lib-intro.tex

%!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,
contract assertions
whose predicates are as described in the hardened precondition
are evaluated with a terminating semantic\iref{basic.contract.eval}.
\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
declared in \ref{\firstlibchapter} through \ref{\lastlibchapter} and \ref{depr}
are provided for exposition only.
The declaration of such an entity
is followed by a comment ending in \expos.

\pnum
The following are provided 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&>()));

  template<class T>
    concept @\defexposconceptnc{constexpr-wrapper-like}@ =                  // \expos
      @\libconcept{convertible_to}@<T, decltype(T::value)> &&
      @\libconcept{equality_comparable_with}@<T, decltype(T::value)> &&
      bool_constant<T() == T::value>::value &&
      bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
}
\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 shown 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 specified in \ref{input.output} and \ref{re} 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 specified 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}
and shall be
a structural type\iref{temp.param} and
a trivially copyable type\iref{class.prop}.
Every constructor of this type
shall have a non-throwing exception specification\iref{except.spec}.

\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,