%!TEX root = std.tex

\rSec0[library]{Library introduction}

\rSec1[library.general]{General}

\pnum

This Clause describes the contents of the

\defnx{\Cpp{} standard library}{library!\Cpp{} standard},

how a well-formed \Cpp{} program makes use of the library, and

how a conforming implementation may provide the entities in the library.

\pnum

The following subclauses describe the method of

description\iref{description} and organization\iref{organization} of the

library. \ref{requirements}, \ref{\firstlibchapter}

through \ref{\lastlibchapter}, and \ref{depr} specify the contents of the

library, as well as library requirements and constraints on both well-formed

\Cpp{} programs and conforming implementations.

\pnum

Detailed specifications for each of the components in the library are in

\ref{\firstlibchapter}--\ref{\lastlibchapter}, as shown in

\tref{library.categories}.

\begin{floattable}{Library categories}{library.categories}

{ll}

\topline

\hdstyle{Clause} & \hdstyle{Category} \\ \capsep

\ref{support} & Language support library \\

\ref{concepts} & Concepts library \\

\ref{diagnostics} & Diagnostics library \\

\ref{mem} & Memory management library \\

\ref{meta} & Metaprogramming library \\

\ref{utilities} & General utilities library \\

\ref{containers} & Containers library \\

\ref{iterators} & Iterators library \\

\ref{ranges} & Ranges library \\

\ref{algorithms} & Algorithms library \\

\ref{strings} & Strings library \\

\ref{text} & Text processing library \\

\ref{numerics} & Numerics library \\

\ref{time} & Time library \\

\ref{input.output} & Input/output library \\

\ref{thread} & Concurrency support library \\

\ref{exec} & Execution control library \\

\end{floattable}

\pnum

The operating system interface described in \IsoPosix{} is

hereinafter called \defn{POSIX}.

\pnum

The language support library\iref{support} provides components that are

required by certain parts of the \Cpp{} language,

such as memory allocation\iref{expr.new,expr.delete} and

exception processing\iref{except}.

\pnum

The concepts library\iref{concepts} describes library components that \Cpp{}

programs may use to perform compile-time validation of template arguments and

perform function dispatch based on properties of types.

\pnum

The diagnostics library\iref{diagnostics} provides a consistent framework for

reporting errors in a \Cpp{} program, including predefined exception classes.

\pnum

The memory management library\iref{mem} provides components for

memory management, including smart pointers and scoped allocators.

\pnum

The metaprogramming library\iref{meta} describes facilities

for use in templates and during constant evaluation,

including type traits, integer sequences, and rational arithmetic.

\pnum

The general utilities library\iref{utilities} includes components used

by other library elements, such as a predefined storage allocator for dynamic

storage management\iref{basic.stc.dynamic}, and components used

as infrastructure

in \Cpp{} programs,

such as tuples and function wrappers.

\pnum

The containers\iref{containers}, iterators\iref{iterators}, ranges\iref{ranges},

and algorithms\iref{algorithms} libraries provide a \Cpp{} program with access

to a subset of the most widely used algorithms and data structures.

\pnum

The strings library\iref{strings} provides support

for manipulating sequences of type \tcode{char},

sequences of type \keyword{char8_t},

sequences of type \keyword{char16_t},

sequences of type \keyword{char32_t},

sequences of type \keyword{wchar_t},

and sequences of any other character-like type.

\pnum

The text processing library\iref{text} provides support for text processing,

including formatting, internationalization support and

regular expression matching and searching.

\pnum

The numerics library\iref{numerics} provides

numeric algorithms and complex number components that extend support for numeric processing.

The

\tcode{valarray}

component provides support for

\textit{n}-at-a-time

processing,

potentially implemented as parallel operations on platforms that support such processing.

The random number component provides facilities for generating pseudo-random numbers.

\pnum

The time library\iref{time} provides

generally useful time utilities.

\pnum

The input/output library\iref{input.output} provides the

\tcode{iostream}

components that are the primary mechanism for \Cpp{} program input and output.

They can be used with other elements of the library, particularly

strings, locales, and iterators.

\pnum

The concurrency support library\iref{thread} provides components to create

and manage threads,

including atomic operations, mutual exclusion, and interthread communication.

\pnum

The execution control library\iref{exec} provides components

supporting execution of function objects.

\rSec1[library.c]{The C standard library}

\pnum

The \Cpp{} standard library also makes available the facilities of the C standard library,

\indextext{library!C standard}%

suitably adjusted to ensure static type safety.

\pnum

The descriptions of many library functions rely on the C standard library for

the semantics of those functions.

In some cases,

the signatures specified in this document

may be different from the signatures in the C standard library,

and additional overloads may be declared in this document,

but the behavior and the preconditions

(including any preconditions implied by the use of

a C \tcode{restrict} qualifier)

are the same unless otherwise stated.

\pnum

A call to a C standard library function is

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

if it raises a floating-point exception other than \tcode{FE_INEXACT}.

The semantics of a call to a C standard library function

evaluated as a core constant expression

are those specified in \IsoC{}, Annex F

\begin{footnote}

See also \IsoC{}, 7.6.

\end{footnote}

to the extent applicable to the floating-point types\iref{basic.fundamental}

that are parameter types of the called function.

\begin{note}

\IsoC{}, Annex F specifies

the conditions under which floating-point exceptions are raised and

the behavior when NaNs and/or infinities are passed as arguments.

\end{note}

\begin{note}

Equivalently, a call to a C standard library function is

a non-constant library call

if \tcode{errno} is set

when \tcode{math_errhandling \& MATH_ERRNO} is \tcode{true}.

\end{note}

\rSec1[description]{Method of description}

\rSec2[description.general]{General}

\pnum

Subclause \ref{description} describes the conventions used to specify the \Cpp{} standard

library. \ref{structure} describes the structure of

\ref{\firstlibchapter} through \ref{\lastlibchapter} and

\ref{depr}. \ref{conventions} describes other editorial conventions.

\rSec2[structure]{Structure of each clause}

\rSec3[structure.elements]{Elements}

\pnum

Each library clause contains the following elements, as applicable:

\begin{footnote}

To

save space, items that do not apply to a Clause are omitted.

For example, if a Clause does not specify any requirements,

there will be no ``Requirements'' subclause.

\end{footnote}

\begin{itemize}

\item Summary

\item Requirements

\item Detailed specifications

\item References to the C standard library

\end{itemize}

\rSec3[structure.summary]{Summary}

\pnum

The Summary provides a synopsis of the category, and introduces the first-level subclauses.

Each subclause also provides a summary, listing the headers specified in the

subclause and the library entities provided in each header.

\pnum

The contents of the summary and the detailed specifications include:

\begin{itemize}

\item macros

\item values

\item types and alias templates

\item classes and class templates

\item functions and function templates

\item objects and variable templates

\item concepts

\end{itemize}

\rSec3[structure.requirements]{Requirements}

\pnum

\indextext{requirements}%

Requirements describe constraints that shall be met by a \Cpp{} program that extends the standard library.

Such extensions are generally one of the following:

\begin{itemize}

\item Template arguments

\item Derived classes

\item Containers, iterators, and algorithms that meet an interface convention or

model a concept

\end{itemize}

\pnum

The string and iostream components use an explicit representation of operations

required of template arguments. They use a class template \tcode{char_traits} to

define these constraints.

\pnum

Interface convention requirements are stated as generally as possible. Instead

of stating ``class \tcode{X} has to define a member function \tcode{operator++()}'', the

interface requires ``for any object \tcode{x} of class \tcode{X}, \tcode{++x} is

defined''. That is, whether the operator is a member is unspecified.

\pnum

Requirements are stated in terms of well-defined expressions that define valid terms of

the types that meet the requirements. For every set of well-defined expression

requirements there is either a named concept or a table that specifies an initial set of the valid expressions and

their semantics. Any generic algorithm\iref{algorithms} that uses the

well-defined expression requirements is described in terms of the valid expressions for

its template type parameters.

\pnum

The library specification uses a typographical convention for naming

requirements. Names in \textit{italic} type that begin with the prefix

\oldconcept{} refer to sets of well-defined expression requirements typically

presented in tabular form, possibly with additional prose semantic requirements.

For example, \oldconcept{Destructible}~(\tref{cpp17.destructible}) is such a named

requirement. Names in \tcode{constant width} type refer to library concepts

which are presented as a concept definition\iref{temp}, possibly with additional

prose semantic requirements. For example,

\libconcept{destructible}\iref{concept.destructible}

is such a named requirement.

\pnum

Template argument requirements are sometimes referenced by name.

See~\ref{type.descriptions}.

\pnum

In some cases the semantic requirements are presented as \Cpp{} code.

Such code is intended as a

specification of equivalence of a construct to another construct, not

necessarily as the way the construct

must be implemented.

\begin{footnote}

Although in some cases the code given is

unambiguously the optimum implementation.

\end{footnote}

\pnum

Required operations of any concept defined in this document need not be

total functions; that is, some arguments to a required operation may

result in the required semantics failing to be met.

\begin{example}

The required \tcode{<} operator of the \libconcept{totally_ordered}

concept\iref{concept.totallyordered} does not meet the

semantic requirements of that concept when operating on NaNs.

\end{example}

This does not affect whether a type models the concept.

\pnum

A declaration may explicitly impose requirements through its associated

constraints\iref{temp.constr.decl}. When the associated constraints refer to a

concept\iref{temp.concept}, the semantic constraints specified for that concept

are additionally imposed on the use of the declaration.

\rSec3[structure.specifications]{Detailed specifications}

\pnum

The detailed specifications each contain the following elements:%

\begin{itemize}

\item name and brief description

\item synopsis (class definition or function declaration, as appropriate)

\item restrictions on template arguments, if any

\item description of class invariants

\item description of function semantics

\end{itemize}

\pnum

Descriptions of class member functions follow the order (as

appropriate):

\begin{footnote}

To save space, items that do not apply to a class are omitted.

For example, if a class does not specify any comparison operator functions, there

will be no ``Comparison operator functions'' subclause.

\end{footnote}

\begin{itemize}

\item constructor(s) and destructor

\item copying, moving \& assignment functions

\item comparison operator functions

\item modifier functions

\item observer functions

\item operators and other non-member functions

\end{itemize}

\pnum

Descriptions of function semantics contain the following elements (as

appropriate):

\begin{footnote}

To save space, elements that do not apply to a function are omitted.

For example, if a function specifies no

preconditions, there will be no \expects element.

\end{footnote}

\begin{itemize}

\item

\constraints

the conditions for the function's participation

in overload resolution\iref{over.match}.

\begin{note}

Failure to meet such a condition results in the function's silent non-viability.

\end{note}

\begin{example}

An implementation can express such a condition

via a \grammarterm{constraint-expression}\iref{temp.constr.decl}.

\end{example}

\item

\mandates

the conditions that, if not met, render the program ill-formed.

\begin{example}

An implementation can express such a condition

via the \grammarterm{constant-expression}

in a \grammarterm{static_assert-declaration}\iref{dcl.pre}.

If the diagnostic is to be emitted only after the function

has been selected by overload resolution,

an implementation can express such a condition

via a \grammarterm{constraint-expression}\iref{temp.constr.decl}

and also define the function as deleted.

\end{example}

\item

\constantwhen

the conditions that are required for a call to the function

to be a constant subexpression\iref{defns.const.subexpr}.

\item

\expects

conditions that the function assumes to hold whenever it is called;

violation of any preconditions results in undefined behavior.

\begin{example}

An implementation can express some such conditions

via the use of a contract assertion,

such as a precondition assertion\iref{dcl.contract.func}.

\end{example}

\item

\hardexpects

conditions that the function assumes to hold whenever it is called.

\begin{itemize}

\item

When invoking the function in a hardened implementation,

prior to any other observable side effects of the function,

one or more contract assertions

whose predicates are as described in the hardened precondition

are evaluated with a checking semantic\iref{basic.contract.eval}.

If any of these assertions is evaluated with a non-terminating semantic

and the contract-violation handler returns,

the program has undefined behavior.

\item

When invoking the function in a non-hardened implementation,

if any hardened precondition is violated,

the program has undefined behavior.

\end{itemize}

\item

\effects

the actions performed by the function.

\item

\sync

the synchronization operations\iref{intro.multithread} applicable to the function.

\item

\ensures

the conditions (sometimes termed observable results)

established by the function.

\begin{example}

An implementation can express some such conditions

via the use of a contract assertion,

such as a postcondition assertion\iref{dcl.contract.func}.

\end{example}

\item

\result

for a \grammarterm{typename-specifier}, a description of the named type;

for an \grammarterm{expression},

a description of the type and value category of the expression;

the expression is an lvalue if the type is an lvalue reference type,

an xvalue if the type is an rvalue reference type, and

a prvalue otherwise.

\item

\returns

a description of the value(s) returned by the function.

\item

\throws

any exceptions thrown by the function, and the conditions that would cause the exception.

\item

\complexity

the time and/or space complexity of the function.

\item

\remarks

additional semantic constraints on the function.

\item

\errors

the error conditions for error codes reported by the function.

\end{itemize}

\pnum

Whenever the \Fundescx{Effects} element specifies that the semantics of some function

\tcode{F} are \term{Equivalent to} some code sequence, then the various elements are

interpreted as follows.

If \tcode{F}'s semantics specifies any \Fundescx{Constraints} or \Fundescx{Mandates} elements,

then those requirements are logically imposed prior to the \term{equivalent-to} semantics.

Next, the semantics of the code sequence are determined by the

\Fundescx{Constraints},

\Fundescx{Mandates},

\Fundescx{Constant When},

\Fundescx{Preconditions},

\Fundescx{Hardened preconditions},

\Fundescx{Effects},

\Fundescx{Synchronization},

\Fundescx{Postconditions},

\Fundescx{Returns},

\Fundescx{Throws},

\Fundescx{Complexity},

\Fundescx{Remarks}, and

\Fundescx{Error conditions}

specified for the function invocations contained in the code sequence.

The value returned from \tcode{F} is specified by \tcode{F}'s \Fundescx{Returns} element,

or if \tcode{F} has no \Fundescx{Returns} element,

a non-\keyword{void} return from \tcode{F} is specified by the

\tcode{return} statements\iref{stmt.return} in the code sequence.

If \tcode{F}'s semantics contains a \Fundescx{Throws},

\Fundescx{Postconditions}, or \Fundescx{Complexity} element,

then that supersedes any occurrences of that element in the code sequence.

\pnum

For non-reserved replacement and handler functions,

\ref{support} specifies two behaviors for the functions in question:

their required and default behavior.

The \defnx{default behavior}{behavior!default}

describes a function definition provided by the implementation.

The \defnx{required behavior}{behavior!required}

describes the semantics of a function definition provided by

either the implementation or a \Cpp{} program.

Where no distinction is explicitly made in the description, the

behavior described is the required behavior.

\pnum

If the formulation of a complexity requirement calls for a negative number of

operations, the actual requirement is zero operations.

\begin{footnote}

This simplifies

the presentation of complexity requirements in some cases.

\end{footnote}

\pnum

Complexity requirements specified in the library clauses are upper bounds,

and implementations that provide better complexity guarantees meet

the requirements.

\pnum

Error conditions specify conditions where a function may fail. The conditions

are listed, together with a suitable explanation, as the \tcode{enum class errc}

constants\iref{syserr}.

\rSec3[structure.see.also]{C library}

\pnum

Paragraphs labeled ``\textsc{See also}'' contain cross-references to the relevant portions

of other standards\iref{intro.refs}.

\rSec2[conventions]{Other conventions}

\rSec3[conventions.general]{General}

\indextext{conventions}%

\pnum

Subclause \ref{conventions} describes several editorial conventions used to describe the contents

of the \Cpp{} standard library.

These conventions are for describing

implementation-defined types\iref{type.descriptions},

and member functions\iref{functions.within.classes}.

\rSec3[expos.only.entity]{Exposition-only entities, etc.}

\pnum

Several entities

defined in \ref{\firstlibchapter} through \ref{\lastlibchapter} and \ref{depr}

are only defined for the purpose of exposition.

The declaration of such an entity

is followed by a comment ending in \expos.

\pnum

The following are defined for exposition only

to aid in the specification of the library:

\indexlibrary{decay-copy@\exposid{decay-copy}}%

\begin{codeblock}

namespace std {

template<class T>

requires @\libconcept{convertible_to}@<T, decay_t<T>>

constexpr decay_t<T> @\exposidnc{decay-copy}@(T&& v) // \expos

noexcept(is_nothrow_convertible_v<T, decay_t<T>>)

{ return std::forward<T>(v); }

constexpr auto @\exposidnc{synth-three-way}@ = // \expos

[]<class T, class U>(const T& t, const U& u)

requires requires {

{ t < u } -> @\exposconcept{boolean-testable}@;

{ u < t } -> @\exposconcept{boolean-testable}@;

}

{

if constexpr (@\libconcept{three_way_comparable_with}@<T, U>) {

return t <=> u;

} else {

if (t < u) return weak_ordering::less;

if (u < t) return weak_ordering::greater;

return weak_ordering::equivalent;

}

};

template<class T, class U = T>

using @\exposidnc{synth-three-way-result}@ = // \expos

decltype(@\exposidnc{synth-three-way}@(declval<T&>(), declval<U&>()));

}

\end{codeblock}

\pnum

An object \tcode{dst} is said to be \defn{decay-copied from}

a subexpression \tcode{src}

if the type of \tcode{dst} is

\begin{codeblock}

decay_t<decltype((src))>

\end{codeblock} and \tcode{dst} is copy-initialized from \tcode{src}.

\rSec3[type.descriptions]{Type descriptions}

\rSec4[type.descriptions.general]{General}

\pnum

The Requirements subclauses may describe names that are used to specify

constraints on template arguments.

\begin{footnote}

Examples

from~\ref{utility.requirements} include:

\oldconcept{EqualityComparable},

\oldconcept{LessThanComparable},

\oldconcept{CopyConstructible}.

Examples from~\ref{iterator.requirements} include:

\oldconcept{InputIterator},

\oldconcept{ForwardIterator}.

\end{footnote}

These names are used in library Clauses

to describe the types that

may be supplied as arguments by a \Cpp{} program when instantiating template components from

the library.

\pnum

Certain types defined in \ref{input.output} are used to describe implementation-defined types.

\indextext{types!implementation-defined}%

They are based on other types, but with added constraints.

\rSec4[enumerated.types]{Enumerated types}

\pnum

Several types defined in \ref{input.output} are

\defnadjx{enumerated}{types}{type}.

Each enumerated type may be implemented as an enumeration or as a synonym for

an enumeration.

\begin{footnote}

Such as an integer type, with constant integer

values\iref{basic.fundamental}.

\end{footnote}

\pnum

The enumerated type \tcode{\placeholder{enumerated}} can be written:

\begin{codeblock}

enum @\placeholder{enumerated}@ { @$\tcode{\placeholder{V}}_{0}$@, @$\tcode{\placeholder{V}}_{1}$@, @$\tcode{\placeholder{V}}_{2}$@, @$\tcode{\placeholder{V}}_{3}$@, @$\ldots$@ };

inline const @$\tcode{\placeholder{enumerated C}}_{0}$@(@$\tcode{\placeholder{V}}_{0}$@);

inline const @$\tcode{\placeholder{enumerated C}}_{1}$@(@$\tcode{\placeholder{V}}_{1}$@);

inline const @$\tcode{\placeholder{enumerated C}}_{2}$@(@$\tcode{\placeholder{V}}_{2}$@);

inline const @$\tcode{\placeholder{enumerated C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}$@);

@\vdots@

\end{codeblock}

\pnum

Here, the names $\tcode{\placeholder{C}}_0$,

$\tcode{\placeholder{C}}_1$, etc.\ represent

\defnx{enumerated elements}{enumerated element}

for this particular enumerated type.

\indextext{type!enumerated}%

All such elements have distinct values.

\rSec4[bitmask.types]{Bitmask types}

\pnum

Several types defined in \ref{\firstlibchapter} through \ref{\lastlibchapter}

and \ref{depr} are

\defnx{bitmask types}{type!bitmask}.

Each bitmask type can be implemented as an

enumerated type that overloads certain operators, as an integer type,

or as a

\tcode{bitset}\iref{template.bitset}.

\indextext{type!enumerated}%

\pnum

The bitmask type \tcode{\placeholder{bitmask}} can be written:

\begin{codeblock}

// For exposition only.

// \tcode{int_type} is an integral type capable of representing all values of the bitmask type.

enum @\placeholder{bitmask}@ : int_type {

@$\tcode{\placeholder{V}}_{0}$@ = 1 << 0, @$\tcode{\placeholder{V}}_{1}$@ = 1 << 1, @$\tcode{\placeholder{V}}_{2}$@ = 1 << 2, @$\tcode{\placeholder{V}}_{3}$@ = 1 << 3, @$\ldots$@

};

inline constexpr @$\tcode{\placeholder{bitmask C}}_{0}$@(@$\tcode{\placeholder{V}}_{0}{}$@);

inline constexpr @$\tcode{\placeholder{bitmask C}}_{1}$@(@$\tcode{\placeholder{V}}_{1}{}$@);

inline constexpr @$\tcode{\placeholder{bitmask C}}_{2}$@(@$\tcode{\placeholder{V}}_{2}{}$@);

inline constexpr @$\tcode{\placeholder{bitmask C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}{}$@);

@\vdots@

constexpr @\placeholder{bitmask}{}@ operator&(@\placeholder{bitmask}{}@ X, @\placeholder{bitmask}{}@ Y) {

return static_cast<@\placeholder{bitmask}{}@>(

static_cast<int_type>(X) & static_cast<int_type>(Y));

}

constexpr @\placeholder{bitmask}{}@ operator|(@\placeholder{bitmask}{}@ X, @\placeholder{bitmask}{}@ Y) {

return static_cast<@\placeholder{bitmask}{}@>(

static_cast<int_type>(X) | static_cast<int_type>(Y));

}

constexpr @\placeholder{bitmask}{}@ operator^(@\placeholder{bitmask}{}@ X, @\placeholder{bitmask}{}@ Y) {

return static_cast<@\placeholder{bitmask}{}@>(

static_cast<int_type>(X) ^ static_cast<int_type>(Y));

}

constexpr @\placeholder{bitmask}{}@ operator~(@\placeholder{bitmask}{}@ X) {

return static_cast<@\placeholder{bitmask}{}@>(~static_cast<int_type>(X));

}

@\placeholder{bitmask}{}@& operator&=(@\placeholder{bitmask}{}@& X, @\placeholder{bitmask}{}@ Y) {

X = X & Y; return X;

}

@\placeholder{bitmask}{}@& operator|=(@\placeholder{bitmask}{}@& X, @\placeholder{bitmask}{}@ Y) {

X = X | Y; return X;

}

@\placeholder{bitmask}{}@& operator^=(@\placeholder{bitmask}{}@& X, @\placeholder{bitmask}{}@ Y) {

X = X ^ Y; return X;

}

\end{codeblock}

\pnum

Here, the names $\tcode{\placeholder{C}}_0$,

$\tcode{\placeholder{C}}_1$, etc.\ represent

\defnx{bitmask elements}{bitmask!element}

for this particular bitmask type.

\indextext{type!bitmask}%

All such elements have distinct, nonzero values such that, for any pair $\tcode{\placeholder{C}}_i$

and $\tcode{\placeholder{C}}_j$ where $i \neq j$, \tcode{$\placeholder{C}_i$ \& $\placeholder{C}_i$} is nonzero and

\tcode{$\placeholder{C}_i$ \& $\placeholder{C}_j$} is zero.

Additionally, the value \tcode{0} is used to represent an \defnx{empty bitmask}{bitmask!empty}, in which no

bitmask elements are set.

\pnum

The following terms apply to objects and values of

bitmask types:

\begin{itemize}

\item

To \defnx{set}{bitmask!value!set}

a value \textit{Y} in an object \textit{X}

is to evaluate the expression \textit{X} \tcode{|=} \textit{Y}.

\item

To \defnx{clear}{bitmask!value!clear}

a value \textit{Y} in an object

\textit{X} is to evaluate the expression \textit{X} \tcode{\&= \~}\textit{Y}.

\item

The value \textit{Y} \defnx{is set}{bitmask!value!is set} in the object

\textit{X} if the expression \textit{X} \tcode{\&} \textit{Y} is nonzero.

\end{itemize}

\rSec4[character.seq]{Character sequences}

\rSec5[character.seq.general]{General}

\pnum

The C standard library makes widespread use

\indextext{library!C standard}%

of characters and character sequences that follow a few uniform conventions:

\begin{itemize}

\item

Properties specified as \defn{locale-specific}

may change during program execution

by a call to \tcode{setlocale(int, const char*)}\iref{clocale.syn}, or

by a change to a \tcode{locale} object,

as described in \ref{locales} and \ref{input.output}.

\item

The \defnadj{execution}{character set} and

the \defnadj{execution}{wide-character set}

are supersets of the basic literal character set\iref{lex.charset}.

The encodings of the execution character sets and

the sets of additional elements (if any) are locale-specific.

Each element of the execution wide-character set is encoded as

a single code unit representable by a value of type \keyword{wchar_t}.

\begin{note}

The encodings of the execution character sets can be unrelated

to any literal encoding.

\end{note}

\item

A \defn{letter} is any of the 26 lowercase or 26

\indextext{lowercase}%

\indextext{uppercase}%

uppercase letters in the basic character set.

\item

The

\defnx{decimal-point character}{character!decimal-point}

is the locale-specific

(single-byte) character used by functions that convert between a (single-byte)

character sequence and a value of one of the floating-point types.

It is used

in the character sequence to denote the beginning of a fractional part.

It is

represented in \ref{\firstlibchapter} through \ref{\lastlibchapter}

and \ref{depr} by a period,

\indextext{period}%

\tcode{'.'},

which is

also its value in the \tcode{"C"}

locale.

\item

A

\defn{character sequence}

is an array object\iref{dcl.array} \tcode{\placeholdernc{A}} that

can be declared as

\tcode{\placeholdernc{T\;A}[\placeholder{N}]},

where \tcode{\placeholder{T}} is any of the types

\tcode{char},

\tcode{unsigned char},

or

\tcode{signed char}\iref{basic.fundamental}, optionally qualified by any combination of

\keyword{const}

or

\tcode{volatile}.

The initial elements of the

array have defined contents up to and including an element determined by some

predicate.

A character sequence can be designated by a pointer value

\tcode{\placeholder{S}} that points to its first element.

\item

\indextext{STATICALLY-WIDEN@\exposid{STATICALLY-WIDEN}}%

Let \exposid{STATICALLY-WIDEN}\tcode{<charT>("...")} be

\tcode{"..."} if \tcode{charT} is \tcode{char} and

\tcode{L"..."} if \tcode{charT} is \keyword{wchar_t}.

\end{itemize}

\rSec5[byte.strings]{Byte strings}

\indextext{string!null-terminated byte|see{\ntbs{}}}%

\pnum

A \defnx{null-terminated byte string}{NTBS@\ntbs{}},

or \ntbs{},

is a character sequence whose highest-addressed element

with defined content has the value zero

(the \defnx{terminating null character}{character!terminating null});

no other element in the sequence has the value zero.

\begin{footnote}

Many of the objects manipulated by

function signatures declared in

\libheaderref{cstring} are character sequences or \ntbs{}s.

The size of some of these character sequences is limited by

a length value, maintained separately from the character sequence.

\end{footnote}

\pnum

The \defnx{length of an \ntbs{}}{NTBS@\ntbs{}!length}

is the number of elements that

precede the terminating null character.

An \defnx{empty \ntbs{}}{NTBS@\ntbs{}!empty}

has a length of zero.

\pnum

The \defnx{value of an \ntbs{}}{NTBS@\ntbs{}!value}

is the sequence of values of the

elements up to and including the terminating null character.

\pnum

A \defnx{static \ntbs{}}{NTBS@\ntbs{}!static}

is an \ntbs{} with

static storage duration.

\begin{footnote}

A \grammarterm{string-literal}, such as

\tcode{"abc"},

is a static \ntbs{}.

\end{footnote}

\rSec5[multibyte.strings]{Multibyte strings}

\pnum

A \defnx{multibyte character}{character!multibyte} is

a sequence of one or more bytes representing the

code unit sequence for an encoded character of the

execution character set.

\indextext{string!null-terminated multibyte|see{\ntmbs{}}}%

\pnum

A \defnx{null-terminated multibyte string}{NTMBS@\ntmbs{}},

or \ntmbs{},

is an \ntbs{} that constitutes a

sequence of valid multibyte characters, beginning and ending in the initial

shift state.

\begin{footnote}

An \ntbs{} that contains characters only from the

basic literal character set is also an \ntmbs{}.

Each multibyte character then

consists of a single byte.

\end{footnote}

\pnum

A \defnx{static \ntmbs{}}{NTMBS@\ntmbs{}!static}

is an \ntmbs{} with static storage duration.

\rSec4[customization.point.object]{Customization Point Object types}

\pnum

A \defnadj{customization point}{object} is a function object\iref{function.objects}

with a literal class type that interacts with program-defined types while

enforcing semantic requirements on that interaction.

\pnum

The type of a customization point object, ignoring cv-qualifiers, shall model

\libconcept{semiregular}\iref{concepts.object}.

\pnum

All instances of a specific customization point object type shall

be equal\iref{concepts.equality}.

The effects of invoking different instances

of a specific customization point object type on the same arguments

are equivalent.

\pnum

The type \tcode{T} of a customization point object,

ignoring \grammarterm{cv-qualifier}s, shall model

\tcode{\libconcept{invocable}<T\&, Args...>},

\tcode{\libconcept{invocable}<const T\&, Args...>},

\tcode{\libconcept{invocable}<T, Args...>}, and

\tcode{\libconcept{invocable}<const T, Args...>}\iref{concept.invocable}

when the types in \tcode{Args...} meet the requirements specified in that

customization point object's definition. When the types of \tcode{Args...} do

not meet the customization point object's requirements, \tcode{T} shall not have

a function call operator that participates in overload resolution.

\pnum

For a given customization point object \tcode{o},

let \tcode{p} be a variable initialized as if by \tcode{auto p = o;}.

Then for any sequence of arguments \tcode{args...},

the following expressions have effects equivalent to \tcode{o(args...)}:

\begin{itemize}

\item \tcode{p(args...)}

\item \tcode{as_const(p)(args...)}

\item \tcode{std::move(p)(args...)}

\item \tcode{std::move(as_const(p))(args...)}

\end{itemize}

\rSec3[alg.func.obj]{Algorithm function objects}

\pnum

An \defn{algorithm function object} is

a customization point object\iref{customization.point.object}

that is specified as one or more overloaded function templates.

The name of these function templates designates

the corresponding algorithm function object.

\pnum

For an algorithm function object \tcode{o},

let $S$ be the corresponding set of function templates.

Then for any sequence of arguments $\tcode{args} \dotsc$,

$\tcode{o(args} \dotsc \tcode{)}$ is expression-equivalent to

$\tcode{s(args} \dotsc \tcode{)}$,

where the result of name lookup for \tcode{s} is the overload set $S$.

\begin{note}

Algorithm function objects are not found by

argument-dependent name lookup\iref{basic.lookup.argdep}.

When found by unqualified name lookup\iref{basic.lookup.unqual}

for the \grammarterm{postfix-expression} in a function call\iref{expr.call},

they inhibit argument-dependent name lookup.

\begin{example}

\begin{codeblock}

void foo() {

using namespace std::ranges;

std::vector<int> vec{1,2,3};

find(begin(vec), end(vec), 2); // \#1

}

\end{codeblock}

The function call expression at \#1 invokes \tcode{std::ranges::find},

not \tcode{std::find}.

\end{example}

\end{note}

\rSec3[functions.within.classes]{Functions within classes}

\pnum

For the sake of exposition, \ref{\firstlibchapter} through \ref{\lastlibchapter}

and \ref{depr} do not describe copy/move constructors, assignment

operators, or (non-virtual) destructors with the same apparent

semantics as those that can be generated

by default\iref{class.copy.ctor,class.copy.assign,class.dtor}.

\indextext{constructor!copy}%

\indextext{operator!assignment}%

\indextext{destructor}%

It is unspecified whether

the implementation provides explicit definitions for such member function

signatures, or for virtual destructors that can be generated by default.

\rSec3[objects.within.classes]{Private members}

\pnum

\ref{\firstlibchapter} through \ref{\lastlibchapter} and

\ref{depr} do not specify the representation of classes, and intentionally

omit specification of class members\iref{class.mem}. An implementation may

define static or non-static class members, or both, as needed to implement the

semantics of the member functions specified in \ref{\firstlibchapter}

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

\pnum

For the sake of exposition,

some subclauses provide representative declarations, and semantic requirements, for

private members of classes that meet the external specifications of the classes.

The declarations for such members are

followed by a comment that ends with \expos, as in:

\begin{codeblock}

streambuf* sb; // \expos

\end{codeblock}

\pnum

An implementation may use any technique that provides equivalent observable behavior.

\rSec3[freestanding.item]{Freestanding items}

\pnum

\indextext{item!freestanding|see{freestanding item}}%

A \defn{freestanding item} is

a declaration, entity, or macro

that is required to be present in

a freestanding implementation and a hosted implementation.

\pnum

Unless otherwise specified,

the requirements on freestanding items for a freestanding implementation

are the same as the corresponding requirements for a hosted implementation,

except that not all of the members of those items are required to be present.

\pnum

Function declarations and function template declarations

followed by a comment that include \textit{freestanding-deleted} are

\defnadjx{freestanding deleted}{functions}{function}.

On freestanding implementations,