15 Library introduction [library]

15.1 General [library.general]

1
#
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.
2
#
The following subclauses describe the definitions, method of description, and organization of the library.
[requirements], [language.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.
3
#
Detailed specifications for each of the components in the library are in [language.support]–[thread], as shown in Table 17.
Table 17 — Library categories
Clause
Category
Language support library
Concepts library
Diagnostics library
General utilities library
Strings library
Localization library
Containers library
Iterators library
Algorithms library
Numerics library
Input/output library
Regular expressions library
Atomic operations library
Thread support library
4
#
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.
5
#
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.
6
#
The diagnostics library ([diagnostics]) provides a consistent framework for reporting errors in a C++ program, including predefined exception classes.
7
#
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.
8
#
The strings library provides support for manipulating text represented as sequences of type char, sequences of type char16_­t, sequences of type char32_­t, sequences of type wchar_­t, and sequences of any other character-like type.
9
#
The localization library provides extended internationalization support for text processing.
10
#
The containers, iterators, and algorithms libraries provide a C++ program with access to a subset of the most widely used algorithms and data structures.
11
#
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.
12
#
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.
13
#
The regular expressions library provides regular expression matching and searching.
14
#
The atomic operations library allows more fine-grained concurrent access to shared data than is possible with locks.
15
#
The thread support library provides components to create and manage threads, including mutual exclusion and interthread communication.

15.2 The C standard library [library.c]

1
#
The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety.
2
#
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.

15.3 Definitions [definitions]

1
#
[ Note
:
[intro.defs] defines additional terms used elsewhere in this document.
— end note
 ]

15.3.1 arbitrary-positional stream [defns.arbitrary.stream]

stream (described in [input.output]) that can seek to any integral position within the length of the stream
[ Note
:
Every arbitrary-positional stream is also a repositional stream.
— end note
 ]

15.3.2 character [defns.character]

⟨[strings], [localization], [input.output], and [re]⟩ object which, when treated sequentially, can represent text
[ Note
:
The term does not mean only char, char16_­t, char32_­t, and wchar_­t objects, but any value that can be represented by a type that provides the definitions specified in these Clauses.
— end note
 ]

15.3.3 character container type [defns.character.container]

class or a type used to represent a character
[ Note
:
It is used for one of the template parameters of the string, iostream, and regular expression class templates.
— end note
 ]

15.3.4 comparison function [defns.comparison]

operator function for any of the equality or relational operators

15.3.5 component [defns.component]

group of library entities directly related as members, parameters, or return types
[ Note
:
For example, the class template basic_­string and the non-member function templates that operate on strings are referred to as the string component.
— end note
 ]

15.3.6 constant subexpression [defns.const.subexpr]

expression whose evaluation as subexpression of a conditional-expression CE ([expr.cond]) would not prevent CE from being a core constant expression

15.3.7 deadlock [defns.deadlock]

situation wherein 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

15.3.8 default behavior [defns.default.behavior.impl]

⟨implementation⟩ specific behavior provided by the implementation, within the scope of the required behavior

15.3.9 default behavior [defns.default.behavior.func]

⟨specification⟩ description of replacement function and handler function semantics

15.3.11 expression-equivalent [defns.expression-equivalent]

expressions that all have the same effects, either are all potentially-throwing ([except.spec]) or are all not potentially-throwing, and either are all constant subexpressions or are all not constant subexpressions
[ Example
:
For a value x of type int and a function f that accepts integer arguments, the expressions f(x + 2), f(2 + x), and f(1 + x + 1) are expression-equivalent.
— end example
 ]

15.3.12 handler function [defns.handler]

non-reserved function whose definition may be provided by a C++ program
[ Note
:
A C++ 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 ([language.support]).
— end note
 ]

15.3.13 iostream class templates [defns.iostream.templates]

templates, defined in [input.output], that take two template arguments
[ Note
:
The arguments are named charT and traits.
The argument charT is a character container class, and the argument traits is a class which defines additional characteristics and functions of the character type represented by charT necessary to implement the iostream class templates.
— end note
 ]

15.3.14 modifier function [defns.modifier]

class member function other than a constructor, assignment operator, or destructor that alters the state of an object of the class

15.3.15 move assignment [defns.move.assign]

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

15.3.16 move construction [defns.move.constr]

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

15.3.17 NTCTS [defns.ntcts]

sequence of values that have character type that precede the terminating null character type value charT()

15.3.18 observer function [defns.observer]

class member function that accesses the state of an object of the class but does not alter that state
[ Note
:
Observer functions are specified as const member functions ([class.this]).
— end note
 ]

15.3.19 program-defined specialization [defns.prog.def.spec]

explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation

15.3.20 program-defined type [defns.prog.def.type]

class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization
[ Note
:
Types defined by the implementation include extensions ([intro.compliance]) and internal types used by the library.
— end note
 ]

15.3.21 referenceable type [defns.referenceable]

type that is either an object type, a function type that does not have cv-qualifiers or a ref-qualifier, or a reference type
[ Note
:
The term describes a type to which a reference can be created, including reference types.
— end note
 ]

15.3.22 replacement function [defns.replacement]

non-reserved function whose definition is provided by a C++ program
[ 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 ([lex.phases]) and resolving the definitions of all translation units ([basic.link]).
— end note
 ]

15.3.23 repositional stream [defns.repositional.stream]

stream (described in [input.output]) that can seek to a position that was previously encountered

15.3.24 required behavior [defns.required.behavior]

description of replacement function and handler function semantics applicable to both the behavior provided by the implementation and the behavior of any such function definition in the program
[ Note
:
If such a function defined in a C++ program fails to meet the required behavior when it executes, the behavior is undefined.
— end note
 ]

15.3.25 reserved function [defns.reserved.function]

function, specified as part of the C++ standard library, that is defined by the implementation
[ Note
:
If a C++ program provides a definition for any reserved function, the results are undefined.
— end note
 ]

15.3.26 stable algorithm [defns.stable]

algorithm that preserves, as appropriate to the particular algorithm, the order of elements
[ Note
:
Requirements for stable algorithms are given in [algorithm.stable].
— end note
 ]

15.3.27 traits class [defns.traits]

class that encapsulates a set of types and functions necessary for class templates and function templates to manipulate objects of types for which they are instantiated

15.3.28 valid but unspecified state [defns.valid]

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
[ Example
:
If an object x of type std​::​vector<int> is in a valid but unspecified state, x.empty() can be called unconditionally, and x.front() can be called only if x.empty() returns false.
— end example
 ]

15.4 Method of description (Informative) [description]

1
#
This subclause describes the conventions used to specify the C++ standard library.
[structure] describes the structure of the normative [language.support] through [thread] and [depr].
[conventions] describes other editorial conventions.

15.4.1 Structure of each clause [structure]

15.4.1.1 Elements [structure.elements]

1
#
Each library clause contains the following elements, as applicable:156
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.

15.4.1.2 Summary [structure.summary]

1
#
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.
2
#
The contents of the summary and the detailed specifications include:

15.4.1.3 Requirements [structure.requirements]

1
#
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 satisfy a concept
2
#
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.
3
#
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.
4
#
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 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.
5
#
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 29) 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.
6
#
Template argument requirements are sometimes referenced by name.
7
#
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
8
#
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 satisfied.
[ Example
:
The required < operator of the StrictTotallyOrdered concept ([concept.stricttotallyordered]) does not meet the semantic requirements of that concept when operating on NaNs.
— end example
 ]
This does not affect whether a type satisfies the concept.
9
#
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.

15.4.1.4 Detailed specifications [structure.specifications]

1
#
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
2
#
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 functions
  • (2.4)
    modifier functions
  • (2.5)
    observer functions
  • (2.6)
    operators and other non-member functions
3
#
Descriptions of function semantics contain the following elements (as appropriate):159
  • (3.1)
    Requires: the preconditions for calling the function.
  • (3.2)
    Constraints: the conditions for the function's participation in overload resolution ([over.match]).
    [ Note
    :
    Failure to meet such a condition results in the function's silent non-viability.
    — end note
     ]
    [ Example
    :
    An implementation might express such a condition via a constraint-expression.
    — end example
     ]
  • (3.3)
    Mandates: the conditions that, if not met, render the program ill-formed.
    [ Example
    :
    An implementation might express such a condition via the constant-expression in a static_assert-declaration.
    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 and also define the function as deleted.
    — end example
     ]
  • (3.4)
    Expects: the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is called.
    [ Example
    :
    An implementation might express such conditions via an attribute such as [[expects]] ([dcl.attr.contract]).
    However, some such conditions might not lend themselves to expression via code.
    — end example
     ]
  • (3.5)
    Effects: the actions performed by the function.
  • (3.6)
    Synchronization: the synchronization operations ([intro.multithread]) applicable to the function.
  • (3.7)
    Ensures: the conditions (sometimes termed observable results or postconditions) established by the function.
  • (3.8)
    Returns: a description of the value(s) returned by the function.
  • (3.9)
    Throws: any exceptions thrown by the function, and the conditions that would cause the exception.
  • (3.10)
    Complexity: the time and/or space complexity of the function.
  • (3.11)
    Remarks: additional semantic constraints on the function.
  • (3.12)
    Error conditions: the error conditions for error codes reported by the function.
4
#
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, Expects, Effects, Synchronization, Ensures, 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, Ensures, or Complexity element, then that supersedes any occurrences of that element in the code sequence.
5
#
For non-reserved replacement and handler functions, [language.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.
6
#
If the formulation of a complexity requirement calls for a negative number of operations, the actual requirement is zero operations.160
7
#
Complexity requirements specified in the library clauses are upper bounds, and implementations that provide better complexity guarantees satisfy the requirements.
8
#
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 functions, there will be no “Comparison functions” subclause.
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 Expects: element.
160)
This simplifies the presentation of complexity requirements in some cases.

15.4.1.5 C library [structure.see.also]

1
#
Paragraphs labeled “See also” contain cross-references to the relevant portions of other standards ([intro.refs]).

15.4.2 Other conventions [conventions]

1
#
This subclause 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.

15.4.2.1 Type descriptions [type.descriptions]

15.4.2.1.1 General [type.descriptions.general]

1
#
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.
2
#
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.

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

1
#
Several types defined in [language.support] through [thread] and [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 exposition only.
[ Example
: 
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
 ]

15.4.2.1.3 Enumerated types [enumerated.types]

1
#
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
2
#
The enumerated type enumerated can be written:
enum enumerated { V, V, V, V, … };

inline const enumerated C(V);
inline const enumerated C(V);
inline const enumerated C(V);
inline const enumerated C(V);
  ⋮
3
#
Here, the names C, C, 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]).

15.4.2.1.4 Bitmask types [bitmask.types]

1
#
Several types defined in [language.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.
2
#
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 = 1 << 0, V = 1 << 1, V = 1 << 2, V = 1 << 3, …
};

inline constexpr bitmask C(V);
inline constexpr bitmask C(V);
inline constexpr bitmask C(V);
inline constexpr bitmask C(V);
  ⋮

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;
}
3
#
Here, the names C, C, etc.
 represent bitmask elements for this particular bitmask type.
All such elements have distinct, nonzero values such that, for any pair C and C where , & is nonzero and & is zero.
Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.
4
#
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.

15.4.2.1.5 Character sequences [character.seq]

1
#
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 [language.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>.

15.4.2.1.5.1 Byte strings [byte.strings]

1
#
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
2
#
The length of an ntbs is the number of elements that precede the terminating null character.
An empty ntbs has a length of zero.
3
#
The value of an ntbs is the sequence of values of the elements up to and including the terminating null character.
4
#
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.
165)
A string literal, such as "abc", is a static ntbs.

15.4.2.1.5.2 Multibyte strings [multibyte.strings]

1
#
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
2
#
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.

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

1
#
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.
2
#
The type of a customization point object shall satisfy Semiregular ([concepts.object]).
3
#
All instances of a specific customization point object type shall be equal ([concepts.equality]).
4
#
The type T of a customization point object shall satisfy 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.
5
#
Each customization point object type constrains its return type to satisfy a particular concept.
6
#
[ Note
:
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
 ]

15.4.2.2 Functions within classes [functions.within.classes]

1
#
For the sake of exposition, [language.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.

15.4.2.3 Operators [operators]

1
#
In this library, whenever a declaration is provided for an operator!=, operator>, operator<=, or operator>= for a type T, its requirements and semantics are as follows, unless explicitly specified otherwise.
🔗
bool operator!=(const T& x, const T& y);
2
#
Requires: Type T is Cpp17EqualityComparable (Table 22).
3
#
Returns: !(x == y).
🔗
bool operator>(const T& x, const T& y);
4
#
Requires: Type T is Cpp17LessThanComparable (Table 23).
5
#
Returns: y < x.
🔗
bool operator<=(const T& x, const T& y);
6
#
Requires: Type T is Cpp17LessThanComparable (Table 23).
7
#
Returns: !(y < x).
🔗
bool operator>=(const T& x, const T& y);
8
#
Requires: Type T is Cpp17LessThanComparable (Table 23).
9
#
Returns: !(x < y).

15.4.2.4 Private members [objects.within.classes]

1
#
[language.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 [language.support] through [thread] and [depr].
2
#
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
3
#
An implementation may use any technique that provides equivalent observable behavior.

15.5 Library-wide requirements [requirements]

1
#
This subclause specifies requirements that apply to the entire C++ standard library.
[language.support] through [thread] and [depr] specify the requirements of individual entities within the library.
2
#
Requirements specified in terms of interactions between threads do not apply to programs having only a single thread of execution.
3
#
Within this subclause, [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.

15.5.1 Library contents and organization [organization]

1
#
[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.

15.5.1.1 Library contents [contents]

1
#
The C++ standard library provides definitions for the entities and macros described in the synopses of the C++ standard library headers ([headers]).
2
#
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
3
#
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]) may also define names within the global namespace.
168)
This gives implementers freedom to use inline namespaces to support multiple configurations of the library.

15.5.1.2 Headers [headers]

1
#
Each element of the C++ standard library is declared or defined (as appropriate) in a header.169
2
#
The C++ standard library provides the C++ library headers, shown in Table 18.
Table 18 — C++ library headers
[t]l <algorithm>
<any>
<array>
<atomic>
<bit>
<bitset>
<charconv>
<chrono>
<codecvt>
<compare>
<complex>
<concepts>
<condition_­variable>
<contract>
<deque>
<exception>
<execution>
<filesystem>
<forward_­list>
<fstream>
<functional>
<future>
<initializer_­list>
<iomanip>
<ios>
<iosfwd>
<iostream>
<istream>
<iterator>
<limits>
<list>
<locale>
<map>
<memory>
<memory_­resource>
<mutex>
<new>
<numeric>
<optional>
<ostream>
<queue>
<random>
<ratio>
<regex>
<scoped_­allocator>
<set>
<shared_­mutex>
<span>
<sstream>
<stack>
<stdexcept>
<streambuf>
<string>
<string_­view>
<strstream>
<syncstream>
<system_­error>
<thread>
<tuple>
<typeindex>
<typeinfo>
<type_­traits>
<unordered_­map>
<unordered_­set>
<utility>
<valarray>
<variant>
<vector>
<version>
3
#
The facilities of the C standard library are provided in the additional headers shown in Table 19.170
Table 19 — C++ headers for C library facilities
[t]l <cassert>
<cctype>
<cerrno>
<cfenv>
<cfloat>
<cinttypes>
<climits>
<clocale>
<cmath>
<csetjmp>
<csignal>
<cstdarg>
<cstddef>
<cstdint>
<cstdio>
<cstdlib>
<cstring>
<ctime>
<cuchar>
<cwchar>
<cwctype>
4
#
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 [language.support] through [thread] and [depr]) are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations.
5
#
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
:
The names defined as macros in C include the following: assert, offsetof, setjmp, va_­arg, va_­end, and va_­start.
— end note
 ]
6
#
Names that are defined as functions in C shall be defined as functions in the C++ standard library.171
7
#
Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers.172
8
#
[depr.c.headers], C standard library headers, describes the effects of using the name.h (C header) form in a C++ program.173
9
#
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.)
10
#
Table 20 lists the Annex K names that may be declared in some header.
These names are also subject to the restrictions of [macro.names].
Table 20 — C standard Annex K names
[t]l abort_­handler_­s
asctime_­s
bsearch_­s
constraint_­handler_­t
ctime_­s
errno_­t
fopen_­s
fprintf_­s
freopen_­s
fscanf_­s
fwprintf_­s
fwscanf_­s
getenv_­s
gets_­s
gmtime_­s
ignore_­handler_­s
localtime_­s
L_­tmpnam_­s
mbsrtowcs_­s
mbstowcs_­s
memcpy_­s
memmove_­s
memset_­s
printf_­s
qsort_­s
RSIZE_­MAX
rsize_­t
scanf_­s
set_­constraint_­handler_­s
snprintf_­s
snwprintf_­s
sprintf_­s
sscanf_­s
strcat_­s
strcpy_­s
strerrorlen_­s
strerror_­s
strlen_­s
strncat_­s
strncpy_­s
strtok_­s
swprintf_­s
swscanf_­s
tmpfile_­s
TMP_­MAX_­S
tmpnam_­s
vfprintf_­s
vfscanf_­s
vfwprintf_­s
vfwscanf_­s
vprintf_­s
vscanf_­s
vsnprintf_­s
vsnwprintf_­s
vsprintf_­s
vsscanf_­s
vswprintf_­s
vswscanf_­s
vwprintf_­s
vwscanf_­s
wcrtomb_­s
wcscat_­s
wcscpy_­s
wcsncat_­s
wcsncpy_­s
wcsnlen_­s
wcsrtombs_­s
wcstok_­s
wcstombs_­s
wctomb_­s
wmemcpy_­s
wmemmove_­s
wprintf_­s
wscanf_­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.

15.5.1.3 Freestanding implementations [compliance]

1
#
Two kinds of implementations are defined: hosted and freestanding ([intro.compliance]).
For a hosted implementation, this document describes the set of available headers.
2
#
A freestanding implementation has an implementation-defined set of headers.
This set shall include at least the headers shown in Table 21.
Table 21 — C++ headers for freestanding implementations
Subclause
Header(s)
Types
<cstddef>
Implementation properties
<cfloat> <limits> <climits> <version>
Integer types
<cstdint>
Start and termination
<cstdlib>
Dynamic memory management
<new>
Type identification
<typeinfo>
Exception handling
<exception>
Initializer lists
<initializer_­list>
Other runtime support
<cstdarg>
Type traits
<type_­traits>
Bit manipulation
<bit>
Atomics
<atomic>
3
#
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 other headers listed in this table shall meet the same requirements as for a hosted implementation.

15.5.2 Using the library [using]

15.5.2.1 Overview [using.overview]

1
#
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.

15.5.2.2 Headers [using.headers]

1
#
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.
2
#
A translation unit may include library headers in any order ([lex]).
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
3
#
A translation unit shall include a header only outside of any declaration or definition, and shall include the header 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.

15.5.2.3 Linkage [using.linkage]

1
#
Entities in the C++ standard library have external linkage.
Unless otherwise specified, objects and functions have the default extern "C++" linkage ([dcl.link]).
2
#
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
3
#
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.

15.5.3 Requirements on types and expressions [utility.requirements]

1
#
[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.

15.5.3.1 Template argument requirements [utility.arg.requirements]

1
#
The template definitions in the C++ standard library refer to various named requirements whose details are set out in Tables 22–29.
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.
2
#
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 22 — Cpp17EqualityComparable requirements
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 23 — Cpp17LessThanComparable requirements
Expression
Return type
Requirement
a < b
convertible to bool
Table 24 — Cpp17DefaultConstructible requirements
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 25 — Cpp17MoveConstructible requirements
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
:
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 26 — Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible)
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 27 — Cpp17MoveAssignable requirements
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
:
 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 28 — Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable)
Expression
Return type
Return value
Post-condition
t = v
T&
t
t is equivalent to v, the value of v is unchanged
Table 29 — Cpp17Destructible requirements
Expression
Post-condition
u.~T()
All resources owned by u are reclaimed, no exception is propagated.

15.5.3.2 Swappable requirements [swappable.requirements]

1
#
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.
2
#
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.
3
#
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:
[ Note
:
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
:
It is unspecified whether a library component that has a swappable requirement includes the header <utility> to ensure an appropriate evaluation context.
— end note
 ]
4
#
An rvalue or lvalue t is swappable if and only if t is swappable with any rvalue or lvalue, respectively, of type T.
5
#
A type X satisfying any of the iterator requirements ([iterator.requirements]) satisfies the Cpp17ValueSwappable requirements if, for any dereferenceable object x of type X, *x is swappable.
[ Example
:
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>(u).
template<class T, class U>
void value_swap(T&& t, U&& u) {
  using std::swap;
  swap(std::forward<T>(t), std::forward<U>(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
 ]

15.5.3.3 Cpp17NullablePointer requirements [nullablepointer.requirements]

1
#
A Cpp17NullablePointer type is a pointer-like type that supports null values.
A type P satisfies the Cpp17NullablePointer requirements if:
  • (1.1)
    P satisfies the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements,
  • (1.2)
    lvalues of type P are swappable,
  • (1.3)
    the expressions shown in Table 30 are valid and have the indicated semantics, and
  • (1.4)
    P satisfies all the other requirements of this subclause.
2
#
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
:
Operations involving indeterminate values may cause undefined behavior.
— end note
 ]
3
#
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.
4
#
No operation which is part of the Cpp17NullablePointer requirements shall exit via an exception.
5
#
In Table 30, 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 30 — Cpp17NullablePointer requirements
Expression
Return type
Operational semantics
P u(np);
Ensures: u == nullptr
P u = np;
P(np)
Ensures: P(np) == nullptr
t = np
P&
Ensures: 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

15.5.3.4 Cpp17Hash requirements [hash.requirements]

1
#
A type H meets the Cpp17Hash requirements if:
  • (1.1)
    it is a function object type ([function.objects]),
  • (1.2)
    it satisfies the Cpp17CopyConstructible (Table 26) and Cpp17Destructible (Table 29) requirements, and
  • (1.3)
    the expressions shown in Table 31 are valid and have the indicated semantics.
2
#
Given Key is an argument type for function objects of type H, in Table 31 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 31 — Cpp17Hash requirements
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
:
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
 ]
[ 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().
— end note
 ]
h(u)
size_­t
Shall not modify u.

15.5.3.5 Cpp17Allocator requirements [allocator.requirements]

1
#
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.
2
#
The class template allocator_­traits supplies a uniform interface to all allocator types.
Table 32 describes the types manipulated through allocators.
Table 33 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 33 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 32 and 33, the use of move and forward always refers to std​::​move and std​::​forward, respectively.
Table 32 — Descriptive variable definitions
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&&
Table 33 — Cpp17Allocator requirements
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
Requires: (*p).m is well-defined.
equivalent to (*p).m
q->m
type of T​::​m
Requires: (*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 n objects of type T but objects are not constructed.
allocate may throw an appropriate exception.176
[ Note
:
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)
Requires: p shall be a value returned by an earlier call to allocate that has not been invalidated by an intervening call to deallocate.
n shall match 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.

Ensures: u == a
X u(b);
Shall not exit via an exception.

Ensures: Y(u) == b, u == X(b)
X u(std​::​move(a));
X u = std​::​move(a);
Shall not exit via an exception.

Ensures: The value of a is unchanged and is equal to u.
X u(std​::​move(b));
Shall not exit via an exception.

Ensures: 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
​::​new ((void*)c) C(forward<​Args>​(args)...)
a.destroy(c)
(not used)
Effects: Destroys the object at c
c->~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
3
#
Note A: The member class template rebind in the table above is effectively a typedef template.
[ Note
:
In general, if the name Allocator is bound to SomeAllocator<T>, then Allocator​::​rebind<U>​::​other is the same type as SomeAllocator<U>, where SomeAllocator<T>​::​value_­type is T and SomeAllocator<U>​::​​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<U>​::​other by default.
For allocator types that are not template instantiations of the above form, no default is provided.
4
#
Note B: If X​::​propagate_­on_­container_­copy_­assignment​::​value is true, X shall satisfy the Cpp17CopyAssignable requirements (Table 28) and the copy operation shall not throw exceptions.
If X​::​propagate_­on_­container_­move_­assignment​::​value is true, X shall satisfy the Cpp17MoveAssignable requirements (Table 27) 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.
5
#
An allocator type X shall satisfy the Cpp17CopyConstructible requirements (Table 26).
The X​::​pointer, X​::​const_­pointer, X​::​void_­pointer, and X​::​const_­void_­pointer types shall satisfy the Cpp17NullablePointer requirements (Table 30).
No constructor, comparison 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 satisfy the requirements for a random access iterator and of a contiguous iterator.
6
#
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.
7
#
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.
8
#
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.
9
#
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.
[ Example
:
The following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 33:
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<U>&);
template<class T, class U>
bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);
— end example
 ]
10
#
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
:
Additionally, the member function allocate for that type may fail by throwing an object of type bad_­alloc.
— end note
 ]
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.

15.5.3.5.1 Allocator completeness requirements [allocator.requirements.completeness]

1
#
If X is an allocator class for type T, X additionally satisfies the allocator completeness requirements if, whether or not T is a complete type:

15.5.4 Constraints on programs [constraints]

15.5.4.1 Overview [constraints.overview]

1
#
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.

15.5.4.2 Namespace use [namespace.constraints]

15.5.4.2.1 Namespace std [namespace.std]

1
#
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.
2
#
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
3
#
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.
4
#
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.
5
#
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.
6
#
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
:
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.
7
#
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
:
This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments.
— end note
 ]
8
#
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.
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 may 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.

15.5.4.2.2 Namespace posix [namespace.posix]

1
#
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.

15.5.4.2.3 Namespaces for future standardization [namespace.future]

1
#
Top level namespaces with a name starting with std and followed by a non-empty sequence of digits are reserved for future standardization.
The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace.
[ Example
:
The top level namespace std2 is reserved for use by future revisions of this International Standard.
— end example
 ]

15.5.4.3 Reserved names [reserved.names]

1
#
The C++ standard library reserves the following kinds of names:
2
#
If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by this Clause, its behavior is undefined.

15.5.4.3.1 Zombie names [zombie.names]

1
#
In namespace std, the following names are reserved for previous standardization:
2
#
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:
3
#
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.
4
#
The header names <ccomplex>, <ciso646>, <cstdalign>, <cstdbool>, and <ctgmath> are reserved for previous standardization.

15.5.4.3.2 Macro names [macro.names]

1
#
A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
2
#
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].

15.5.4.3.3 External linkage [extern.names]

1
#
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.
2
#
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
3
#
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.
4
#
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.

15.5.4.3.4 Types [extern.types]

1
#
For each type T from the C standard library,182 the types ​::​T and std​::​T are reserved to the implementation and, when defined, ​::​T shall be identical to std​::​T.
182)
These types are clock_­t, div_­t, FILE, fpos_­t, lconv, ldiv_­t, mbstate_­t, ptrdiff_­t, sig_­atomic_­t, size_­t, time_­t, tm, va_­list, wctrans_­t, wctype_­t, and wint_­t.

15.5.4.3.5 User-defined literal suffixes [usrlit.suffix]

1
#
Literal suffix identifiers that do not start with an underscore are reserved for future standardization.

15.5.4.4 Headers [alt.headers]

1
#
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.

15.5.4.5 Derived classes [derived.classes]

1
#
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]).

15.5.4.6 Replacement functions [replacement.functions]

1
#
[language.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 ([definitions]).
2
#
A C++ program may provide the definition for any of the following dynamic memory allocation function signatures declared in header <new> ([basic.stc.dynamic], [support.dynamic]):
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&)
3
#
The program's definitions are used instead of the default versions supplied by the implementation ([support.dynamic]).
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.

15.5.4.7 Handler functions [handler.functions]

1
#
The C++ standard library provides a default version of the following handler function ([language.support]):
2
#
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): See also subclauses [alloc.errors], Storage allocation errors, and [support.exception], Exception handling.
3
#
A C++ program can get a pointer to the current handler function by calling the following functions:
4
#
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.

15.5.4.8 Other functions [res.on.functions]

1
#
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.
2
#
In particular, the effects are undefined in the following cases:
  • (2.1)
    for replacement functions, 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.

15.5.4.9 Function arguments [res.on.arguments]

1
#
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
    :
    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
    :
    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
     ]

15.5.4.10 Library object access [res.on.objects]

1
#
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
:
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
 ]
2
#
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
:
This applies even to objects such as mutexes intended for thread synchronization.
— end note
 ]

15.5.4.11 Requires paragraph [res.on.required]

1
#
Violation of any preconditions specified in a function's Requires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception when the precondition is violated.
2
#
Violation of any preconditions specified in a function's Expects: element results in undefined behavior.

15.5.4.12 Semantic requirements [res.on.requirements]

1
#
If the semantic requirements of a declaration's constraints ([structure.requirements]) are not satisfied at the point of use, the program is ill-formed, no diagnostic required.

15.5.5 Conforming implementations [conforming]

15.5.5.1 Overview [conforming.overview]

1
#
Subclause [conforming] describes the constraints upon, and latitude of, implementations of the C++ standard library.
2
#
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].

15.5.5.2 Headers [res.on.headers]

1
#
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.
2
#
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]).
3
#
The C standard library headers shall include only their corresponding C++ standard library header, as described in [headers].

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

1
#
The names and global function signatures described in [contents] are reserved to the implementation.
2
#
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.

15.5.5.4 Non-member functions [global.functions]

1
#
It is unspecified whether any non-member functions in the C++ standard library are defined as inline.
2
#
A call to a non-member function signature described in [language.support] through [thread] and [depr] shall behave as if the implementation declared no additional non-member function signatures.183
3
#
An implementation shall not declare a non-member function signature with additional default arguments.
4
#
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
:
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
 ]
183)
A valid C++ program always calls the expected library non-member function.
An implementation may also define additional non-member functions that would otherwise not be called by a valid C++ program.

15.5.5.5 Member functions [member.functions]

1
#
It is unspecified whether any member functions in the C++ standard library are defined as inline.
2
#
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
:
For instance, an implementation may 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
 ]

15.5.5.6 Constexpr functions and constructors [constexpr.functions]

1
#
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.

15.5.5.7 Requirements for stable algorithms [algorithm.stable]

1
#
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).

15.5.5.8 Reentrancy [reentrancy]

1
#
Except where explicitly specified in this document, it is implementation-defined which functions in the C++ standard library may be recursively reentered.

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

1
#
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.
2
#
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.
3
#
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.
4
#
[ Note
:
This means, for example, that implementations can't use a static object 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
 ]
5
#
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.
6
#
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
:
In particular, container operations that invalidate iterators conflict with operations on iterators associated with that container.
— end note
 ]
7
#
Implementations may share their own internal objects between threads if the objects are not visible to users and are protected against data races.
8
#
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.
9
#
[ Note
:
This allows implementations to parallelize operations if there are no visible side effects.
— end note
 ]

15.5.5.10 Protection within classes [protection.within.classes]

1
#
It is unspecified whether any function signature or class described in [language.support] through [thread] and [depr] is a friend of another class in the C++ standard library.

15.5.5.11 Derived classes [derivation]

1
#
An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.
2
#
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.
3
#
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.184
4
#
All types specified in the C++ standard library shall be non-final types unless otherwise specified.
184)
There is an implicit exception to this rule for types that are described as synonyms for basic integral types, such as size_­t and streamoff.

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

1
#
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.
2
#
Functions from the C standard library shall not throw exceptions185 except when such a function calls a program-supplied function that throws an exception.186
3
#
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.
4
#
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.187
Implementations should report errors by throwing exceptions of or derived from the standard exception classes ([bad.alloc], [support.exception], [std.exceptions]).
5
#
An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.
185)
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.
186)
The functions qsort() and bsearch() ([alg.c.library]) meet this condition.
187)
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.

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

1
#
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.
[ Note
:
Other libraries are strongly encouraged to do the same, since not doing so may result in accidental use of pointers that are not safely derived.
Libraries that store pointers outside the user's address space should make it appear that they are stored and retrieved from a traceable pointer location.
— end note
 ]

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

1
#
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
:
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
 ]

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

1
#
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.