquire fence, a release fence, or both an acquire and release fence. In

addition, there are relaxed atomic operations, which are not synchronization

operations, and atomic read-modify-write operations, which have special

characteristics. \enternote For example, a call that acquires a mutex will

perform an acquire operation on the locations comprising the mutex.

Correspondingly, a call that releases the same mutex will perform a release

operation on those same locations. Informally, performing a release operation on

\term{A} forces prior

\indextext{side effects}%

side effects on other memory locations to become visible

to other threads that later perform a consume or an acquire operation on

\term{A}. ``Relaxed'' atomic operations are not synchronization operations even

though, like synchronization operations, they cannot contribute to data races.

\exitnote