\bigskip\noindent\textlarger{\textbf{Programming languages --- \Cpp{}}}

\rSec0[intro.scope]{Scope}

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\indextext{scope|(}%

This document specifies requirements for implementations

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

an implementation implements the language, so this document also

defines \Cpp{}. Other requirements and relaxations of the first

requirement appear at various places within this document.

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

programming language as described in \IsoC{}.

\Cpp{} provides many facilities

beyond those provided by C, including additional data types,

classes, templates, exceptions, namespaces, operator

overloading, function name overloading, references, free store

management operators, and additional library facilities.%

\indextext{scope|)}

\rSec0[intro.refs]{Normative references}%

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

\indextext{references!normative|(}%

The following documents are referred to in the text

in such a way that some or all of their content

constitutes requirements

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 ISO/IEC 2382, \doccite{Information technology --- Vocabulary}

\item ISO 8601-1:2019, \doccite{Date and time --- Representations for information interchange --- Part 1: Basic rules}

\item \IsoC{}, \doccite{Information technology --- Programming languages --- C}

\item \IsoPosix{}, \doccite{Information Technology --- Portable Operating System Interface (POSIX\textregistered)\begin{footnote}

POSIX\textregistered\ is a registered trademark of

the Institute of Electrical and Electronic Engineers, Inc.

This information is given for the convenience of users of this document and

does not constitute an endorsement by ISO or IEC of this product.

\end{footnote}

Base Specifications, Issue 7}

\item \IsoPosix{}/Cor 1:2013, \doccite{Information Technology --- Portable Operating System Interface

(POSIX\textregistered) Base Specifications, Issue 7 --- Technical Corrigendum 1}

\item \IsoPosix{}/Cor 2:2017, \doccite{Information Technology --- Portable Operating System Interface

(POSIX\textregistered) Base Specifications, Issue 7 --- Technical Corrigendum 2}

\item \IsoFloatUndated{}:2020, \doccite{Information technology --- Microprocessor Systems --- Floating-Point arithmetic}

\item ISO 80000-2:2019, \doccite{Quantities and units --- Part 2: Mathematics}

\item Ecma International, \doccite{ECMAScript

\begin{footnote}

ECMAScript\textregistered\ is a registered trademark of Ecma

International.

This information is given for the convenience of users of this document and

does not constitute an endorsement by ISO or IEC of this product.

\end{footnote}

Language Specification},

Standard Ecma-262, third edition, 1999.

The Unicode Consortium. \doccite{The Unicode Standard}, Version 15.1.

Available from: \url{https://www.unicode.org/versions/Unicode15.1.0/}

\end{itemize}

\indextext{references!normative|)}

\rSec0[intro.defs]{Terms and definitions}

\indextext{definitions|(}%

For the purposes of this document,

the terms and definitions

given in ISO/IEC 2382, ISO 80000-2:2019,

and the following apply.

ISO and IEC maintain terminology databases

for use in standardization

at the following addresses:

\begin{itemize}

\item ISO Online browsing platform: available at \url{https://www.iso.org/obp}

\item IEC Electropedia: available at \url{https://www.electropedia.org/}

\end{itemize}

\indexdefn{access}%

\definition{access}{defns.access}

\defncontext{execution-time action}

read or modify the value of an object

\begin{defnote}

Only glvalues of scalar type\iref{basic.types.general} can be used to access objects.

Reads of scalar objects are described in \ref{conv.lval} and

modifications of scalar objects are described in

\ref{expr.assign}, \ref{expr.post.incr}, and \ref{expr.pre.incr}.

Attempts to read or modify an object of class type

typically invoke a constructor\iref{class.ctor}

or assignment operator\iref{class.copy.assign};

such invocations do not themselves constitute accesses,

although they may involve accesses of scalar subobjects.

\end{defnote}

\indexdefn{argument}%

\indexdefn{argument!function call expression}

\definition{argument}{defns.argument}

\defncontext{function call expression}

expression or \grammarterm{braced-init-list}

in the comma-separated list bounded by the parentheses

\indexdefn{argument}%

\indexdefn{argument!function-like macro}%

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

\defncontext{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}

\defncontext{throw expression} operand of \keyword{throw}

\indexdefn{argument}%

\indexdefn{argument!template instantiation}%

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

\defncontext{template instantiation}

\grammarterm{constant-expression},

\grammarterm{type-id}, or

\grammarterm{id-expression} in the comma-separated

list bounded by the angle brackets

\indexdefn{block (execution)}%

\definition{block}{defns.block}

\defncontext{execution}

wait for some condition (other than for the implementation to execute

the execution steps of the thread of execution) to be satisfied before

continuing execution past the blocking operation

\indexdefn{block (statement)}%

\definition{block}{defns.block.stmt}

\defncontext{statement}

compound statement

\indexdefn{C!standard library}%

\definition{C standard library}{defns.c.lib}

library described in \IsoC{}, Clause 7

\begin{defnote}

With the qualifications noted in \ref{\firstlibchapter}

through \ref{\lastlibchapter} and in \ref{diff.library},

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

\end{defnote}

\definition{character}{defns.character}

\indexdefn{character}%

\defncontext{library}

object which,

when treated sequentially,

can represent text

\begin{defnote}

The term does not mean only

\tcode{char},

\keyword{char8_t},

\keyword{char16_t},

\keyword{char32_t},

and

\keyword{wchar_t}

objects\iref{basic.fundamental},

but any value that can be represented by a type

that provides the definitions specified in

\ref{strings}, \ref{localization}, \ref{input.output}, or~\ref{re}.

\end{defnote}

\definition{character container type}{defns.character.container}

\defncontext{library}

\indexdefn{type!character container}%

class or a type used to

represent a \termref{defns.character}{character}{}

\begin{defnote}

It is used for one of the template parameters of \tcode{char_traits}

and the class templates which use that,

such as the string, iostream, and regular expression class templates.

\end{defnote}

\definition{collating element}{defns.regex.collating.element}

\indexdefn{collating element}%

sequence of one or more \termref{defns.character}{character}{s} within the

current locale that collate as if they were a single character

\definition{component}{defns.component}

\defncontext{library}

\indexdefn{component}%

group of library entities directly related as members, \termref{defns.parameter}{parameter}{s}, or

return types

\begin{defnote}

For example, the class template \tcode{basic_string}

and the non-member function templates

that operate on strings are referred to as the string component.

\end{defnote}

\indexdefn{behavior!conditionally-supported}%

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

program construct that an implementation is not required to support

\begin{defnote}

Each implementation documents all conditionally-supported

constructs that it does not support.

\end{defnote}

\definition{constant evaluation}{defns.const.eval}

\indexdefn{constant evaluation}%

evaluation that is performed as part of evaluating an expression

as a core constant expression\iref{expr.const}

\definition{constant subexpression}{defns.const.subexpr}

\indexdefn{constant subexpression}%

expression whose evaluation as subexpression of a

\grammarterm{conditional-expression}

\tcode{CE} would not prevent \tcode{CE}

from being a core constant expression

\definition{deadlock}{defns.deadlock}

\defncontext{library}

\indexdefn{deadlock}%

situation wherein

one or more threads are unable to continue execution because each is

\termref{defns.block}{block}{ed} waiting for one or more of the others to satisfy some condition

\definition{default behavior}{defns.default.behavior.impl}

\indexdefn{behavior!default}%

\defncontext{library implementation}

specific behavior provided by the implementation,

within the scope of the \termref{defns.required.behavior}{required behavior}{}

\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}

\defncontext{glvalue} type of the most derived object to which the

glvalue refers

\begin{example}

If a pointer\iref{dcl.ptr} \tcode{p} whose type is ``pointer to

class \tcode{B}'' is pointing to a base class subobject

of class \tcode{B}, whose most derived object is of class \tcode{D}, derived

from \tcode{B}\iref{class.derived}, the dynamic type of the

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

treated similarly.

\end{example}

\indexdefn{type!dynamic}%

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

\defncontext{prvalue} \termref{defns.static.type}{static type}{} of the prvalue expression

\indexdefn{behavior!erroneous}%

\definition{erroneous behavior}{defns.erroneous}

well-defined behavior that the implementation is recommended to diagnose

\begin{defnote}

Erroneous behavior is always the consequence of incorrect program code.

Implementations are allowed, but not required,

to diagnose it\iref{intro.compliance.general}.

Evaluation of a constant expression\iref{expr.const}

never exhibits behavior specified as erroneous in \ref{intro} through \ref{\lastcorechapter}.

\end{defnote}

\definition{expression-equivalent}{defns.expression.equivalent}

\defncontext{library}

\indexdefn{expression-equivalent}%

expressions that all have the same effects,

either

are all potentially-throwing or

are all not potentially-throwing,

and

either

are all \termref{defns.const.subexpr}{constant subexpression}{s} or

are all not constant subexpressions

\begin{example}

For a value \tcode{x} of type \tcode{int}

and a function \tcode{f} that accepts integer arguments,

the expressions

\tcode{f(x + 2)},

\tcode{f(2 + x)},

and

\tcode{f(1 + x + 1)}

are expression-equivalent.

\end{example}

\definition{finite state machine}{defns.regex.finite.state.machine}

\defncontext{regular expression}

\indexdefn{finite state machine}%

unspecified data structure that is used to

represent a \termref{defns.regex.regular.expression}{regular expression}{}, and which permits efficient matches

against the regular expression to be obtained

\definition{format specifier}{defns.regex.format.specifier}

\defncontext{regular expression}

\indexdefn{format specifier}%

sequence of one or more \termref{defns.character}{character}{s} that is expected to be

replaced with some part of a \termref{defns.regex.regular.expression}{regular expression}{} match

\definition{handler function}{defns.handler}

\defncontext{library}

\indexdefn{function!handler}%

non-reserved function whose definition may be provided by a \Cpp{} program

\begin{defnote}

A \Cpp{} program may designate a handler function at various points in its execution by

supplying a pointer to the function when calling any of the library functions that install

handler functions (see \ref{support}).

\end{defnote}

\indexdefn{program!ill-formed}%

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

program that is not well-formed\iref{defns.well.formed}

\indexdefn{behavior!implementation-defined}%

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

behavior, for a \termref{defns.well.formed}{well-formed program}{} construct and correct data, that

depends on the implementation and that each implementation documents

\definition{implementation-defined strict total order over pointers}

{defns.order.ptr}

\indexdefn{pointer!strict total order}%

\defncontext{library}

\impldef{strict total order over pointer values}

strict total ordering over all pointer values

such that the ordering is consistent with the partial order

imposed by the built-in operators

\tcode{<}, \tcode{>}, \tcode{<=}, \tcode{>=}, and \tcode{<=>}

\indexdefn{limits!implementation}%

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

restriction 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

\definition{matched}{defns.regex.matched}

\defncontext{regular expression}

\indexdefn{matched}%

\indexdefn{regular expression!matched}%

condition when a sequence of zero or more \termref{defns.character}{character}{s}

correspond to a sequence of characters defined by the pattern

\definition{modifier function}{defns.modifier}

\defncontext{library}

\indexdefn{function!modifier}%

class member function other than a constructor,

assignment operator, or destructor

that alters the state of an object of the class

\definition{move assignment}{defns.move.assign}

\defncontext{library}

\indexdefn{assignment!move}%

assignment of an rvalue of some object type to a modifiable lvalue of the same type

\definition{move construction}{defns.move.constr}

\defncontext{library}

\indexdefn{construction!move}%

direct-initialization of an object of some type with an rvalue of the same type

\indexdefn{library call!non-constant}%

\definition{non-constant library call}{defns.nonconst.libcall}

invocation of a library function that,

as part of evaluating any expression \tcode{E},

prevents \tcode{E} from being a core constant expression

\definition{NTCTS}{defns.ntcts}

\defncontext{library}

\indexdefn{NTCTS}%

\indexdefn{string!null-terminated character type}%

sequence of values that have

\termref{defns.character}{character}{} type

that precede the terminating null character type

value

\tcode{charT()}

\definition{observer function}{defns.observer}

\defncontext{library}

\indexdefn{function!observer}%

class member function that accesses the state of an object of the class

but does not alter that state

\begin{defnote}

Observer functions are specified as

\keyword{const}

member functions.

\end{defnote}

\indexdefn{parameter}%

\indexdefn{parameter!function}%

\indexdefn{parameter!catch clause}%

\definition{parameter}{defns.parameter}

\defncontext{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}

\defncontext{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}

\defncontext{template} member of a \grammarterm{template-parameter-list}

\definition{primary equivalence class}{defns.regex.primary.equivalence.class}

\defncontext{regular expression}

\indexdefn{primary equivalence class}%

set of one or more \termref{defns.character}{character}{s} which

share the same primary sort key: that is the sort key weighting that

depends only upon character shape, and not accents, case, or

locale-specific tailorings

\definition{program-defined specialization}{defns.prog.def.spec}

\defncontext{library}

\indexdefn{specialization!program-defined}%

explicit template specialization or partial specialization

that is not part of the \Cpp{} standard library and

not defined by the implementation

\definition{program-defined type}{defns.prog.def.type}

\defncontext{library}

\indexdefn{type!program-defined}%

non-closure class type or enumeration type

that is not part of the \Cpp{} standard library and

not defined by the implementation,

or a closure type of a non-implementation-provided lambda expression,

or an instantiation of a \termref{defns.prog.def.spec}{program-defined specialization}{}

\begin{defnote}

Types defined by the implementation include

extensions\iref{intro.compliance} and internal types used by the library.

\end{defnote}

\definition{projection}{defns.projection}

\indexdefn{projection}%

\defncontext{library}

transformation that an algorithm applies

before inspecting the values of elements

\begin{example}

\begin{codeblock}

std::pair<int, std::string_view> pairs[] = {{2, "foo"}, {1, "bar"}, {0, "baz"}};

std::ranges::sort(pairs, std::ranges::less{}, [](auto const& p) { return p.first; });

\end{codeblock}

sorts the pairs in increasing order of their \tcode{first} members:

\begin{codeblock}

{{0, "baz"}, {1, "bar"}, {2, "foo"}}

\end{codeblock}

\end{example}

\definition{referenceable type}{defns.referenceable}

\indexdefn{type!referenceable}%

type that is either an

object type, a function type that does not have cv-qualifiers or a

\grammarterm{ref-qualifier}, or a reference type

\begin{defnote}

The term describes a type to which a reference can be created,

including reference types.

\end{defnote}

\definition{regular expression}{defns.regex.regular.expression}

pattern that selects specific strings

from a set of \termref{defns.character}{character}{} strings

\definition{replacement function}{defns.replacement}

\defncontext{library}

\indexdefn{function!replacement}%

non-reserved function

whose definition is provided by a \Cpp{} program

\begin{defnote}

Only one definition for such a function is in effect for the duration of the program's

execution, as the result of creating the program\iref{lex.phases} and resolving the

definitions of all translation units\iref{basic.link}.

\end{defnote}

\definition{required behavior}{defns.required.behavior}

\defncontext{library}

\indexdefn{behavior!required}%

description of \termref{defns.replacement}{replacement function}{}

and \termref{defns.handler}{handler function}{} semantics

applicable to both the behavior provided by the implementation and

the behavior of any such function definition in the program

\begin{defnote}

If such a function defined in a \Cpp{} program fails to meet the required

behavior when it executes, the behavior is undefined.%

\indextext{undefined}

\end{defnote}

\definition{reserved function}{defns.reserved.function}

\defncontext{library}

\indexdefn{function!reserved}%

function, specified as part of the \Cpp{} standard library, that is defined by the

implementation

\begin{defnote}

If a \Cpp{} program provides a definition for any reserved function, the results are undefined.%

\indextext{undefined}

\end{defnote}

\definition{runtime-undefined behavior}{defns.undefined.runtime}

\indexdefn{behavior!runtime-undefined}%

behavior that is undefined except when it occurs during constant evaluation

\begin{defnote}

During constant evaluation,

\begin{itemize}

it is

\impldef{whether runtime-undefined behavior results in the expression being deemed non-constant}

whether runtime-undefined behavior results in the expression being deemed non-constant

(as specified in~\ref{expr.const}) and

runtime-undefined behavior has no other effect.

\end{itemize}

\end{defnote}

\indexdefn{signature}%

\definition{signature}{defns.signature}

\defncontext{function}

name,

parameter-type-list,

and enclosing namespace

\begin{defnote}

Signatures are used as a basis for

name mangling and linking.

\end{defnote}

\indexdefn{signature}%

\definition{signature}{defns.signature.friend}

\defncontext{non-template friend function with trailing \grammarterm{requires-clause}}

name,

parameter-type-list,

enclosing class,

and

trailing \grammarterm{requires-clause}

\indexdefn{signature}%

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

\defncontext{function template}

name,

parameter-type-list,

enclosing namespace,

return type,

\termref{defns.signature.template.head}{signature}{} of the \grammarterm{template-head},

and

trailing \grammarterm{requires-clause} (if any)

\indexdefn{signature}%

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

\defncontext{friend function template with constraint involving enclosing template parameters}

name,

parameter-type-list,

return type,

enclosing class,

\termref{defns.signature.template.head}{signature}{} of the \grammarterm{template-head},

and

trailing \grammarterm{requires-clause} (if any)

\indexdefn{signature}%

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

\defncontext{function template specialization} \termref{defns.signature.templ}{signature}{} of the template of which it is a specialization

and its template \termref{defns.argument.templ}{argument}{s} (whether explicitly specified or deduced)

\indexdefn{signature}%

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

\defncontext{class member function}

name,

parameter-type-list,

class of which the function is a member,

\cv-qualifiers (if any),

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

and

trailing \grammarterm{requires-clause} (if any)

\indexdefn{signature}%

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

\defncontext{class member function template}

name,

parameter-type-list,

class of which the function is a member,

\cv-qualifiers (if any),

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

return type (if any),

\termref{defns.signature.template.head}{signature}{} of the \grammarterm{template-head},

and

trailing \grammarterm{requires-clause} (if any)

\indexdefn{signature}%

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

\defncontext{class member function template specialization} \termref{defns.signature.member.templ}{signature}{} of the member function template

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

\indexdefn{signature}%

\definition{signature}{defns.signature.template.head}

\defncontext{\grammarterm{template-head}}

template \termref{defns.parameter.templ}{parameter}{} list,

excluding template parameter names and default \termref{defns.argument.templ}{argument}{s},

and

\grammarterm{requires-clause} (if any)

\definition{stable algorithm}{defns.stable}

\defncontext{library}

\indexdefn{algorithm!stable}%

\indexdefn{stable algorithm}%

algorithm that preserves, as appropriate to the particular algorithm, the order

of elements

\begin{defnote}

Requirements for stable algorithms are given in \ref{algorithm.stable}.

\end{defnote}

\indexdefn{type!static}%

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

type of an expression resulting from

analysis of the program without considering execution semantics

\begin{defnote}

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.

\end{defnote}

\definition{sub-expression}{defns.regex.subexpression}

\defncontext{regular expression}

\indexdefn{sub-expression!regular expression}%

subset of a \termref{defns.regex.regular.expression}{regular expression}{} that has

been marked by parentheses

\definition{traits class}{defns.traits}

\defncontext{library}

\indexdefn{traits}%

class that encapsulates a set of types and functions necessary for class templates and

function templates to manipulate objects of types for which they are instantiated

\indexdefn{unblock}%

\definition{unblock}{defns.unblock}

satisfy a condition that one or more \termref{defns.block}{blocked}{} threads of execution are waiting for

\indexdefn{behavior!undefined}%

\definition{undefined behavior}{defns.undefined}

behavior for which this document

imposes no requirements

\begin{defnote}

Undefined behavior may be expected when

this document omits any explicit

definition of behavior or when a program uses an incorrect construct or invalid 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

\termref{defns.diagnostic}{diagnostic message}{}), to terminating a translation or execution (with the

issuance of a diagnostic message). Many incorrect program constructs do

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

Evaluation of a constant expression\iref{expr.const} never exhibits behavior explicitly

specified as undefined in \ref{intro} through \ref{\lastcorechapter}.

\end{defnote}

\indexdefn{behavior!unspecified}%

\definition{unspecified behavior}{defns.unspecified}

behavior, for a \termref{defns.well.formed}{well-formed program}{} construct and correct data, that

depends on the implementation

\begin{defnote}

The implementation is not required to

document which behavior occurs. The range of

possible behaviors is usually delineated by this document.

\end{defnote}

\definition{valid but unspecified state}{defns.valid}

\defncontext{library}

\indexdefn{valid but unspecified state}%

value of an object that is not specified except that the object's invariants are

met and operations on the object behave as specified for its type

\begin{example}

If an object \tcode{x} of type \tcode{std::vector<int>} is in a

valid but unspecified state, \tcode{x.empty()} can be called unconditionally,

and \tcode{x.front()} can be called only if \tcode{x.empty()} returns

\tcode{false}.

\end{example}

\indexdefn{program!well-formed}%

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

\Cpp{} program constructed according to the syntax and semantic rules

\indextext{definitions|)}

\rSec0[intro]{General principles}

\rSec1[intro.compliance]{Implementation compliance}%

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

\indexdefn{conditionally-supported behavior|see{behavior, con\-ditionally-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, im\-plementation-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-spe\-cific}}%

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

\rSec2[intro.compliance.general]{General}

\indextext{conformance requirements|(}%

\indextext{conformance requirements!general|(}%

The set of

\defn{diagnosable rules}

consists of all syntactic and semantic rules in this document

except for those rules containing an explicit notation that

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

``undefined behavior''.

\indextext{conformance requirements!method of description}%

Although this document 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}

If a program contains no violations of the rules in

\ref{lex} through \ref{\lastlibchapter} as well as those specified in \ref{depr},

a conforming implementation shall accept and correctly execute

\begin{footnote}

``Correct execution'' can include undefined behavior

and erroneous behavior, depending on

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

\end{footnote}

that program,

except when the implementation's limitations (see below) are exceeded.

\indextext{behavior!undefined}%

If a program contains a violation of a rule for which no diagnostic is required,

this document places no requirement on implementations

with respect to that program.

\indextext{message!diagnostic}%

\indextext{contract evaluation semantics!checking}%

\indextext{contract evaluation semantics!terminating}%

Otherwise, if a program contains

\begin{itemize}

a violation of any diagnosable rule,

a preprocessing translation unit with

a \tcode{\#warning} preprocessing directive\iref{cpp.error},

an occurrence

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

the implementation does not support that construct, or

a contract assertion\iref{basic.contract.eval}

evaluated with a checking semantic

in a manifestly constant-evaluated context\iref{expr.const}

resulting in a contract violation,

\end{itemize}

a conforming implementation

shall issue at least one diagnostic message.

\end{itemize}

\begin{note}

During template argument deduction and substitution,

certain constructs that in other contexts require a diagnostic

are treated differently;

see~\ref{temp.deduct}.

\end{note}

Furthermore, a conforming implementation

shall not accept

\begin{itemize}

a preprocessing translation unit containing

a \tcode{\#error} preprocessing directive\iref{cpp.error},

a translation unit with

a \grammarterm{static_assert-declaration} that fails\iref{dcl.pre}, or

a contract assertion evaluated with a terminating semantic\iref{basic.contract.eval}

in a manifestly constant-evaluated context\iref{expr.const}

resulting in a contract violation.

\end{itemize}

\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\iref{class.access} are not

specified, but each implementation shall supply them to complete the

definitions according to the description in the library Clauses.

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

Clauses specify declarations. Implementations shall supply definitions

consistent with the descriptions in the library Clauses.

A \Cpp{} translation unit\iref{lex.phases}

obtains access to the names defined in the library by

including the appropriate standard library header or importing

the appropriate standard library named header unit\iref{using.headers}.

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

external linkage\iref{basic.link}. The implementation provides

definitions for standard library entities, as necessary, while combining

translation units to form a complete \Cpp{} program\iref{lex.phases}.%

\indextext{conformance requirements!library|)}

An implementation is either a

\defnadj{hosted}{implementation} or a

\defnadj{freestanding}{implementation}.

A freestanding

implementation is one in which execution may take place without the benefit of

an operating system.

A hosted implementation

supports all the facilities described in this document, while

a freestanding implementation

supports the entire \Cpp{} language

described in \ref{lex} through \ref{\lastcorechapter} and

the subset of the library facilities described in \ref{compliance}.

It is

\impldef{whether the implementation is a hardened implementation}

whether the implementation is a

\defnadj{hardened}{implementation}.

If it is a hardened implementation,

violating a hardened precondition

results in a contract violation\iref{structure.specifications}.

An implementation is encouraged to document its limitations in

the size or complexity of the programs it can successfully process,

if possible and where known.

\ref{implimits} lists some quantities that can be subject to limitations and

a potential minimum supported value for each quantity.

A conforming implementation may use an implementation-defined version

of the Unicode Standard that is a later version than the one

referenced in \ref{intro.refs}.

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 document.

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

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.

\begin{footnote}

This documentation also defines implementation-defined behavior;

see~\ref{intro.abstract}.

\end{footnote}

\indextext{conformance requirements!general|)}%

\indextext{conformance requirements|)}%

\rSec2[intro.abstract]{Abstract machine}

\indextext{program execution|(}%

\indextext{program execution!abstract machine}%

The semantic descriptions in this document define a

parameterized nondeterministic abstract machine. This document

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.

\begin{footnote}

This provision is

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

disregard any requirement of this document 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.

\end{footnote}

\indextext{behavior!implementation-defined}%

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

document as implementation-defined behavior (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.

\begin{footnote}

This documentation also includes

conditionally-supported constructs and locale-specific behavior.

See~\ref{intro.compliance.general}.

\end{footnote}

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}%

Certain other aspects and operations of the abstract machine are

described in this document as unspecified behavior (for example,

order of evaluation of arguments in a function call\iref{expr.call}).

Where possible, this

document 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}%

Certain other operations are described in this document as

undefined behavior (for example, the effect of

attempting to modify a const object).

Certain events in the execution of a program

are termed \defnadj{observable}{checkpoints}.

\begin{note}

A call to \tcode{std::observable_checkpoint}\iref{utility.undefined}

is an observable checkpoint,

as are certain parts of

the evaluation of contract assertions\iref{basic.contract}.

\end{note}

\indextext{program!well-formed}%

\indextext{behavior!observable}%

The \defnadj{defined}{prefix} of an execution

comprises the operations $O$

for which for every undefined operation $U$

there is an observable checkpoint $C$

such that $O$ happens before $C$ and

$C$ happens before $U$.

\begin{note}

The undefined behavior that arises from a data race\iref{intro.races}

occurs on all participating threads.

\end{note}

A conforming implementation executing a well-formed program shall

produce the observable behavior

of the defined prefix

of one of the possible executions

of the corresponding instance

of the abstract machine with the

same program and the same input.

\indextext{behavior!undefined}%

If the selected execution contains an undefined operation,

the implementation executing that program with that input

may produce arbitrary additional observable behavior afterwards.

If the execution contains an operation specified as having erroneous behavior,

the implementation is permitted to issue a diagnostic and

is permitted to terminate the execution

at an unspecified time after that operation.