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%!TEX root = std.tex
\rSec0[library]{Library introduction}
\rSec1[library.general]{General}
\pnum
This Clause describes the contents of the
\term{\Cpp standard library},
\indextext{library!C++ 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 definitions~(\ref{definitions}), method of
description~(\ref{description}), and organization~(\ref{organization}) of the
library. Clause~\ref{requirements}, Clauses~\ref{\firstlibchapter}
through~\ref{\lastlibchapter}, and Annex~\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
Clauses~\ref{\firstlibchapter}--\ref{\lastlibchapter}, as shown in
Table~\ref{tab:library.categories}.
\begin{libsumtabbase}{Library categories}{tab:library.categories}{Clause}{Category}
\ref{language.support} & & Language support library \\
\ref{diagnostics} & & Diagnostics library \\
\ref{utilities} & & General utilities library \\
\ref{strings} & & Strings library \\
\ref{localization} & & Localization library \\
\ref{containers} & & Containers library \\
\ref{iterators} & & Iterators library \\
\ref{algorithms} & & Algorithms library \\
\ref{numerics} & & Numerics library \\
\ref{input.output} & & Input/output library \\
\ref{re} & & Regular expressions library \\
\ref{atomics} & & Atomic operations library \\
\ref{thread} & & Thread support library \\
\end{libsumtabbase}
\pnum
The language support library (Clause~\ref{language.support}) provides components that are
required by certain parts of the \Cpp language, such as memory allocation~(\ref{expr.new},
\ref{expr.delete}) and exception processing (Clause~\ref{except}).
\pnum
The diagnostics library (Clause~\ref{diagnostics}) provides a consistent framework for
reporting errors in a \Cpp program, including predefined exception classes.
\pnum
The general utilities library (Clause~\ref{utilities}) includes components used
by other library elements, such as a predefined storage allocator for dynamic
storage management~(\ref{basic.stc.dynamic}), and components used
as infrastructure
in \Cpp programs,
such as tuples, function wrappers, and time facilities.
\pnum
The strings library (Clause~\ref{strings}) provides support for manipulating text represented
as sequences of type
\tcode{char},
sequences of type
\tcode{char16_t},
sequences of type
\tcode{char32_t},
sequences of type
\tcode{wchar_t},
and sequences of any other character-like type.
\pnum
The localization library (Clause~\ref{localization}) provides extended internationalization
support for text processing.
\pnum
The containers (Clause~\ref{containers}), iterators (Clause~\ref{iterators}),
and algorithms (Clause~\ref{algorithms}) libraries provide a \Cpp program with access
to a subset of the most widely used algorithms and data structures.
\pnum
The numerics library (Clause~\ref{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 input/output library (Clause~\ref{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 regular expressions library (Clause~\ref{re}) provides regular expression matching and searching.
\pnum
The atomic operations library (Clause~\ref{atomics}) allows more fine-grained
concurrent access to shared data than is possible with locks.
\pnum
The thread support library (Clause~\ref{thread}) provides components to create
and manage threads, including mutual exclusion and interthread communication.
\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 International Standard
may be different from the signatures in the C standard library,
and additional overloads may be declared in this International Standard,
but the behavior and the preconditions
(including any preconditions implied by the use of an
ISO C \tcode{restrict} qualifier)
are the same unless otherwise stated.
\rSec1[definitions]{Definitions}
\pnum
\begin{note}
Clause \ref{intro.defs} defines additional terms used elsewhere in this International Standard.
\end{note}
\def\definition{\definitionx{\subsection}}%
\definition{arbitrary-positional stream}{defns.arbitrary.stream}
\indexdefn{stream!arbitrary-positional}%
a stream (described in Clause~\ref{input.output}) that can seek to any integral position within
the length of the stream\\
\begin{note} Every arbitrary-positional stream is also a repositional stream. \end{note}
\definition{character}{defns.character}
\indexdefn{character}%
\defncontext{Clauses~\ref{strings}, \ref{localization}, \ref{input.output}, and~\ref{re}}
any object which,
when treated sequentially,
can represent text\\
\begin{note}
The term does not mean only
\tcode{char},
\tcode{char16_t},
\tcode{char32_t},
and
\tcode{wchar_t}
objects,
but any value that can be represented by a type
that provides the definitions specified in these Clauses.
\end{note}
\definition{character container type}{defns.character.container}
\indexdefn{type!character container}%
a class or a type used to
represent a
\term{character}\\
\begin{note}
It is used for one of the template parameters of the string,
iostream, and regular expression class templates.
A character container type is a POD~(\ref{basic.types}) type.
\end{note}
\definition{comparison function}{defns.comparison}
\indexdefn{function!comparison}%
an operator function~(\ref{over.oper}) for any of the equality~(\ref{expr.eq}) or
relational~(\ref{expr.rel}) operators
\definition{component}{defns.component}
\indexdefn{component}%
a group of library entities directly related as members, parameters, or
return types\\
\begin{note}
For example, the class template
\tcode{basic_string}
and the non-member
function templates
that operate on
strings are referred to as the
\term{string component}.
\end{note}
\definition{constant subexpression}{defns.const.subexpr}
\indexdefn{constant subexpression}%
an expression whose evaluation as subexpression of a
\grammarterm{conditional-expression}
\tcode{CE}~(\ref{expr.cond}) would not prevent \tcode{CE}
from being a core constant expression~(\ref{expr.const})
\definition{deadlock}{defns.deadlock}
\indexdefn{deadlock}%
one or more threads are unable to continue execution because each is
blocked waiting for one or more of the others to satisfy some condition
\definition{default behavior}{defns.default.behavior.impl}
\indexdefn{behavior!default}%
\defncontext{implementation}
any specific behavior provided by the implementation,
within the scope of the
\term{required behavior}
\definition{default behavior}{defns.default.behavior.func}
\indexdefn{behavior!default}%
\defncontext{specification}
a description of
\term{replacement function}
and
\term{handler function}
semantics
\definition{direct-non-list-initialization}{defns.direct-non-list-init}
\indexdefn{direct-non-list-initialization}%
a direct-initialization~(\ref{dcl.init})
that is not list-initialization~(\ref{dcl.init.list})
\definition{handler function}{defns.handler}
\indexdefn{function!handler}%
a
\term{non-reserved function}
whose definition may be provided by a \Cpp program\\
\begin{note}
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 (Clause~\ref{language.support}).
\end{note}
\definition{iostream class templates}{defns.iostream.templates}
templates, defined in Clause~\ref{input.output},
that take two template arguments\\
\begin{note}
The arguments are named
\tcode{charT}
and
\tcode{traits}.
The argument
\tcode{charT}
is a character container class,
and the argument
\tcode{traits}
is a class which defines additional characteristics and functions
of the character type represented by
\tcode{charT}
necessary to implement the iostream class templates.
\end{note}
\definition{modifier function}{defns.modifier}
\indexdefn{function!modifier}%
a class member function~(\ref{class.mfct}) other than a constructor,
assignment operator, or destructor
that alters the state of an object of the class
\definition{move assignment}{defns.move.assign}
\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}
\indexdefn{construction!move}%
direct-initialization of an object of some type with an rvalue of the same type
\definition{NTCTS}{defns.ntcts}
\indexdefn{NTCTS}%
\indexdefn{string!null-terminated character type}%
a sequence of values that have
\term{character type}
that precede the terminating null character type
value
\tcode{charT()}
\definition{observer function}{defns.observer}
\indexdefn{function!observer}%
a class member function~(\ref{class.mfct}) that accesses the state of an object of the class
but does not alter that state\\
\begin{note}
Observer functions are specified as
\tcode{const}
member functions~(\ref{class.this}).
\end{note}
\definition{referenceable type}{defns.referenceable}
\indexdefn{type!referenceable}%
an object type, a function type that does not have cv-qualifiers or a
\grammarterm{ref-qualifier}, or a reference type
\begin{note} The term describes a type to which a reference can be created,
including reference types. \end{note}
\definition{replacement function}{defns.replacement}
\indexdefn{function!replacement}%
a
\term{non-reserved function}
whose definition is provided by a \Cpp program\\
\begin{note}
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~(\ref{lex.phases}) and resolving the
definitions of all translation units~(\ref{basic.link}).
\end{note}
\definition{repositional stream}{defns.repositional.stream}
\indexdefn{stream!repositional}%
a stream (described in Clause~\ref{input.output}) that can seek to a position that was
previously encountered
\definition{required behavior}{defns.required.behavior}
\indexdefn{behavior!required}%
a description of
\term{replacement function}
and
\term{handler function}
semantics
applicable to both the behavior provided by the implementation and
the behavior of any such function definition in the program\\
\begin{note}
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{note}
\definition{reserved function}{defns.reserved.function}
\indexdefn{function!reserved}%
a function, specified as part of the \Cpp standard library, that must be defined by the
implementation\\
\begin{note}
If a \Cpp program provides a definition for any reserved function, the results are undefined.%
\indextext{undefined}
\end{note}
\definition{stable algorithm}{defns.stable}
\indexdefn{algorithm!stable}%
\indexdefn{stable algorithm}%
an algorithm that preserves, as appropriate to the particular algorithm, the order
of elements\\
\begin{note} Requirements for stable algorithms are given in~\ref{algorithm.stable}. \end{note}
\definition{traits class}{defns.traits}
\indexdefn{traits}%
a 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
\definition{valid but unspecified state}{defns.valid}
\indexdefn{valid but unspecified state}%
a 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}
\rSec1[description]{Method of description (Informative)}
\pnum
This subclause describes the conventions used to specify the \Cpp standard
library. \ref{structure} describes the structure of the normative
Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter} and
Annex~\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:\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.}
\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
Paragraphs labeled ``Note(s):'' or ``Example(s):'' are informative, other paragraphs
are normative.
\pnum
The contents of the summary and the detailed specifications include:
\begin{itemize}
\item macros
\item values
\item types
\item classes and class templates
\item functions and function templates
\item objects
\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
\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 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 satisfy the requirements. For every set of well-defined expression
requirements there is a table that specifies an initial set of the valid expressions and
their semantics. Any generic algorithm (Clause~\ref{algorithms}) that uses the
well-defined expression requirements is described in terms of the valid expressions for
its template type parameters.
\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.\footnote{Although in some cases the code given is
unambiguously the optimum implementation.}
\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):\footnote{To save space, items that do not apply to a class are omitted.
For example, if a class does not specify any comparison functions, there
will be no ``Comparison functions'' subclause.}
\begin{itemize}
\item constructor(s) and destructor
\item copying, moving \& assignment functions
\item comparison 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):\footnote{To save space, items that do not apply to a function are omitted.
For example, if a function does not specify any
further
preconditions, there will be no \requires paragraph.}
\begin{itemize}
\item \requires the preconditions for calling the function
\item \effects the actions performed by the function
\item \sync the synchronization operations~(\ref{intro.multithread}) applicable to the function
\item \postconditions the observable results established by the function
\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 \effects element specifies that the semantics of some function
\tcode{F} are \techterm{Equivalent to} some code sequence, then the various elements are
interpreted as follows. If \tcode{F}'s semantics specifies a \requires element, then
that requirement is logically imposed prior to the \techterm{equivalent-to} semantics.
Next, the semantics of the code sequence are determined by the \requires, \effects,
\sync, \postconditions, \returns, \throws, \complexity, \remarks, and \errors
specified for the function invocations contained in the code sequence. The value
returned from \tcode{F} is specified by \tcode{F}'s \returns element, or if \tcode{F}
has no \returns element, a non-\tcode{void} return from \tcode{F} is specified by the
\tcode{return} statements in the code sequence.
If \tcode{F}'s semantics contains a \throws,
\postconditions, or \complexity element, then that supersedes any occurrences of that
element in the code sequence.
\pnum
For non-reserved replacement and handler functions,
Clause~\ref{language.support} specifies two behaviors for the functions in question:
their required and default behavior.
The
\term{default behavior}
describes a function definition provided by the implementation.
\indextext{behavior!default}%
The
\term{required behavior}
describes the semantics of a function definition provided by
\indextext{behavior!required}%
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.\footnote{This simplifies
the presentation of complexity requirements in some cases.}
\pnum
Complexity requirements specified in the library clauses are upper bounds,
and implementations that provide better complexity guarantees satisfy
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~(\ref{syserr}).
\rSec3[structure.see.also]{C library}
\pnum
Paragraphs labeled ``\xref'' contain cross-references to the relevant portions
of this International Standard and the ISO C standard.
\rSec2[conventions]{Other conventions}
\indextext{conventions}%
\pnum
This subclause describes several editorial conventions used to describe the contents
of the \Cpp standard library.
These conventions are for describing
implementation-defined types~(\ref{type.descriptions}),
and member functions~(\ref{functions.within.classes}).
\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.\footnote{Examples
from~\ref{utility.requirements} include:
\tcode{EqualityComparable},
\tcode{LessThanComparable},
\tcode{CopyConstructible}.
Examples from~\ref{iterator.requirements} include:
\tcode{InputIterator},
\tcode{ForwardIterator}.}
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 Clause~\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[expos.only.types]{Exposition-only types}
\pnum
Several types defined in Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter}
and Annex~\ref{depr} that are used as function parameter or return types are defined
for the purpose of exposition only in order to capture their language linkage. The
declarations of such types are followed by a comment ending in \textit{exposition only}.
\begin{example}
\begin{codeblock}
namespace std {
extern "C" using @\placeholdernc{some-handler}@ = int(int, void*, double); // \expos
}
\end{codeblock}
The type placeholder \tcode{\placeholder{some-handler}} can now be used to specify a function
that takes a callback parameter with C language linkage.
\end{example}
\rSec4[enumerated.types]{Enumerated types}
\pnum
Several types defined in Clause~\ref{input.output} are
\term{enumerated types}.
\indextext{type!enumerated}%
Each enumerated type may be implemented as an enumeration or as a synonym for
an enumeration.\footnote{Such as an integer type, with constant integer
values~(\ref{basic.fundamental}).}
\pnum
The enumerated type \tcode{\placeholder{enumerated}} can be written:
\begin{codeblock}
enum @\placeholder{enumerated}@ { @$\tcode{\placeholder{V}}_{0}$@, @$\tcode{\placeholder{V}}_{1}$@, @$\tcode{\placeholder{V}}_{2}$@, @$\tcode{\placeholder{V}}_{3}$@, ..... };
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 @$\tcode{\placeholder{enumerated C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}$@);
.....
\end{codeblock}
\pnum
Here, the names $\tcode{\placeholder{C}}_0$,
$\tcode{\placeholder{C}}_1$, etc.\ represent
\term{enumerated elements}
for this particular enumerated type.
\indextext{type!enumerated}%
All such elements have distinct values.
\rSec4[bitmask.types]{Bitmask types}
\pnum
Several types defined in Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter}
and Annex~\ref{depr} are
\term{bitmask types}.
\indextext{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}~(\ref{template.bitset}).
\indextext{type!enumerated}%
\pnum
The bitmask type \term{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, .....
};
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 @$\tcode{\placeholder{bitmask C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}{}$@);
.....
constexpr @\term{bitmask}{}@ operator&(@\term{bitmask}{}@ X, @\term{bitmask}{}@ Y) {
return static_cast<@\term{bitmask}{}@>(
static_cast<int_type>(X) & static_cast<int_type>(Y));
}
constexpr @\term{bitmask}{}@ operator|(@\term{bitmask}{}@ X, @\term{bitmask}{}@ Y) {
return static_cast<@\term{bitmask}{}@>(
static_cast<int_type>(X) | static_cast<int_type>(Y));
}
constexpr @\term{bitmask}{}@ operator^(@\term{bitmask}{}@ X, @\term{bitmask}{}@ Y){
return static_cast<@\term{bitmask}{}@>(
static_cast<int_type>(X) ^ static_cast<int_type>(Y));
}
constexpr @\term{bitmask}{}@ operator~(@\term{bitmask}{}@ X){
return static_cast<@\term{bitmask}{}@>(~static_cast<int_type>(X));
}
@\term{bitmask}{}@& operator&=(@\term{bitmask}{}@& X, @\term{bitmask}{}@ Y){
X = X & Y; return X;
}
@\term{bitmask}{}@& operator|=(@\term{bitmask}{}@& X, @\term{bitmask}{}@ Y) {
X = X | Y; return X;
}
@\term{bitmask}{}@& operator^=(@\term{bitmask}{}@& X, @\term{bitmask}{}@ Y) {
X = X ^ Y; return X;
}
\end{codeblock}
\pnum
Here, the names $\tcode{\placeholder{C}}_0$,
$\tcode{\placeholder{C}}_1$, etc.\ represent
\term{bitmask elements}
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.
\indextext{bitmask!empty}%
Additionally, the value \tcode{0} is used to represent an \term{empty bitmask}, in which no
bitmask elements are set.
\pnum
The following terms apply to objects and values of
bitmask types:
\begin{itemize}
\item
To
\term{set}
a value \textit{Y} in an object \textit{X}
is to evaluate the expression \textit{X} \tcode{|=} \textit{Y}.
\item
To
\term{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}
\term{is set}
in the object
\textit{X} if the expression \textit{X} \tcode{\&} \textit{Y} is nonzero.
\end{itemize}
\rSec4[character.seq]{Character sequences}
\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
A
\term{letter}
is any of the 26 lowercase or 26
\indextext{lowercase}%
\indextext{uppercase}%
uppercase letters in the basic execution character set.
\item
The
\term{decimal-point character}
is the
\indextext{character!decimal-point}%
(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 Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter}
and Annex~\ref{depr} by a period,
\indextext{period}%
\tcode{'.'},
which is
also its value in the \tcode{"C"}
locale, but may change during program
execution by a call to
\tcode{setlocale(int, const char*)},\footnote{declared in
\tcode{<clocale>}~(\ref{c.locales}).
\indextext{\idxcode{setlocale}}%
\indexlibrary{\idxcode{setlocale}}%
\indextext{\idxhdr{clocale}}%
\indexlibrary{\idxhdr{clocale}}}
or by a change to a
\tcode{locale}
object, as described in Clauses~\ref{locales} and~\ref{input.output}.
\item
A
\term{character sequence}
is an array object~(\ref{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}~(\ref{basic.fundamental}), optionally qualified by any combination of
\tcode{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.
\end{itemize}
\rSec5[byte.strings]{Byte strings}
\pnum
A
\indextext{string!null-terminated byte}%
\indextext{NTBS}%
\term{null-terminated byte string},
or \ntbs,
is a character sequence whose highest-addressed element
with defined content has the value zero
(the
\term{terminating null}
character); no other element in the sequence has the value zero.%
\indextext{\idxhdr{cstring}}%
\indexlibrary{\idxhdr{cstring}}%
\indextext{NTBS}\footnote{Many of the objects manipulated by
function signatures declared in
\tcode{<cstring>}~(\ref{c.strings}) are character sequences or \ntbs{}s.
\indextext{\idxhdr{cstring}}%
\indexlibrary{\idxhdr{cstring}}%
The size of some of these character sequences is limited by
a length value, maintained separately from the character sequence.}
\pnum
The
\term{length} of an \ntbs
is the number of elements that
precede the terminating null character.
\indextext{NTBS}%
An
\term{empty} \ntbs
has a length of zero.
\pnum
The
\term{value} of an \ntbs
is the sequence of values of the
elements up to and including the terminating null character.
\indextext{NTBS}%
\pnum
A
\indextext{NTBS}%
\indextext{NTBS!static}%
\term{static} \ntbs
is an \ntbs with
static storage duration.\footnote{A string literal, such as
\tcode{"abc"},
is a static \ntbs.}
\rSec5[multibyte.strings]{Multibyte strings}
\pnum
\indextext{NTBS}%
\indextext{NTMBS}%
A \defnx{null-terminated multibyte string}{string!null-terminated multibyte},
or \ntmbs, is an \ntbs that constitutes a
sequence of valid multibyte characters, beginning and ending in the initial
shift state.\footnote{An \ntbs that contains characters only from the
basic execution character set is also an \ntmbs.
Each multibyte character then
consists of a single byte.}
\pnum
A
\term{static} \ntmbs
is an \ntmbs with static storage duration.
\indextext{NTMBS!static}%
\indextext{NTMBS}%
\rSec3[functions.within.classes]{Functions within classes}
\pnum
For the sake of exposition, Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter}
and Annex~\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~(\ref{class.ctor}, \ref{class.dtor}, \ref{class.copy}).
\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.
\pnum
For the sake of exposition, the library clauses sometimes annotate
constructors with \EXPLICIT{}. Such a constructor is conditionally declared
as either explicit or non-explicit~(\ref{class.conv.ctor}).
\begin{note} This is typically implemented by declaring two such constructors,
of which at most one participates in overload resolution. \end{note}
\rSec3[objects.within.classes]{Private members}
\pnum
Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter} and
Annex~\ref{depr} do not specify the representation of classes, and intentionally
omit specification of class members~(\ref{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 Clauses~\ref{\firstlibchapter}
through~\ref{\lastlibchapter} and Annex~\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.
\rSec1[requirements]{Library-wide requirements}
\pnum
This subclause specifies requirements that apply to the entire \Cpp standard library.
Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter} and Annex~\ref{depr}
specify the requirements of individual entities within the library.
\pnum
Requirements specified in terms of interactions between threads do not apply to
programs having only a single thread of execution.
\pnum
Within this subclause, \ref{organization} describes the library's contents and
organization, \ref{using} describes how well-formed \Cpp programs gain access to library
entities,
\ref{utility.requirements} describes constraints on types and functions used with
the \Cpp standard library,
\ref{constraints} describes constraints on well-formed \Cpp programs, and
\ref{conforming} describes constraints on conforming implementations.
\rSec2[organization]{Library contents and organization}
\pnum
\ref{contents} describes the entities and macros defined in the \Cpp standard library.
\ref{headers} lists the standard library headers and some constraints on those headers.
\ref{compliance} lists requirements for a freestanding implementation of the \Cpp
standard library.
\rSec3[contents]{Library contents}
\pnum
The \Cpp standard library provides definitions
for the entities and macros described in the synopses
of the \Cpp standard library headers~(\ref{headers}).
\pnum
All library entities except
\tcode{operator new}
and
\tcode{operator delete}
are defined within the namespace
\tcode{std}
or namespaces nested within namespace
\tcode{std}.\footnote{The C standard library headers (Annex~\ref{depr.c.headers}) also define
names within the global namespace, while the \Cpp headers for C library
facilities~(\ref{headers}) may also define names within the global namespace.}%
\indextext{namespace}
It is unspecified whether names declared in a specific namespace are declared
directly in that namespace or in an inline namespace inside that
namespace.\footnote{This gives implementers freedom to use inline namespaces to
support multiple configurations of the library.}
\pnum
Whenever a name \tcode{x} defined in the standard library is mentioned,
the name \tcode{x} is assumed to be fully qualified as
\tcode{::std::x},
unless explicitly described otherwise. For example, if the \effects section
for library function \tcode{F} is described as calling library function \tcode{G},
the function
\tcode{::std::G}
is meant.
\rSec3[headers]{Headers}
\pnum
Each element of the \Cpp standard library is declared or defined (as appropriate) in a
\term{header}.\footnote{A header is not necessarily a source file, nor are the
sequences delimited by \tcode{<} and \tcode{>} in header names necessarily valid source
file names~(\ref{cpp.include}).}
\pnum
The \Cpp standard library provides the
\defnx{\Cpp library headers}{header!C++ library},
shown in Table~\ref{tab:cpp.library.headers}.
\begin{floattable}{\Cpp library headers}{tab:cpp.library.headers}
{llll}
\topline
\tcode{<algorithm>} &
\tcode{<future>} &
\tcode{<numeric>} &
\tcode{<strstream>} \\
\tcode{<any>} &
\tcode{<initializer_list>} &
\tcode{<optional>} &
\tcode{<system_error>} \\
\tcode{<array>} &
\tcode{<iomanip>} &
\tcode{<ostream>} &
\tcode{<thread>} \\
\tcode{<atomic>} &
\tcode{<ios>} &
\tcode{<queue>} &
\tcode{<tuple>} \\
\tcode{<bitset>} &
\tcode{<iosfwd>} &
\tcode{<random>} &
\tcode{<type_traits>} \\
\tcode{<chrono>} &
\tcode{<iostream>} &
\tcode{<ratio>} &