16 Library introduction [library]

16.1 General [library.general]

This Clause describes the contents of the C++ standard library, how a well-formed C++ program makes use of the library, and how a conforming implementation may provide the entities in the library.

The following subclauses describe the method of description ([description]) and organization ([organization]) of the library.

[requirements], [support] through [thread], and [depr] specify the contents of the library, as well as library requirements and constraints on both well-formed C++ programs and conforming implementations.

Detailed specifications for each of the components in the library are in [support]–[thread], as shown in Table 20 .

Table 20: Library categories [tab:library.categories]

🔗	Clause	Category
🔗	[support]	Language support library
🔗	[concepts]	Concepts library
🔗	[diagnostics]	Diagnostics library
🔗	[utilities]	General utilities library
🔗	[strings]	Strings library
🔗	[containers]	Containers library
🔗	[iterators]	Iterators library
🔗	[ranges]	Ranges library
🔗	[algorithms]	Algorithms library
🔗	[numerics]	Numerics library
🔗	[time]	Time library
🔗	[localization]	Localization library
🔗	[input.output]	Input/output library
🔗	[re]	Regular expressions library
🔗	[atomics]	Atomic operations library
🔗	[thread]	Thread support library

The language support library provides components that are required by certain parts of the C++ language, such as memory allocation ([expr.new], [expr.delete]) and exception processing .

The concepts library ([concepts]) describes library components that C++ programs may use to perform compile-time validation of template arguments and perform function dispatch based on properties of types.

The diagnostics library ([diagnostics]) provides a consistent framework for reporting errors in a C++ program, including predefined exception classes.

The general utilities library includes components used by other library elements, such as a predefined storage allocator for dynamic storage management, and components used as infrastructure in C++ programs, such as tuples, function wrappers, and time facilities.

The strings library ([strings]) provides support for manipulating text represented as sequences of type char, sequences of type char8_t, sequences of type char16_t, sequences of type char32_t, sequences of type wchar_t, and sequences of any other character-like type.

The localization library provides extended internationalization support for text processing.

The containers ([containers]), iterators ([iterators]), ranges ([ranges]), and algorithms ([algorithms]) libraries provide a C++ program with access to a subset of the most widely used algorithms and data structures.

The numerics library provides numeric algorithms and complex number components that extend support for numeric processing.

The valarray component provides support for 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.

The input/output library provides the iostream components that are the primary mechanism for C++ program input and output.

They can be used with other elements of the library, particularly strings, locales, and iterators.

The regular expressions library provides regular expression matching and searching.

The atomic operations library allows more fine-grained concurrent access to shared data than is possible with locks.

The thread support library provides components to create and manage threads, including mutual exclusion and interthread communication.

16.2 The C standard library [library.c]

The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety.

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 an ISO C restrict qualifier) are the same unless otherwise stated.

16.3 Method of description [description]

16.3.1 General [description.general]

Subclause [description] describes the conventions used to specify the C++ standard library.

[structure] describes the structure of [support] through [thread] and [depr].

[conventions] describes other editorial conventions.

16.3.2 Structure of each clause [structure]

16.3.2.1 Elements [structure.elements]

Each library clause contains the following elements, as applicable:156

(1.1)
Summary
(1.2)
Requirements
(1.3)
Detailed specifications
(1.4)
References to the C standard library

156)

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.

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16.3.2.2 Summary [structure.summary]

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.

The contents of the summary and the detailed specifications include:

(2.1)
macros
(2.2)
values
(2.3)
types and alias templates
(2.4)
classes and class templates
(2.5)
functions and function templates
(2.6)
objects and variable templates
(2.7)
concepts

16.3.2.3 Requirements [structure.requirements]

Requirements describe constraints that shall be met by a C++ program that extends the standard library.

Such extensions are generally one of the following:

(1.1)
Template arguments
(1.2)
Derived classes
(1.3)
Containers, iterators, and algorithms that meet an interface convention or model a concept

The string and iostream components use an explicit representation of operations required of template arguments.

They use a class template char_traits to define these constraints.

Interface convention requirements are stated as generally as possible.

Instead of stating “class X has to define a member function operator++()”, the interface requires “for any object x of class X, ++x is defined”.

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

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 ([algorithms]) that uses the well-defined expression requirements is described in terms of the valid expressions for its template type parameters.

The library specification uses a typographical convention for naming requirements.

Names in italic type that begin with the prefix Cpp17 refer to sets of well-defined expression requirements typically presented in tabular form, possibly with additional prose semantic requirements.

For example, Cpp17Destructible (Table 32) is such a named requirement.

Names in constant width type refer to library concepts which are presented as a concept definition ([temp]), possibly with additional prose semantic requirements.

For example, destructible ([concept.destructible]) is such a named requirement.

Template argument requirements are sometimes referenced by name.

See [type.descriptions].

In some cases the semantic requirements are presented as C++ 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.157

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.

[Example 1:

The required < operator of the totally_ordered concept ([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.

A declaration may explicitly impose requirements through its associated constraints ([temp.constr.decl]).

When the associated constraints refer to a concept ([temp.concept]), the semantic constraints specified for that concept are additionally imposed on the use of the declaration.

157)

Although in some cases the code given is unambiguously the optimum implementation.

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16.3.2.4 Detailed specifications [structure.specifications]

The detailed specifications each contain the following elements:

(1.1)
name and brief description
(1.2)
synopsis (class definition or function declaration, as appropriate)
(1.3)
restrictions on template arguments, if any
(1.4)
description of class invariants
(1.5)
description of function semantics

Descriptions of class member functions follow the order (as appropriate):158

(2.1)
constructor(s) and destructor
(2.2)
copying, moving & assignment functions
(2.3)
comparison operator functions
(2.4)
modifier functions
(2.5)
observer functions
(2.6)
operators and other non-member functions

Descriptions of function semantics contain the following elements (as appropriate):159

(3.1)
Constraints: the conditions for the function's participation in overload resolution ([over.match]).

[Note 1:
Failure to meet such a condition results in the function's silent non-viability.
— end note]

[Example 1:
An implementation might express such a condition via a constraint-expression ([temp.constr.decl]).
— end example]
(3.2)
Mandates: the conditions that, if not met, render the program ill-formed.

[Example 2:
An implementation might express such a condition via the constant-expression in a static_assert-declaration ([dcl.pre]).

If the diagnostic is to be emitted only after the function has been selected by overload resolution, an implementation might express such a condition via a constraint-expression ([temp.constr.decl]) and also define the function as deleted.
— end example]
(3.3)
Preconditions: the conditions that the function assumes to hold whenever it is called; violation of any preconditions results in undefined behavior.
(3.4)
Effects: the actions performed by the function.
(3.5)
Synchronization: the synchronization operations ([intro.multithread]) applicable to the function.
(3.6)
Postconditions: the conditions (sometimes termed observable results) established by the function.
(3.7)
Returns: a description of the value(s) returned by the function.
(3.8)
Throws: any exceptions thrown by the function, and the conditions that would cause the exception.
(3.9)
Complexity: the time and/or space complexity of the function.
(3.10)
Remarks: additional semantic constraints on the function.
(3.11)
Error conditions: the error conditions for error codes reported by the function.

Whenever the Effects element specifies that the semantics of some function F are Equivalent to some code sequence, then the various elements are interpreted as follows.

If F's semantics specifies any Constraints or Mandates elements, then those requirements are logically imposed prior to the equivalent-to semantics.

Next, the semantics of the code sequence are determined by the Constraints, Mandates, Preconditions, Effects, Synchronization, Postconditions, Returns, Throws, Complexity, Remarks, and Error conditions specified for the function invocations contained in the code sequence.

The value returned from F is specified by F's Returns element, or if F has no Returns element, a non-void return from F is specified by the return statements ([stmt.return]) in the code sequence.

If F's semantics contains a Throws, Postconditions, or Complexity element, then that supersedes any occurrences of that element in the code sequence.

For non-reserved replacement and handler functions, [support] specifies two behaviors for the functions in question: their required and default behavior.

The default behavior describes a function definition provided by the implementation.

The required behavior describes the semantics of a function definition provided by either the implementation or a C++ program.

Where no distinction is explicitly made in the description, the behavior described is the required behavior.

If the formulation of a complexity requirement calls for a negative number of operations, the actual requirement is zero operations.160

Complexity requirements specified in the library clauses are upper bounds, and implementations that provide better complexity guarantees meet the requirements.

Error conditions specify conditions where a function may fail.

The conditions are listed, together with a suitable explanation, as the enum class errc constants ([syserr]).

158)

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.

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159)

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 Preconditions: element.

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160)

This simplifies the presentation of complexity requirements in some cases.

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16.3.2.5 C library [structure.see.also]

Paragraphs labeled “See also” contain cross-references to the relevant portions of other standards ([intro.refs]).

16.3.3 Other conventions [conventions]

16.3.3.1 General [conventions.general]

Subclause [conventions] describes several editorial conventions used to describe the contents of the C++ standard library.

These conventions are for describing implementation-defined types, and member functions .

16.3.3.2 Exposition-only functions [expos.only.func]

Several function templates defined in [support] through [thread] and [depr] are only defined for the purpose of exposition.

The declaration of such a function is followed by a comment ending in exposition only.

The following are defined for exposition only to aid in the specification of the library: template<class T> constexpr decay_t<T> decay-copy(T&& v) noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only { return std::forward<T>(v); } constexpr auto synth-three-way = []<class T, class U>(const T& t, const U& u) requires requires { { t boolean-testable; { u < t } -> boolean-testable; } { if constexpr (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 synth-three-way-result = decltype(synth-three-way(declval<T&>(), declval<U&>()));

16.3.3.3 Type descriptions [type.descriptions]

16.3.3.3.1 General [type.descriptions.general]

The Requirements subclauses may describe names that are used to specify constraints on template arguments.161

These names are used in library Clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.

Certain types defined in [input.output] are used to describe implementation-defined types.

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

161)

Examples from [utility.requirements] include: Cpp17EqualityComparable, Cpp17LessThanComparable, Cpp17CopyConstructible.

Examples from [iterator.requirements] include: Cpp17InputIterator, Cpp17ForwardIterator.

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16.3.3.3.2 Exposition-only types [expos.only.types]

Several types defined in [support] through [thread] and [depr] are defined for the purpose of exposition.

The declaration of such a type is followed by a comment ending in exposition only.

[Example 1: namespace std { extern "C" using some-handler = int(int, void*, double); // exposition only }

The type placeholder some-handler can now be used to specify a function that takes a callback parameter with C language linkage.

— end example]

16.3.3.3.3 Enumerated types [enumerated.types]

Several types defined in [input.output] are enumerated types.

Each enumerated type may be implemented as an enumeration or as a synonym for an enumeration.162

The enumerated type enumerated can be written: enum enumerated {

V_{0}

V_{1}

V_{2}

V_{3}

, … }; inline const

{enumerated C}_{0}

(

V_{0}

); inline const

{enumerated C}_{1}

(

V_{1}

); inline const

{enumerated C}_{2}

(

V_{2}

); inline const

{enumerated C}_{3}

(

V_{3}

); ⋮

Here, the names

C_{0}

C_{1}

, etc. represent enumerated elements for this particular enumerated type.

All such elements have distinct values.

162)

Such as an integer type, with constant integer values ([basic.fundamental]).

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16.3.3.3.4 Bitmask types [bitmask.types]

Several types defined in [support] through [thread] and [depr] are bitmask types.

Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset .

The bitmask type bitmask can be written: // For exposition only. // int_type is an integral type capable of representing all values of the bitmask type. enum bitmask : int_type {

V_{0}

= 1 << 0,

V_{1}

= 1 << 1,

V_{2}

= 1 << 2,

V_{3}

= 1 << 3, … }; inline constexpr

{bitmask C}_{0}

(

V_{0}

); inline constexpr

{bitmask C}_{1}

(

V_{1}

); inline constexpr

{bitmask C}_{2}

(

V_{2}

); inline constexpr

{bitmask C}_{3}

(

V_{3}

); ⋮ constexpr bitmask operator&(bitmask X, bitmask Y) { return static_cast<bitmask>( static_cast<int_type>(X) & static_cast<int_type>(Y)); } constexpr bitmask operator|(bitmask X, bitmask Y) { return static_cast<bitmask>( static_cast<int_type>(X) | static_cast<int_type>(Y)); } constexpr bitmask operator^(bitmask X, bitmask Y){ return static_cast<bitmask>( static_cast<int_type>(X) ^ static_cast<int_type>(Y)); } constexpr bitmask operator~(bitmask X){ return static_cast<bitmask>(~static_cast<int_type>(X)); } bitmask& operator&=(bitmask& X, bitmask Y){ X = X & Y; return X; } bitmask& operator|=(bitmask& X, bitmask Y) { X = X | Y; return X; } bitmask& operator^=(bitmask& X, bitmask Y) { X = X ^ Y; return X; }

Here, the names

C_{0}

C_{1}

, etc. represent bitmask elements for this particular bitmask type.

All such elements have distinct, nonzero values such that, for any pair

C_{i}

and

C_{j}

where

i \neq j

C_{i}

C_{i}

is nonzero and

C_{i}

C_{j}

is zero.

Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.

The following terms apply to objects and values of bitmask types:

(4.1)
To set a value Y in an object X is to evaluate the expression X |= Y.
(4.2)
To clear a value Y in an object X is to evaluate the expression X &= ~Y.
(4.3)
The value Y is set in the object X if the expression X & Y is nonzero.

16.3.3.3.5 Character sequences [character.seq]

16.3.3.3.5.1 General [character.seq.general]

The C standard library makes widespread use of characters and character sequences that follow a few uniform conventions:

(1.1)
A letter is any of the 26 lowercase or 26 uppercase letters in the basic execution character set.
(1.2)
The decimal-point character is the (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 [support] through [thread] and [depr] by a period, '.', which is also its value in the "C" locale, but may change during program execution by a call to setlocale(int, const char*),163 or by a change to a locale object, as described in [locales] and [input.output].
(1.3)
A character sequence is an array object A that can be declared as T A[N], where T is any of the types char, unsigned char, or signed char ([basic.fundamental]), optionally qualified by any combination of const or 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 S that points to its first element.

163)

declared in <clocale>.

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16.3.3.3.5.2 Byte strings [byte.strings]

A null-terminated byte string, or ntbs, is a character sequence whose highest-addressed element with defined content has the value zero (the terminating null character); no other element in the sequence has the value zero.164

The length of an ntbs is the number of elements that precede the terminating null character.

An empty ntbs has a length of zero.

The value of an ntbs is the sequence of values of the elements up to and including the terminating null character.

A static ntbs is an ntbs with static storage duration.165

164)

Many of the objects manipulated by function signatures declared in <cstring> are character sequences or ntbss.

The size of some of these character sequences is limited by a length value, maintained separately from the character sequence.

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165)

A string-literal, such as "abc", is a static ntbs.

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16.3.3.3.5.3 Multibyte strings [multibyte.strings]

A null-terminated multibyte string, or ntmbs, is an ntbs that constitutes a sequence of valid multibyte characters, beginning and ending in the initial shift state.166

A static ntmbs is an ntmbs with static storage duration.

166)

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.

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16.3.3.3.6 Customization Point Object types [customization.point.object]

A customization point object is a function object ([function.objects]) with a literal class type that interacts with program-defined types while enforcing semantic requirements on that interaction.

The type of a customization point object, ignoring cv-qualifiers, shall model semiregular ([concepts.object]).

All instances of a specific customization point object type shall be equal ([concepts.equality]).

The type T of a customization point object shall model invocable<const T&, Args...> ([concept.invocable]) when the types in Args... meet the requirements specified in that customization point object's definition.

When the types of Args... do not meet the customization point object's requirements, T shall not have a function call operator that participates in overload resolution.

Each customization point object type constrains its return type to model a particular concept.

[Note 1:

Many of the customization point objects in the library evaluate function call expressions with an unqualified name which results in a call to a program-defined function found by argument dependent name lookup ([basic.lookup.argdep]).

To preclude such an expression resulting in a call to unconstrained functions with the same name in namespace std, customization point objects specify that lookup for these expressions is performed in a context that includes deleted overloads matching the signatures of overloads defined in namespace std.

When the deleted overloads are viable, program-defined overloads need be more specialized ([temp.func.order]) or more constrained ([temp.constr.order]) to be used by a customization point object.

— end note]

16.3.3.4 Functions within classes [functions.within.classes]

For the sake of exposition, [support] through [thread] and [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 ([class.copy.ctor], [class.copy.assign], [class.dtor]).

It is unspecified whether the implementation provides explicit definitions for such member function signatures, or for virtual destructors that can be generated by default.

16.3.3.5 Private members [objects.within.classes]

[support] through [thread] and [depr] do not specify the representation of classes, and intentionally omit specification of class members .

An implementation may define static or non-static class members, or both, as needed to implement the semantics of the member functions specified in [support] through [thread] and [depr].

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 exposition only, as in: streambuf* sb; // exposition only

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

16.4 Library-wide requirements [requirements]

16.4.1 General [requirements.general]

Subclause [requirements] specifies requirements that apply to the entire C++ standard library.

[support] through [thread] and [depr] specify the requirements of individual entities within the library.

Requirements specified in terms of interactions between threads do not apply to programs having only a single thread of execution.

[organization] describes the library's contents and organization, [using] describes how well-formed C++ programs gain access to library entities, [utility.requirements] describes constraints on types and functions used with the C++ standard library, [constraints] describes constraints on well-formed C++ programs, and [conforming] describes constraints on conforming implementations.

16.4.2 Library contents and organization [organization]

16.4.2.1 General [organization.general]

[contents] describes the entities and macros defined in the C++ standard library.

[headers] lists the standard library headers and some constraints on those headers.

[compliance] lists requirements for a freestanding implementation of the C++ standard library.

16.4.2.2 Library contents [contents]

The C++ standard library provides definitions for the entities and macros described in the synopses of the C++ standard library headers ([headers]), unless otherwise specified.

All library entities except operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.167

It is unspecified whether names declared in a specific namespace are declared directly in that namespace or in an inline namespace inside that namespace.168

Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise.

For example, if the Effects: element for library function F is described as calling library function G, the function ::std::G is meant.

167)

The C standard library headers ([depr.c.headers]) also define names within the global namespace, while the C++ headers for C library facilities ([headers]) can also define names within the global namespace.

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168)

This gives implementers freedom to use inline namespaces to support multiple configurations of the library.

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16.4.2.3 Headers [headers]

Each element of the C++ standard library is declared or defined (as appropriate) in a .169

The C++ standard library provides the , shown in Table 21 .

Table 21: C++ library headers [tab:headers.cpp]

🔗	<algorithm>	<forward_list>	<numbers>	<string>
🔗	<any>	<fstream>	<numeric>	<string_view>
🔗	<array>	<functional>	<optional>	<strstream>
🔗	<atomic>	<future>	<ostream>	<syncstream>
🔗	<barrier>	<initializer_list>	<queue>	<system_error>
🔗	<bit>	<iomanip>	<random>	<thread>
🔗	<bitset>	<ios>	<ranges>	<tuple>
🔗	<charconv>	<iosfwd>	<ratio>	<typeindex>
🔗	<chrono>	<iostream>	<regex>	<typeinfo>
🔗	<codecvt>	<istream>	<scoped_allocator>	<type_traits>
🔗	<compare>	<iterator>	<semaphore>	<unordered_map>
🔗	<complex>	<latch>	<set>	<unordered_set>
🔗	<concepts>	<limits>	<shared_mutex>	<utility>
🔗	<condition_variable>	<list>	<source_location>	<valarray>
🔗	<coroutine>	<locale>	<span>	<variant>
🔗	<deque>	<map>	<sstream>	<vector>
🔗	<exception>	<memory>	<stack>	<version>
🔗	<execution>	<memory_resource>	<stdexcept>
🔗	<filesystem>	<mutex>	<stop_token>
🔗	<format>	<new>	<streambuf>

The facilities of the C standard library are provided in the additional headers shown in Table 22 .170

Table 22: C++ headers for C library facilities [tab:headers.cpp.c]

🔗	<cassert>	<cfenv>	<climits>	<csetjmp>	<cstddef>	<cstdlib>	<cuchar>
🔗	<cctype>	<cfloat>	<clocale>	<csignal>	<cstdint>	<cstring>	<cwchar>
🔗	<cerrno>	<cinttypes>	<cmath>	<cstdarg>	<cstdio>	<ctime>	<cwctype>

The headers listed in Table 21, or, for a freestanding implementation, the subset of such headers that are provided by the implementation, are collectively known as the importable C++ library headers.

[Note 1:

Importable C++ library headers can be imported as module units ([module.import]).

— end note]

[Example 1: import <vector>; // imports the <vector> header unit std::vector<int> vi; // OK — end example]

Except as noted in [library] through [thread] and [depr], the contents of each header cname is the same as that of the corresponding header name.h as specified in the C standard library .

In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope of the namespace std.

It is unspecified whether these names (including any overloads added in [support] through [thread] and [depr]) are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations ([namespace.udecl]).

Names which are defined as macros in C shall be defined as macros in the C++ standard library, even if C grants license for implementation as functions.

[Note 2:

The names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.

— end note]

Names that are defined as functions in C shall be defined as functions in the C++ standard library.171

Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers.172

[depr.c.headers], C standard library headers, describes the effects of using the name.h (C header) form in a C++ program.173

Annex K of the C standard describes a large number of functions, with associated types and macros, which “promote safer, more secure programming” than many of the traditional C library functions.

The names of the functions have a suffix of _s; most of them provide the same service as the C library function with the unsuffixed name, but generally take an additional argument whose value is the size of the result array.

If any C++ header is included, it is implementation-defined whether any of these names is declared in the global namespace.

(None of them is declared in namespace std.)

Table 23 lists the Annex K names that may be declared in some header.

These names are also subject to the restrictions of [macro.names].

Table 23: C standard Annex K names [tab:c.annex.k.names]

🔗	abort_handler_s	mbstowcs_s	strncat_s	vswscanf_s
🔗	asctime_s	memcpy_s	strncpy_s	vwprintf_s
🔗	bsearch_s	memmove_s	strtok_s	vwscanf_s
🔗	constraint_handler_t	memset_s	swprintf_s	wcrtomb_s
🔗	ctime_s	printf_s	swscanf_s	wcscat_s
🔗	errno_t	qsort_s	tmpfile_s	wcscpy_s
🔗	fopen_s	RSIZE_MAX	TMP_MAX_S	wcsncat_s
🔗	fprintf_s	rsize_t	tmpnam_s	wcsncpy_s
🔗	freopen_s	scanf_s	vfprintf_s	wcsnlen_s
🔗	fscanf_s	set_constraint_handler_s	vfscanf_s	wcsrtombs_s
🔗	fwprintf_s	snprintf_s	vfwprintf_s	wcstok_s
🔗	fwscanf_s	snwprintf_s	vfwscanf_s	wcstombs_s
🔗	getenv_s	sprintf_s	vprintf_s	wctomb_s
🔗	gets_s	sscanf_s	vscanf_s	wmemcpy_s
🔗	gmtime_s	strcat_s	vsnprintf_s	wmemmove_s
🔗	ignore_handler_s	strcpy_s	vsnwprintf_s	wprintf_s
🔗	localtime_s	strerrorlen_s	vsprintf_s	wscanf_s
🔗	L_tmpnam_s	strerror_s	vsscanf_s
🔗	mbsrtowcs_s	strlen_s	vswprintf_s

169)

A header is not necessarily a source file, nor are the sequences delimited by < and > in header names necessarily valid source file names ([cpp.include]).

⮥

170)

It is intentional that there is no C++ header for any of these C headers: <stdatomic.h>, <stdnoreturn.h>, <threads.h>.

⮥

171)

This disallows the practice, allowed in C, of providing a masking macro in addition to the function prototype.

The only way to achieve equivalent inline behavior in C++ is to provide a definition as an extern inline function.

⮥

172)

In particular, including the standard header <iso646.h> has no effect.

⮥

173)

The ".h" headers dump all their names into the global namespace, whereas the newer forms keep their names in namespace std.

Therefore, the newer forms are the preferred forms for all uses except for C++ programs which are intended to be strictly compatible with C.

⮥

16.4.2.4 Freestanding implementations [compliance]

Two kinds of implementations are defined: hosted and freestanding ([intro.compliance]); the kind of the implementation is implementation-defined.

For a hosted implementation, this document describes the set of available headers.

A freestanding implementation has an implementation-defined set of headers.

This set shall include at least the headers shown in Table 24 .

Table 24: C++ headers for freestanding implementations [tab:headers.cpp.fs]

🔗		Subclause	Header
🔗	[support.types]	Types	<cstddef>
🔗	[support.limits]	Implementation properties	<cfloat>, <climits>, <limits>, <version>
🔗	[cstdint]	Integer types	<cstdint>
🔗	[support.start.term]	Start and termination	<cstdlib>
🔗	[support.dynamic]	Dynamic memory management	<new>
🔗	[support.rtti]	Type identification	<typeinfo>
🔗	[support.srcloc]	Source location	<source_location>
🔗	[support.exception]	Exception handling	<exception>
🔗	[support.initlist]	Initializer lists	<initializer_list>
🔗	[cmp]	Comparisons	<compare>
🔗	[support.coroutine]	Coroutines support	<coroutine>
🔗	[support.runtime]	Other runtime support	<cstdarg>
🔗	[concepts]	Concepts library	<concepts>
🔗	[meta]	Type traits	<type_traits>
🔗	[bit]	Bit manipulation	<bit>
🔗	[atomics]	Atomics	<atomic>

The supplied version of the header <cstdlib> shall declare at least the functions abort, atexit, at_quick_exit, exit, and quick_exit ([support.start.term]).

The supplied version of the header <atomic> shall meet the same requirements as for a hosted implementation except that support for always lock-free integral atomic types ([atomics.lockfree]) is implementation-defined, and whether or not the type aliases atomic_signed_lock_free and atomic_unsigned_lock_free are defined ([atomics.alias]) is implementation-defined.

The other headers listed in this table shall meet the same requirements as for a hosted implementation.

16.4.3 Using the library [using]

16.4.3.1 Overview [using.overview]

Subclause [using] describes how a C++ program gains access to the facilities of the C++ standard library.

[using.headers] describes effects during translation phase 4, while [using.linkage] describes effects during phase 8 .

16.4.3.2 Headers [using.headers]

The entities in the C++ standard library are defined in headers, whose contents are made available to a translation unit when it contains the appropriate #include preprocessing directive ([cpp.include]) or the appropriate import declaration ([module.import]).

A translation unit may include library headers in any order ([lex.separate]).

Each may be included more than once, with no effect different from being included exactly once, except that the effect of including either <cassert> or <assert.h> depends each time on the lexically current definition of NDEBUG.174

A translation unit shall include a header only outside of any declaration or definition and, in the case of a module unit, only in its global-module-fragment, and shall include the header or import the corresponding header unit lexically before the first reference in that translation unit to any of the entities declared in that header.

No diagnostic is required.

174)

This is the same as the C standard library.

⮥

16.4.3.3 Linkage [using.linkage]

Entities in the C++ standard library have external linkage .

Unless otherwise specified, objects and functions have the default extern "C++" linkage ([dcl.link]).

Whether a name from the C standard library declared with external linkage has extern "C" or extern "C++" linkage is implementation-defined.

It is recommended that an implementation use extern "C++" linkage for this purpose.175

Objects and functions defined in the library and required by a C++ program are included in the program prior to program startup.

175)

The only reliable way to declare an object or function signature from the C standard library is by including the header that declares it, notwithstanding the latitude granted in 7.1.4 of the C Standard.

⮥

16.4.4 Requirements on types and expressions [utility.requirements]

16.4.4.1 General [utility.requirements.general]

[utility.arg.requirements] describes requirements on types and expressions used to instantiate templates defined in the C++ standard library.

[swappable.requirements] describes the requirements on swappable types and swappable expressions.

[nullablepointer.requirements] describes the requirements on pointer-like types that support null values.

[hash.requirements] describes the requirements on hash function objects.

[allocator.requirements] describes the requirements on storage allocators.

16.4.4.2 Template argument requirements [utility.arg.requirements]

The template definitions in the C++ standard library refer to various named requirements whose details are set out in Tables 25–32 .

In these tables, T is an object or reference type to be supplied by a C++ program instantiating a template; a, b, and c are values of type (possibly const) T; s and t are modifiable lvalues of type T; u denotes an identifier; rv is an rvalue of type T; and v is an lvalue of type (possibly const) T or an rvalue of type const T.

In general, a default constructor is not required.

Certain container class member function signatures specify T() as a default argument.

T() shall be a well-defined expression ([dcl.init]) if one of those signatures is called using the default argument .

Table 25: Cpp17EqualityComparable requirements [tab:cpp17.equalitycomparable]

🔗	Expression	Return type	Requirement
🔗	a == b	convertible to bool	== is an equivalence relation, that is, it has the following properties: For all a, a == a. If a == b, then b == a. If a == b and b == c, then a == c.

Table 26: Cpp17LessThanComparable requirements [tab:cpp17.lessthancomparable]

🔗	Expression	Return type	Requirement
🔗	a < b	convertible to bool	< is a strict weak ordering relation ([alg.sorting])

Table 27: Cpp17DefaultConstructible requirements [tab:cpp17.defaultconstructible]

🔗	Expression	Post-condition
🔗	T t;	object t is default-initialized
🔗	T u{};	object u is value-initialized or aggregate-initialized
🔗	T() T{}	an object of type T is value-initialized or aggregate-initialized

Table 28: Cpp17MoveConstructible requirements [tab:cpp17.moveconstructible]

🔗	Expression	Post-condition
🔗	T u = rv;	u is equivalent to the value of rv before the construction
🔗	T(rv)	T(rv) is equivalent to the value of rv before the construction
🔗	rv's state is unspecified [Note 1: rv must still meet the requirements of the library component that is using it. The operations listed in those requirements must work as specified whether rv has been moved from or not. — end note]

Table 29: Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible) [tab:cpp17.copyconstructible]

🔗	Expression	Post-condition
🔗	T u = v;	the value of v is unchanged and is equivalent to u
🔗	T(v)	the value of v is unchanged and is equivalent to T(v)

Table 30: Cpp17MoveAssignable requirements [tab:cpp17.moveassignable]

🔗	Expression	Return type	Return value	Post-condition
🔗	t = rv	T&	t	If t and rv do not refer to the same object, t is equivalent to the value of rv before the assignment
🔗	rv's state is unspecified. [Note 2: rv must still meet the requirements of the library component that is using it, whether or not t and rv refer to the same object. The operations listed in those requirements must work as specified whether rv has been moved from or not. — end note]

Table 31: Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable) [tab:cpp17.copyassignable]

🔗	Expression	Return type	Return value	Post-condition
🔗	t = v	T&	t	t is equivalent to v, the value of v is unchanged

Table 32: Cpp17Destructible requirements [tab:cpp17.destructible]

🔗	Expression	Post-condition
🔗	u.~T()	All resources owned by u are reclaimed, no exception is propagated.
🔗	[Note 3: Array types and non-object types are not Cpp17Destructible. — end note]

16.4.4.3 Swappable requirements [swappable.requirements]

This subclause provides definitions for swappable types and expressions.

In these definitions, let t denote an expression of type T, and let u denote an expression of type U.

An object t is swappable with an object u if and only if:

(2.1)
the expressions swap(t, u) and swap(u, t) are valid when evaluated in the context described below, and
(2.2)
these expressions have the following effects:
- (2.2.1)
  the object referred to by t has the value originally held by u and
- (2.2.2)
  the object referred to by u has the value originally held by t.

The context in which swap(t, u) and swap(u, t) are evaluated shall ensure that a binary non-member function named “swap” is selected via overload resolution on a candidate set that includes:

(3.1)
the two swap function templates defined in <utility> and
(3.2)
the lookup set produced by argument-dependent lookup .

[Note 1:

If T and U are both fundamental types or arrays of fundamental types and the declarations from the header <utility> are in scope, the overall lookup set described above is equivalent to that of the qualified name lookup applied to the expression std::swap(t, u) or std::swap(u, t) as appropriate.

— end note]

[Note 2:

It is unspecified whether a library component that has a swappable requirement includes the header <utility> to ensure an appropriate evaluation context.

— end note]

An rvalue or lvalue t is swappable if and only if t is swappable with any rvalue or lvalue, respectively, of type T.

A type X meeting any of the iterator requirements ([iterator.requirements]) meets the Cpp17ValueSwappable requirements if, for any dereferenceable object x of type X, *x is swappable.

[Example 1:

User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows: #include <utility> // Requires: std::forward<T>(t) shall be swappable with std::forward(u). template<class T, class U> void value_swap(T&& t, U&& u) { using std::swap; swap(std::forward<T>(t), std::forward(u)); // OK: uses “swappable with” conditions // for rvalues and lvalues } // Requires: lvalues of T shall be swappable. template<class T> void lv_swap(T& t1, T& t2) { using std::swap; swap(t1, t2); // OK: uses swappable conditions for lvalues of type T } namespace N { struct A { int m; }; struct Proxy { A* a; }; Proxy proxy(A& a) { return Proxy{ &a }; } void swap(A& x, Proxy p) { std::swap(x.m, p.a->m); // OK: uses context equivalent to swappable // conditions for fundamental types } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry constraint } int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }

— end example]

16.4.4.4 Cpp17NullablePointer requirements [nullablepointer.requirements]

A Cpp17NullablePointer type is a pointer-like type that supports null values.

A type P meets the Cpp17NullablePointer requirements if:

(1.1)
P meets the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements,
(1.2)
lvalues of type P are swappable,
(1.3)
the expressions shown in Table 33 are valid and have the indicated semantics, and
(1.4)
P meets all the other requirements of this subclause.

A value-initialized object of type P produces the null value of the type.

The null value shall be equivalent only to itself.

A default-initialized object of type P may have an indeterminate value.

[Note 1:

Operations involving indeterminate values might cause undefined behavior.

— end note]

An object p of type P can be contextually converted to bool .

The effect shall be as if p != nullptr had been evaluated in place of p.

No operation which is part of the Cpp17NullablePointer requirements shall exit via an exception.

In Table 33, u denotes an identifier, t denotes a non-const lvalue of type P, a and b denote values of type (possibly const) P, and np denotes a value of type (possibly const) std::nullptr_t.

Table 33: Cpp17NullablePointer requirements [tab:cpp17.nullablepointer]

🔗	Expression	Return type	Operational semantics
🔗	P u(np);		Postconditions: u == nullptr
🔗	P u = np;
🔗	P(np)		Postconditions: P(np) == nullptr
🔗	t = np	P&	Postconditions: t == nullptr
🔗	a != b	contextually convertible to bool	!(a == b)
🔗	a == np	contextually convertible to bool	a == P()
🔗	np == a
🔗	a != np	contextually convertible to bool	!(a == np)
🔗	np != a

16.4.4.5 Cpp17Hash requirements [hash.requirements]

A type H meets the Cpp17Hash requirements if:

(1.1)
it is a function object type ([function.objects]),
(1.2)
it meets the Cpp17CopyConstructible (Table 29) and Cpp17Destructible (Table 32) requirements, and
(1.3)
the expressions shown in Table 34 are valid and have the indicated semantics.

Given Key is an argument type for function objects of type H, in Table 34 h is a value of type (possibly const) H, u is an lvalue of type Key, and k is a value of a type convertible to (possibly const) Key.

Table 34: Cpp17Hash requirements [tab:cpp17.hash]

🔗	Expression	Return type	Requirement
🔗	h(k)	size_t	The value returned shall depend only on the argument k for the duration of the program. [Note 1: Thus all evaluations of the expression h(k) with the same value for k yield the same result for a given execution of the program. — end note] For two different values t1 and t2, the probability that h(t1) and h(t2) compare equal should be very small, approaching 1.0 / numeric_limits<size_t>::max().
🔗	h(u)	size_t	Shall not modify u.

16.4.4.6 Cpp17Allocator requirements [allocator.requirements]

16.4.4.6.1 General [allocator.requirements.general]

The library describes a standard set of requirements for allocators, which are class-type objects that encapsulate the information about an allocation model.

This information includes the knowledge of pointer types, the type of their difference, the type of the size of objects in this allocation model, as well as the memory allocation and deallocation primitives for it.

All of the string types, containers (except array), string buffers and string streams ([input.output]), and match_results are parameterized in terms of allocators.

Table 35: Descriptive variable definitions [tab:allocator.req.var]

🔗	Variable	Definition
🔗	T, U, C	any cv-unqualified object type ([basic.types])
🔗	X	an allocator class for type T
🔗	Y	the corresponding allocator class for type U
🔗	XX	the type allocator_traits<X>
🔗	YY	the type allocator_traits<Y>
🔗	a, a1, a2	lvalues of type X
🔗	u	the name of a variable being declared
🔗	b	a value of type Y
🔗	c	a pointer of type C* through which indirection is valid
🔗	p	a value of type XX::pointer, obtained by calling a1.allocate, where a1 == a
🔗	q	a value of type XX::const_pointer obtained by conversion from a value p
🔗	r	a value of type T& obtained by the expression *p
🔗	w	a value of type XX::void_pointer obtained by conversion from a value p
🔗	x	a value of type XX::const_void_pointer obtained by conversion from a value q or a value w
🔗	y	a value of type XX::const_void_pointer obtained by conversion from a result value of YY::allocate, or else a value of type (possibly const) std::nullptr_t
🔗	n	a value of type XX::size_type
🔗	Args	a template parameter pack
🔗	args	a function parameter pack with the pattern Args&&

The class template allocator_traits ([allocator.traits]) supplies a uniform interface to all allocator types.

Table 35 describes the types manipulated through allocators.

Table 36 describes the requirements on allocator types and thus on types used to instantiate allocator_traits.

A requirement is optional if the last column of Table 36 specifies a default for a given expression.

Within the standard library allocator_traits template, an optional requirement that is not supplied by an allocator is replaced by the specified default expression.

A user specialization of allocator_traits may provide different defaults and may provide defaults for different requirements than the primary template.

Within Tables 35 and 36, the use of move and forward always refers to std::move and std::forward, respectively.

Table 36: Cpp17Allocator requirements [tab:cpp17.allocator]

🔗	Expression	Return type	Assertion/note	Default
🔗			pre-/post-condition
🔗	X::pointer			T*
🔗	X::const_pointer		X::pointer is convertible to X::const_pointer	pointer_traits<X::pointer>::rebind<const T>
🔗	X::void_pointer Y::void_pointer		X::pointer is convertible to X::void_pointer. X::void_pointer and Y::void_pointer are the same type.	pointer_traits<X::pointer>::rebind<void>
🔗	X::const_void_pointer Y::const_void_pointer		X::pointer, X::const_pointer, and X::void_pointer are convertible to X::const_void_pointer. X::const_void_pointer and Y::const_void_pointer are the same type.	pointer_traits<X::pointer>::rebind<const void>
🔗	X::value_type	Identical to T
🔗	X::size_type	unsigned integer type	a type that can represent the size of the largest object in the allocation model	make_unsigned_t<X::difference_type>
🔗	X::difference_type	signed integer type	a type that can represent the difference between any two pointers in the allocation model	pointer_traits<X::pointer>::difference_type
🔗	typename X::template rebind<U>::other	Y	For all U (including T), Y::template rebind<T>::other is X.	See Note A, below.
🔗	*p	T&
🔗	*q	const T&	q refers to the same object as p.
🔗	p->m	type of T::m	Preconditions: (p).m is well-defined. equivalent to (p).m
🔗	q->m	type of T::m	Preconditions: (q).m is well-defined. equivalent to (q).m
🔗	static_cast<X::pointer>(w)	X::pointer	static_cast<X::pointer>(w) == p
🔗	static_cast<X::const_pointer>(x)	X::const_pointer	static_cast< X::const_pointer>(x) == q
🔗	pointer_traits<X::pointer>::pointer_to(r)	X::pointer	same as p
🔗	a.allocate(n)	X::pointer	Memory is allocated for an array of n T and such an object is created but array elements are not constructed. [Example 1: When reusing storage denoted by some pointer value p, launder(reinterpret_cast<T>(new (p) byte[n sizeof(T)])) can be used to implicitly create a suitable array object and obtain a pointer to it. — end example] allocate may throw an appropriate exception.176 [Note 1: If n == 0, the return value is unspecified. — end note]
🔗	a.allocate(n, y)	X::pointer	Same as a.allocate(n). The use of y is unspecified, but it is intended as an aid to locality.	a.allocate(n)
🔗	a.deallocate(p,n)	(not used)	Preconditions: p is a value returned by an earlier call to allocate that has not been invalidated by an intervening call to deallocate. n matches the value passed to allocate to obtain this memory. Throws: Nothing.
🔗	a.max_size()	X::size_type	the largest value that can meaningfully be passed to X::allocate()	numeric_limits<size_type>::max() / sizeof(value_type)
🔗	a1 == a2	bool	Returns true only if storage allocated from each can be deallocated via the other. operator== shall be reflexive, symmetric, and transitive, and shall not exit via an exception.
🔗	a1 != a2	bool	same as !(a1 == a2)
🔗	a == b	bool	same as a == Y::rebind<T>::other(b)
🔗	a != b	bool	same as !(a == b)
🔗	X u(a); X u = a;		Shall not exit via an exception. Postconditions: u == a
🔗	X u(b);		Shall not exit via an exception. Postconditions: Y(u) == b, u == X(b)
🔗	X u(std::move(a)); X u = std::move(a);		Shall not exit via an exception. Postconditions: The value of a is unchanged and is equal to u.
🔗	X u(std::move(b));		Shall not exit via an exception. Postconditions: u is equal to the prior value of X(b).
🔗	a.construct(c, args)	(not used)	Effects: Constructs an object of type C at c.	construct_at(c, std::forward<Args>(args)...)
🔗	a.destroy(c)	(not used)	Effects: Destroys the object at c	destroy_at(c)
🔗	a.select_on_container_copy_construction()	X	Typically returns either a or X().	return a;
🔗	X::propagate_on_container_copy_assignment	Identical to or derived from true_type or false_type	true_type only if an allocator of type X should be copied when the client container is copy-assigned. See Note B, below.	false_type
🔗	X::propagate_on_container_move_assignment	Identical to or derived from true_type or false_type	true_type only if an allocator of type X should be moved when the client container is move-assigned. See Note B, below.	false_type
🔗	X::propagate_on_- container_swap	Identical to or derived from true_type or false_type	true_type only if an allocator of type X should be swapped when the client container is swapped. See Note B, below.	false_type
🔗	X::is_always_equal	Identical to or derived from true_type or false_type	true_type only if the expression a1 == a2 is guaranteed to be true for any two (possibly const) values a1, a2 of type X.	is_empty<X>::type

Note A: The member class template rebind in the table above is effectively a typedef template.

[Note 2:

In general, if the name Allocator is bound to SomeAllocator<T>, then Allocator::rebind::other is the same type as SomeAllocator, where SomeAllocator<T>::value_type is T and SomeAllocator::value_type is U.

— end note]

If Allocator is a class template instantiation of the form SomeAllocator<T, Args>, where Args is zero or more type arguments, and Allocator does not supply a rebind member template, the standard allocator_traits template uses SomeAllocator<U, Args> in place of Allocator::rebind::other by default.

For allocator types that are not template instantiations of the above form, no default is provided.

Note B: If X::propagate_on_container_copy_assignment::value is true, X shall meet the Cpp17CopyAssignable requirements (Table 31) and the copy operation shall not throw exceptions.

If X::propagate_on_container_move_assignment::value is true, X shall meet the Cpp17MoveAssignable requirements (Table 30) and the move operation shall not throw exceptions.

If X::propagate_on_container_swap::value is true, lvalues of type X shall be swappable and the swap operation shall not throw exceptions.

An allocator type X shall meet the Cpp17CopyConstructible requirements (Table 29).

The X::pointer, X::const_pointer, X::void_pointer, and X::const_void_pointer types shall meet the Cpp17NullablePointer requirements (Table 33).

No constructor, comparison operator function, copy operation, move operation, or swap operation on these pointer types shall exit via an exception.

X::pointer and X::const_pointer shall also meet the requirements for a Cpp17RandomAccessIterator ([random.access.iterators]) and the additional requirement that, when a and (a + n) are dereferenceable pointer values for some integral value n, addressof(*(a + n)) == addressof(*a) + n is true.

Let x1 and x2 denote objects of (possibly different) types X::void_pointer, X::const_void_pointer, X::pointer, or X::const_pointer.

Then, x1 and x2 are equivalently-valued pointer values, if and only if both x1 and x2 can be explicitly converted to the two corresponding objects px1 and px2 of type X::const_pointer, using a sequence of static_casts using only these four types, and the expression px1 == px2 evaluates to true.

Let w1 and w2 denote objects of type X::void_pointer.

Then for the expressions w1 == w2 w1 != w2 either or both objects may be replaced by an equivalently-valued object of type X::const_void_pointer with no change in semantics.

Let p1 and p2 denote objects of type X::pointer.

Then for the expressions p1 == p2 p1 != p2 p1 < p2 p1 <= p2 p1 >= p2 p1 > p2 p1 - p2 either or both objects may be replaced by an equivalently-valued object of type X::const_pointer with no change in semantics.

An allocator may constrain the types on which it can be instantiated and the arguments for which its construct or destroy members may be called.

If a type cannot be used with a particular allocator, the allocator class or the call to construct or destroy may fail to instantiate.

If the alignment associated with a specific over-aligned type is not supported by an allocator, instantiation of the allocator for that type may fail.

The allocator also may silently ignore the requested alignment.

[Note 3:

Additionally, the member function allocate for that type can fail by throwing an object of type bad_alloc.

— end note]

[Example 2:

The following is an allocator class template supporting the minimal interface that meets the requirements of Table 36: template<class Tp> struct SimpleAllocator { typedef Tp value_type; SimpleAllocator(ctor args); template<class T> SimpleAllocator(const SimpleAllocator<T>& other); [[nodiscard]] Tp* allocate(std::size_t n); void deallocate(Tp* p, std::size_t n); }; template<class T, class U> bool operator==(const SimpleAllocator<T>&, const SimpleAllocator&); template<class T, class U> bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator&);

— end example]

176)

It is intended that a.allocate be an efficient means of allocating a single object of type T, even when sizeof(T) is small.

That is, there is no need for a container to maintain its own free list.

⮥

16.4.4.6.2 Allocator completeness requirements [allocator.requirements.completeness]

If X is an allocator class for type T, X additionally meets the allocator completeness requirements if, whether or not T is a complete type:

(1.1)
X is a complete type, and
(1.2)
all the member types of allocator_traits<X> other than value_type are complete types.

16.4.5 Constraints on programs [constraints]

16.4.5.1 Overview [constraints.overview]

Subclause [constraints] describes restrictions on C++ programs that use the facilities of the C++ standard library.

The following subclauses specify constraints on the program's use of namespaces, its use of various reserved names, its use of headers, its use of standard library classes as base classes ([derived.classes]), its definitions of replacement functions, and its installation of handler functions during execution.

16.4.5.2 Namespace use [namespace.constraints]

16.4.5.2.1 Namespace std [namespace.std]

Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.

Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.177

The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

The behavior of a C++ program is undefined if it declares

(4.1)
an explicit specialization of any member function of a standard library class template, or
(4.2)
an explicit specialization of any member function template of a standard library class or class template, or
(4.3)
an explicit or partial specialization of any member class template of a standard library class or class template, or
(4.4)
a deduction guide for any standard library class template.

A program may explicitly instantiate a class template defined in the standard library only if the declaration (a) depends on the name of at least one program-defined type and (b) the instantiation meets the standard library requirements for the original template.

Let F denote a standard library function ([global.functions]), a standard library static member function, or an instantiation of a standard library function template.

Unless F is designated an addressable function, the behavior of a C++ program is unspecified (possibly ill-formed) if it explicitly or implicitly attempts to form a pointer to F.

[Note 1:

Possible means of forming such pointers include application of the unary & operator ([expr.unary.op]), addressof ([specialized.addressof]), or a function-to-pointer standard conversion ([conv.func]).

— end note]

Moreover, the behavior of a C++ program is unspecified (possibly ill-formed) if it attempts to form a reference to F or if it attempts to form a pointer-to-member designating either a standard library non-static member function ([member.functions]) or an instantiation of a standard library member function template.

Other than in namespace std or in a namespace within namespace std, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point.178

[Note 2:

This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments.

— end note]

A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).

177)

Any library code that instantiates other library templates must be prepared to work adequately with any user-supplied specialization that meets the minimum requirements of this document.

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178)

Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document.

Therefore an implementation can elect, under the as-if rule ([intro.execution]), to provide any customization point in the form of an instantiated function object ([function.objects]) even though the customization point's specification is in the form of a function template.

The template parameters of each such function object and the function parameters and return type of the object's operator() must match those of the corresponding customization point's specification.

⮥

16.4.5.2.2 Namespace posix [namespace.posix]

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified.

The namespace posix is reserved for use by ISO/IEC 9945 and other POSIX standards.

16.4.5.2.3 Namespaces for future standardization [namespace.future]

Top-level namespaces whose namespace-name consists of std followed by one or more digits ([lex.name]) are reserved for future standardization.

The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace.

[Example 1:

The top-level namespace std2 is reserved for use by future revisions of this International Standard.

— end example]

16.4.5.3 Reserved names [reserved.names]

16.4.5.3.1 General [reserved.names.general]

The C++ standard library reserves the following kinds of names:

(1.1)
macros
(1.2)
global names
(1.3)
names with external linkage

If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by [library], its behavior is undefined.

16.4.5.3.2 Zombie names [zombie.names]

In namespace std, the following names are reserved for previous standardization:

(1.1)
auto_ptr,
(1.2)
auto_ptr_ref,
(1.3)
binary_function,
(1.4)
binary_negate,
(1.5)
bind1st,
(1.6)
bind2nd,
(1.7)
binder1st,
(1.8)
binder2nd,
(1.9)
const_mem_fun1_ref_t,
(1.10)
const_mem_fun1_t,
(1.11)
const_mem_fun_ref_t,
(1.12)
const_mem_fun_t,
(1.13)
get_temporary_buffer,
(1.14)
get_unexpected,
(1.15)
gets,
(1.16)
is_literal_type,
(1.17)
is_literal_type_v,
(1.18)
mem_fun1_ref_t,
(1.19)
mem_fun1_t,
(1.20)
mem_fun_ref_t,
(1.21)
mem_fun_ref,
(1.22)
mem_fun_t,
(1.23)
mem_fun,
(1.24)
not1,
(1.25)
not2,
(1.26)
pointer_to_binary_function,
(1.27)
pointer_to_unary_function,
(1.28)
ptr_fun,
(1.29)
random_shuffle,
(1.30)
raw_storage_iterator,
(1.31)
result_of,
(1.32)
result_of_t,
(1.33)
return_temporary_buffer,
(1.34)
set_unexpected,
(1.35)
unary_function,
(1.36)
unary_negate,
(1.37)
uncaught_exception,
(1.38)
unexpected, and
(1.39)
unexpected_handler.

The following names are reserved as member types for previous standardization, and may not be used as a name for object-like macros in portable code:

(2.1)
argument_type,
(2.2)
first_argument_type,
(2.3)
io_state,
(2.4)
open_mode,
(2.5)
second_argument_type, and
(2.6)
seek_dir.

The name stossc is reserved as a member function for previous standardization, and may not be used as a name for function-like macros in portable code.

The header names <ccomplex>, <ciso646>, <cstdalign>, <cstdbool>, and <ctgmath> are reserved for previous standardization.

16.4.5.3.3 Macro names [macro.names]

A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.

A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in [dcl.attr], except that the names likely and unlikely may be defined as function-like macros ([cpp.replace]).

16.4.5.3.4 External linkage [extern.names]

Each name declared as an object with external linkage in a header is reserved to the implementation to designate that library object with external linkage,179 both in namespace std and in the global namespace.

Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.180

Each name from the C standard library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.

Each function signature from the C standard library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage,181 or as a name of namespace scope in the global namespace.

179)

The list of such reserved names includes errno, declared or defined in <cerrno>.

⮥

180)

The list of such reserved function signatures with external linkage includes setjmp(jmp_buf), declared or defined in <csetjmp>, and va_end(va_list), declared or defined in <cstdarg>.

⮥

181)

The function signatures declared in <cuchar>, <cwchar>, and <cwctype> are always reserved, notwithstanding the restrictions imposed in subclause 4.5.1 of Amendment 1 to the C Standard for these headers.

⮥

16.4.5.3.5 Types [extern.types]

For each type T from the C standard library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.

16.4.5.3.6 User-defined literal suffixes [usrlit.suffix]

Literal suffix identifiers that do not start with an underscore are reserved for future standardization.

16.4.5.4 Headers [alt.headers]

If a file with a name equivalent to the derived file name for one of the C++ standard library headers is not provided as part of the implementation, and a file with that name is placed in any of the standard places for a source file to be included, the behavior is undefined.

16.4.5.5 Derived classes [derived.classes]

Virtual member function signatures defined for a base class in the C++ standard library may be overridden in a derived class defined in the program ([class.virtual]).

16.4.5.6 Replacement functions [replacement.functions]

[support] through [thread] and [depr] describe the behavior of numerous functions defined by the C++ standard library.

Under some circumstances, however, certain of these function descriptions also apply to replacement functions defined in the program.

A C++ program may provide the definition for any of the following dynamic memory allocation function signatures declared in header <new> ([basic.stc.dynamic], [new.syn]): operator new(std::size_t) operator new(std::size_t, std::align_val_t) operator new(std::size_t, const std::nothrow_t&) operator new(std::size_t, std::align_val_t, const std::nothrow_t&) operator delete(void*) operator delete(void*, std::size_t) operator delete(void*, std::align_val_t) operator delete(void*, std::size_t, std::align_val_t) operator delete(void*, const std::nothrow_t&) operator delete(void*, std::align_val_t, const std::nothrow_t&) operator new[](std::size_t) operator new[](std::size_t, std::align_val_t) operator new[](std::size_t, const std::nothrow_t&) operator new[](std::size_t, std::align_val_t, const std::nothrow_t&) operator delete[](void*) operator delete[](void*, std::size_t) operator delete[](void*, std::align_val_t) operator delete[](void*, std::size_t, std::align_val_t) operator delete[](void*, const std::nothrow_t&) operator delete[](void*, std::align_val_t, const std::nothrow_t&)

The program's definitions are used instead of the default versions supplied by the implementation ([new.delete]).

Such replacement occurs prior to program startup ([basic.def.odr], [basic.start]).

The program's declarations shall not be specified as inline.

No diagnostic is required.

16.4.5.7 Handler functions [handler.functions]

The C++ standard library provides a default version of the following handler function ([support]):

(1.1)
terminate_handler

A C++ program may install different handler functions during execution, by supplying a pointer to a function defined in the program or the library as an argument to (respectively):

(2.1)
set_new_handler
(2.2)
set_terminate

See also subclauses [alloc.errors], Storage allocation errors, and [support.exception], Exception handling.

A C++ program can get a pointer to the current handler function by calling the following functions:

(3.1)
get_new_handler
(3.2)
get_terminate

Calling the set_* and get_* functions shall not incur a data race.

A call to any of the set_* functions shall synchronize with subsequent calls to the same set_* function and to the corresponding get_* function.

16.4.5.8 Other functions [res.on.functions]

In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program.

If these components do not meet their requirements, this document places no requirements on the implementation.

In particular, the effects are undefined in the following cases:

(2.1)
For replacement functions ([new.delete]), if the installed replacement function does not implement the semantics of the applicable Required behavior: paragraph.
(2.2)
For handler functions ([new.handler], [terminate.handler]), if the installed handler function does not implement the semantics of the applicable Required behavior: paragraph.
(2.3)
For types used as template arguments when instantiating a template component, if the operations on the type do not implement the semantics of the applicable Requirements subclause ([allocator.requirements], [container.requirements], [iterator.requirements], [algorithms.requirements], [numeric.requirements]).

Operations on such types can report a failure by throwing an exception unless otherwise specified.
(2.4)
If any replacement function or handler function or destructor operation exits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.
(2.5)
If an incomplete type ([basic.types]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.

16.4.5.9 Function arguments [res.on.arguments]

Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

(1.1)
If an argument to a function has an invalid value (such as a value outside the domain of the function or a pointer invalid for its intended use), the behavior is undefined.
(1.2)
If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
(1.3)
If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument.

[Note 1:
If the parameter is a generic parameter of the form T&& and an lvalue of type A is bound, the argument binds to an lvalue reference ([temp.deduct.call]) and thus is not covered by the previous sentence.
— end note]

[Note 2:
If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g., by calling the function with the argument std::move(x)), the program is effectively asking that function to treat that lvalue as a temporary object.

The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue.
— end note]

16.4.5.10 Library object access [res.on.objects]

The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race.

The conditions under which this may occur are specified in [res.on.data.races].

[Note 1:

Modifying an object of a standard library type that is shared between threads risks undefined behavior unless objects of that type are explicitly specified as being shareable without data races or the user supplies a locking mechanism.

— end note]

If an object of a standard library type is accessed, and the beginning of the object's lifetime does not happen before the access, or the access does not happen before the end of the object's lifetime, the behavior is undefined unless otherwise specified.

[Note 2:

This applies even to objects such as mutexes intended for thread synchronization.

— end note]

16.4.5.11 Semantic requirements [res.on.requirements]

A sequence Args of template arguments is said to model a concept C if Args satisfies C ([temp.constr.decl]) and meets all semantic requirements (if any) given in the specification of C.

If the validity or meaning of a program depends on whether a sequence of template arguments models a concept, and the concept is satisfied but not modeled, the program is ill-formed, no diagnostic required.

If the semantic requirements of a declaration's constraints ([structure.requirements]) are not modeled at the point of use, the program is ill-formed, no diagnostic required.

16.4.6 Conforming implementations [conforming]

16.4.6.1 Overview [conforming.overview]

Subclause [conforming] describes the constraints upon, and latitude of, implementations of the C++ standard library.

An implementation's use of headers is discussed in [res.on.headers], its use of macros in [res.on.macro.definitions], non-member functions in [global.functions], member functions in [member.functions], data race avoidance in [res.on.data.races], access specifiers in [protection.within.classes], class derivation in [derivation], and exceptions in [res.on.exception.handling].

16.4.6.2 Headers [res.on.headers]

A C++ header may include other C++ headers.

A C++ header shall provide the declarations and definitions that appear in its synopsis.

A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.

Certain types and macros are defined in more than one header.

Every such entity shall be defined such that any header that defines it may be included after any other header that also defines it ([basic.def.odr]).

The C standard library headers shall include only their corresponding C++ standard library header, as described in [headers].

16.4.6.3 Restrictions on macro definitions [res.on.macro.definitions]

The names and global function signatures described in [contents] are reserved to the implementation.

All object-like macros defined by the C standard library and described in this Clause as expanding to integral constant expressions are also suitable for use in #if preprocessing directives, unless explicitly stated otherwise.

16.4.6.4 Non-member functions [global.functions]

It is unspecified whether any non-member functions in the C++ standard library are defined as inline .

A call to a non-member function signature described in [support] through [thread] and [depr] shall behave as if the implementation declared no additional non-member function signatures.182

An implementation shall not declare a non-member function signature with additional default arguments.

Unless otherwise specified, calls made by functions in the standard library to non-operator, non-member functions do not use functions from another namespace which are found through argument-dependent name lookup ([basic.lookup.argdep]).

[Note 1:

The phrase “unless otherwise specified” applies to cases such as the swappable with requirements ([swappable.requirements]).

The exception for overloaded operators allows argument-dependent lookup in cases like that of ostream_iterator::operator=:

Effects: *out_stream << value; if (delim != 0) *out_stream << delim; return *this;

— end note]

182)

A valid C++ program always calls the expected library non-member function.

An implementation can also define additional non-member functions that would otherwise not be called by a valid C++ program.

⮥

16.4.6.5 Member functions [member.functions]

It is unspecified whether any member functions in the C++ standard library are defined as inline .

For a non-virtual member function described in the C++ standard library, an implementation may declare a different set of member function signatures, provided that any call to the member function that would select an overload from the set of declarations described in this document behaves as if that overload were selected.

[Note 1:

For instance, an implementation can add parameters with default values, or replace a member function with default arguments with two or more member functions with equivalent behavior, or add additional signatures for a member function name.

— end note]

16.4.6.6 Friend functions [hidden.friends]

Whenever this document specifies a friend declaration of a function or function template within a class or class template definition, that declaration shall be the only declaration of that function or function template provided by an implementation.

[Note 1:

In particular, an implementation is not allowed to provide an additional declaration of that function or function template at namespace scope.

— end note]

[Note 2:

Such a friend function or function template declaration is known as a hidden friend, as it is visible neither to ordinary unqualified lookup ([basic.lookup.unqual]) nor to qualified lookup ([basic.lookup.qual]).

— end note]

16.4.6.7 Constexpr functions and constructors [constexpr.functions]

This document explicitly requires that certain standard library functions are constexpr ([dcl.constexpr]).

An implementation shall not declare any standard library function signature as constexpr except for those where it is explicitly required.

Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.

16.4.6.8 Requirements for stable algorithms [algorithm.stable]

When the requirements for an algorithm state that it is “stable” without further elaboration, it means:

(1.1)
For the sort algorithms the relative order of equivalent elements is preserved.
(1.2)
For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
(1.3)
For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

16.4.6.9 Reentrancy [reentrancy]

Except where explicitly specified in this document, it is implementation-defined which functions in the C++ standard library may be recursively reentered.

16.4.6.10 Data race avoidance [res.on.data.races]

This subclause specifies requirements that implementations shall meet to prevent data races .

Every standard library function shall meet each requirement unless otherwise specified.

Implementations may prevent data races in cases other than those specified below.

A C++ standard library function shall not directly or indirectly access objects ([intro.multithread]) accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's arguments, including this.

A C++ standard library function shall not directly or indirectly modify objects ([intro.multithread]) accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments, including this.

[Note 1:

This means, for example, that implementations can't use an object with static storage duration for internal purposes without synchronization because it could cause a data race even in programs that do not explicitly share objects between threads.

— end note]

A C++ standard library function shall not access objects indirectly accessible via its arguments or via elements of its container arguments except by invoking functions required by its specification on those container elements.

Operations on iterators obtained by calling a standard library container or string member function may access the underlying container, but shall not modify it.

[Note 2:

In particular, container operations that invalidate iterators conflict with operations on iterators associated with that container.

— end note]

Implementations may share their own internal objects between threads if the objects are not visible to users and are protected against data races.

Unless otherwise specified, C++ standard library functions shall perform all operations solely within the current thread if those operations have effects that are visible to users.

[Note 3:

This allows implementations to parallelize operations if there are no visible side effects.

— end note]

16.4.6.11 Protection within classes [protection.within.classes]

It is unspecified whether any function signature or class described in [support] through [thread] and [depr] is a friend of another class in the C++ standard library.

16.4.6.12 Derived classes [derivation]

An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.

Certain classes defined in the C++ standard library are required to be derived from other classes in the C++ standard library.

An implementation may derive such a class directly from the required base or indirectly through a hierarchy of base classes with names reserved to the implementation.

In any case:

(3.1)
Every base class described as virtual shall be virtual;
(3.2)
Every base class not specified as virtual shall not be virtual;
(3.3)
Unless explicitly stated otherwise, types with distinct names shall be distinct types.183

All types specified in the C++ standard library shall be non-final types unless otherwise specified.

183)

There is an implicit exception to this rule for types that are described as synonyms for basic integral types, such as size_t ([support.types]) and streamoff ([stream.types]).

⮥

16.4.6.13 Restrictions on exception handling [res.on.exception.handling]

Any of the functions defined in the C++ standard library can report a failure by throwing an exception of a type described in its Throws: paragraph, or of a type derived from a type named in the Throws: paragraph that would be caught by an exception handler for the base type.

Functions from the C standard library shall not throw exceptions 184 except when such a function calls a program-supplied function that throws an exception.185

Destructor operations defined in the C++ standard library shall not throw exceptions.

Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification.

Functions defined in the C++ standard library that do not have a Throws: paragraph but do have a potentially-throwing exception specification may throw implementation-defined exceptions.186

Implementations should report errors by throwing exceptions of or derived from the standard exception classes ([bad.alloc], [support.exception], [std.exceptions]).

An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.

184)

That is, the C library functions can all be treated as if they are marked noexcept.

This allows implementations to make performance optimizations based on the absence of exceptions at runtime.

⮥

185)

The functions qsort() and bsearch() ([alg.c.library]) meet this condition.

⮥

186)

In particular, they can report a failure to allocate storage by throwing an exception of type bad_alloc, or a class derived from bad_alloc ([bad.alloc]).

⮥

16.4.6.14 Restrictions on storage of pointers [res.on.pointer.storage]

Objects constructed by the standard library that may hold a user-supplied pointer value or an integer of type std::intptr_t shall store such values in a traceable pointer location ([basic.stc.dynamic.safety]).

16.4.6.15 Value of error codes [value.error.codes]

Certain functions in the C++ standard library report errors via a std::error_code object.

That object's category() member shall return std::system_category() for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere.

The implementation shall define the possible values of value() for each of these error categories.

[Example 1:

For operating systems that are based on POSIX, implementations should define the std::system_category() values as identical to the POSIX errno values, with additional values as defined by the operating system's documentation.

Implementations for operating systems that are not based on POSIX should define values identical to the operating system's values.

For errors that do not originate from the operating system, the implementation may provide enums for the associated values.

— end example]

16.4.6.16 Moved-from state of library types [lib.types.movedfrom]

Objects of types defined in the C++ standard library may be moved from ([class.copy.ctor]).

Move operations may be explicitly specified or implicitly generated.

Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.