[requirements]
16 Library introduction [library]
16.4 Library-wide requirements [requirements]
16.4.1 General [requirements.general]
16.4.2 Library contents and organization [organization]
16.4.2.1 General [organization.general]
16.4.2.2 Library contents [contents]
16.4.2.3 Headers [headers]
16.4.2.4 Modules [std.modules]
16.4.2.5 Freestanding implementations [compliance]
16.4.3 Using the library [using]
16.4.3.1 Overview [using.overview]
16.4.3.2 Headers [using.headers]
16.4.3.3 Linkage [using.linkage]
16.4.4 Requirements on types and expressions [utility.requirements]
16.4.4.1 General [utility.requirements.general]
16.4.4.2 Template argument requirements [utility.arg.requirements]
16.4.4.3 Swappable requirements [swappable.requirements]
16.4.4.4 Cpp17NullablePointer requirements [nullablepointer.requirements]
16.4.4.5 Cpp17Hash requirements [hash.requirements]
16.4.4.6 Cpp17Allocator requirements [allocator.requirements]
16.4.4.6.1 General [allocator.requirements.general]
16.4.4.6.2 Allocator completeness requirements [allocator.requirements.completeness]
16.4.5 Constraints on programs [constraints]
16.4.5.1 Overview [constraints.overview]
16.4.5.2 Namespace use [namespace.constraints]
16.4.5.2.1 Namespace std [namespace.std]
16.4.5.2.2 Namespace posix [namespace.posix]
16.4.5.2.3 Namespaces for future standardization [namespace.future]
16.4.5.3 Reserved names [reserved.names]
16.4.5.3.1 General [reserved.names.general]
16.4.5.3.2 Zombie names [zombie.names]
16.4.5.3.3 Macro names [macro.names]
16.4.5.3.4 External linkage [extern.names]
16.4.5.3.5 Types [extern.types]
16.4.5.3.6 User-defined literal suffixes [usrlit.suffix]
16.4.5.4 Headers [alt.headers]
16.4.5.5 Derived classes [derived.classes]
16.4.5.6 Replacement functions [replacement.functions]
16.4.5.7 Handler functions [handler.functions]
16.4.5.8 Other functions [res.on.functions]
16.4.5.9 Function arguments [res.on.arguments]
16.4.5.10 Library object access [res.on.objects]
16.4.5.11 Semantic requirements [res.on.requirements]
16.4.6 Conforming implementations [conforming]
16.4.6.1 Overview [conforming.overview]
16.4.6.2 Headers [res.on.headers]
16.4.6.3 Restrictions on macro definitions [res.on.macro.definitions]
16.4.6.4 Non-member functions [global.functions]
16.4.6.5 Member functions [member.functions]
16.4.6.6 Friend functions [hidden.friends]
16.4.6.7 Constexpr functions and constructors [constexpr.functions]
16.4.6.8 Requirements for stable algorithms [algorithm.stable]
16.4.6.9 Reentrancy [reentrancy]
16.4.6.10 Data race avoidance [res.on.data.races]
16.4.6.11 Properties of library classes [library.class.props]
16.4.6.12 Protection within classes [protection.within.classes]
16.4.6.13 Derived classes [derivation]
16.4.6.14 Restrictions on exception handling [res.on.exception.handling]
16.4.6.15 Contract assertions [res.contract.assertions]
16.4.6.16 Value of error codes [value.error.codes]
16.4.6.17 Moved-from state of library types [lib.types.movedfrom]
16.4.1 General [requirements.general]
Subclause [requirements] specifies requirements that apply to the entire C++ standard library.
[support] through [exec] 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.134
It is unspecified whether names declared in a specific namespace are declared directly in that namespace or in an inline namespace inside that namespace.135
Whenever an unqualified name other than swap, make_error_code, make_error_condition, from_stream, or submdspan_mapping is used in the specification of a declaration D in [support] through [exec] or [depr], its meaning is established as-if by performing unqualified name lookup ([basic.lookup.unqual]) in the context of D.
Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup ([basic.lookup.qual]) in the context of D.
[Example 1:
The reference to is_array_v in the specification of std::to_array ([array.creation]) refers to ::std::is_array_v.
— end example]
The meaning of the unqualified name swap is established in an overload resolution context for swappable values ([swappable.requirements]).
The meanings of the unqualified names make_error_code, make_error_condition, from_stream, and submdspan_mapping are established as-if by performing argument-dependent lookup ([basic.lookup.argdep]).
16.4.2.4 Modules [std.modules]
The C++ standard library provides the following C++ library modules.
The named module std exports declarations in namespace std that are provided by the importable C++ library headers (Table 24 or the subset provided by a freestanding implementation) and the C++ headers for C library facilities (Table 25).
It additionally exports declarations in the global namespace for the storage allocation and deallocation functions that are provided by .
The named module std.compat exports the same declarations as the named module std, and additionally exports
- declarations in the global namespace corresponding to the declarations in namespace std that are provided by the C++ headers for C library facilities (Table 25), except the explicitly excluded declarations described in [support.c.headers.other] and
- declarations provided by the headers and .
It is unspecified to which module a declaration in the standard library is attached.
[Note 1:
Conforming implementations ensure that mixing #include and import does not result in conflicting attachments ([basic.link]).
— end note]
Recommended practice: Implementations should ensure such attachments do not preclude further evolution or decomposition of the standard library modules.
A declaration in the standard library denotes the same entity regardless of whether it was made reachable through including a header, importing a header unit, or importing a C++ library module.
Recommended practice: Implementations should avoid exporting any other declarations from the C++ library modules.
16.4.2.5 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 27.
For each of the headers listed in Table 27, a freestanding implementation provides at least the freestanding items ([freestanding.item]) declared in the header.
The hosted library facilities are the set of facilities described in this document that are required for hosted implementations, but not required for freestanding implementations.
A freestanding implementation provides a (possibly empty) implementation-defined subset of the hosted library facilities.
Unless otherwise specified, the requirements on each declaration, entity, and macro provided in this way are the same as the corresponding requirements for a hosted implementation, except that not all of the members of the namespaces are required to be present.
A freestanding implementation provides deleted definitions ([dcl.fct.def.delete]) for a (possibly empty) implementation-defined subset of the namespace-scope functions and function templates from the hosted library facilities.
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.
16.4.3.3 Linkage [using.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.142
Objects and functions defined in the library and required by a C++ program are included in the program prior to program startup.
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 28–35.
In these tables,
- T denotes an object or reference type to be supplied by a C++ program instantiating a template,
- a, b, and c denote values of type (possibly const) T,
- s and t denote modifiable lvalues of type T,
- u denotes an identifier,
- rv denotes an rvalue of type T, and
- v denotes 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 28 — Cpp17EqualityComparable requirements [tab:cpp17.equalitycomparable]
decltype(a == b) models boolean-testable | == is an equivalence relation, that is, it has the following properties:
|
Table 30 — Cpp17DefaultConstructible requirements [tab:cpp17.defaultconstructible]
object t is default-initialized | |
object u is value-initialized or aggregate-initialized | |
an object of type T is value-initialized or aggregate-initialized |
Table 31 — Cpp17MoveConstructible requirements [tab:cpp17.moveconstructible]
u is equivalent to the value of rv before the construction | |
T(rv) is equivalent to the value of rv before the construction | |
Table 32 — Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible) [tab:cpp17.copyconstructible]
the value of v is unchanged and is equivalent to u | |
the value of v is unchanged and is equivalent to T(v) |
Table 33 — 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 |
Table 35 — Cpp17Destructible requirements [tab:cpp17.destructible]
No exception is propagated. | |
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
- the expressions swap(t, u) and swap(u, t) are valid when evaluated in the context described below, and
- these expressions have the following effects:
- the object referred to by t has the value originally held by u and
- 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:
- the two swap function templates defined in and
- 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 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]
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 meets the Cpp17Swappable requirements if lvalues of type X are swappable.
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 <cassert> #include <utility> template<class T, class U> void value_swap(T&& t, U&& u) { using std::swap; swap(std::forward<T>(t), std::forward<U>(u)); } template<class T> void lv_swap(T& t1, T& t2) { using std::swap; swap(t1, t2); } 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); } void swap(Proxy p, A& x) { swap(x, p); } } 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
- P meets the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, Cpp17Swappable, and Cpp17Destructible requirements,
- the expressions shown in Table 36 are valid and have the indicated semantics, and
- 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 or erroneous value.
[Note 1:
Operations involving indeterminate values can cause undefined behavior, and operations involving erroneous values can cause erroneous behavior ([basic.indet]).
— end note]
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 36, 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 36 — 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 | decltype(a != b) models boolean-testable | !(a == b) |
a == np | decltype(a == np) and decltype(np == a) each model boolean-testable | a == P() |
np == a | ||
a != np | decltype(a != np) and decltype(np != a) each model boolean-testable | !(a == np) |
np != a |
16.4.4.5 Cpp17Hash requirements [hash.requirements]
A type H meets the Cpp17Hash requirements if
- it is a function object type ([function.objects]),
- it meets the Cpp17CopyConstructible (Table 32) and Cpp17Destructible (Table 35) requirements, and
- the expressions shown in Table 37 are valid and have the indicated semantics.
Given Key is an argument type for function objects of type H, in Table 37 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 37 — Cpp17Hash requirements [tab:cpp17.hash]
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(). | ||
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 ([strings]), containers ([containers]) (except array and inplace_vector), string buffers and string streams ([input.output]), and match_results are parameterized in terms of allocators.
- T, U, C denote any cv-unqualified object type ([basic.types.general]),
- X denotes an allocator class for type T,
- Y denotes the corresponding allocator class for type U,
- XX denotes the type allocator_traits<X>,
- YY denotes the type allocator_traits<Y>,
- a, a1, a2 denote lvalues of type X,
- u denotes the name of a variable being declared,
- b denotes a value of type Y,
- c denotes a pointer of type C* through which indirection is valid,
- p denotes a value of type XX::pointer obtained by calling a1.allocate, where a1 == a,
- q denotes a value of type XX::const_pointer obtained by conversion from a value p,
- r denotes a value of type T& obtained by the expression *p,
- w denotes a value of type XX::void_pointer obtained by conversion from a value p,
- x denotes a value of type XX::const_void_pointer obtained by conversion from a value q or a value w,
- y denotes 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 denotes a value of type XX::size_type,
- Args denotes a template parameter pack, and
- args denotes a function parameter pack with the pattern Args&&.
The class template allocator_traits ([allocator.traits]) supplies a uniform interface to all allocator types.
This subclause describes the requirements on allocator types and thus on types used to instantiate allocator_traits.
A requirement is optional if a default for a given type or expression is specified.
Within the standard library allocator_traits template, an optional requirement that is not supplied by an allocator is replaced by the specified default type or expression.
typename X::const_pointer
Mandates: XX::pointer is convertible to XX::const_pointer.
Remarks: Default: pointer_traits<XX::pointer>::rebind<const T>
typename X::void_pointer
typename Y::void_pointer
Mandates: XX::pointer is convertible to XX::void_pointer.
XX::void_pointer and YY::void_pointer are the same type.
Remarks: Default: pointer_traits<XX::pointer>::rebind<void>
typename X::const_void_pointer
typename Y::const_void_pointer
Mandates: XX::pointer, XX::const_pointer, and XX::void_pointer are convertible to XX::const_void_pointer.
XX::const_void_pointer and YY::const_void_pointer are the same type.
Remarks: Default: pointer_traits<XX::pointer>::rebind<const void>
Result: An unsigned integer type that can represent the size of the largest object in the allocation model.
Remarks: Default: make_unsigned_t<XX::difference_type>
typename X::difference_type
Result: A signed integer type that can represent the difference between any two pointers in the allocation model.
Remarks: Default: pointer_traits<XX::pointer>::difference_type
typename X::rebind<U>::other
Postconditions: For all U (including T), YY::rebind_alloc<T> is X.
Remarks: 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.
[Note 2:
The member class template rebind of X is effectively a typedef template.
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]
Postconditions: *q refers to the same object as *p.
Preconditions: (*p).m is well-defined.
Effects: Equivalent to (*p).m.
Preconditions: (*q).m is well-defined.
Effects: Equivalent to (*q).m.
static_cast<XX::pointer>(w)
Postconditions: static_cast<XX::pointer>(w) == p.
static_cast<XX::const_pointer>(x)
Result: XX::const_pointer
Postconditions: static_cast<XX::const_pointer>(x) == q.
pointer_traits<XX::pointer>::pointer_to(r)
Postconditions: Same as p.
Effects: Memory is allocated for an array of n T and such an object is created but array elements are not constructed.
Throws: allocate may throw an appropriate exception.
[Note 3:
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.
— end note]
Remarks: If n == 0, the return value is unspecified.
Effects: Same as a.allocate(n).
The use of y is unspecified, but it is intended as an aid to locality.
Remarks: Default: a.allocate(n)
Result: allocation_result<XX::pointer, XX::size_type>
Returns: allocation_result<XX::pointer, XX::size_type>{ptr, count} where ptr is memory allocated for an array of count T and such an object is created but array elements are not constructed, such that count ≥ n.
If n == 0, the return value is unspecified.
Throws: allocate_at_least may throw an appropriate exception.
Remarks: Default: {a.allocate(n), n}.
Preconditions:
p has not been invalidated by an intervening call to deallocate.
Returns: The largest value n that can meaningfully be passed to a.allocate(n).
Remarks: Default: numeric_limits<size_type>::max() / sizeof(value_type)
Returns: true only if storage allocated from each can be deallocated via the other.
Remarks: operator== shall be reflexive, symmetric, and transitive.
Returns: a == YY::rebind_alloc<T>(b).
Postconditions: Y(u) == b and u == X(b).
X u(std::move(a));
X u = std::move(a);
Postconditions: The value of a is unchanged and is equal to u.
Postconditions: u is equal to the prior value of X(b).
Effects: Constructs an object of type C at c.
Remarks: Default: construct_at(c, std::forward<Args>(args)...)
Effects: Destroys the object at c.
Remarks: Default: destroy_at(c)
a.select_on_container_copy_construction()
Returns: Typically returns either a or X().
Remarks: Default: return a;
typename X::propagate_on_container_copy_assignment
Result: Identical to or derived from true_type or false_type.
Returns: true_type only if an allocator of type X should be copied when the client container is copy-assigned; if so, X shall meet the Cpp17CopyAssignable requirements (Table 34) and the copy operation shall not throw exceptions.
Remarks: Default: false_type
typename X::propagate_on_container_move_assignment
Result: Identical to or derived from true_type or false_type.
Returns: true_type only if an allocator of type X should be moved when the client container is move-assigned; if so, X shall meet the Cpp17MoveAssignable requirements (Table 33) and the move operation shall not throw exceptions.
Remarks: Default: false_type
typename X::propagate_on_container_swap
Result: Identical to or derived from true_type or false_type.
Returns: true_type only if an allocator of type X should be swapped when the client container is swapped; if so, X shall meet the Cpp17Swappable requirements ([swappable.requirements]) and the swap operation shall not throw exceptions.
Remarks: Default: false_type
typename X::is_always_equal
Result: Identical to or derived from true_type or false_type.
Returns: true_type only if the expression a1 == a2 is guaranteed to be true for any two (possibly const) values a1, a2 of type X.
Remarks: Default: is_empty<X>::type
An allocator type X shall meet the Cpp17CopyConstructible requirements (Table 32).
The XX::pointer, XX::const_pointer, XX::void_pointer, and XX::const_void_pointer types shall meet the Cpp17NullablePointer requirements (Table 36).
No constructor, comparison operator function, copy operation, move operation, or swap operation on these pointer types shall exit via an exception.
XX::pointer and XX::const_pointer shall also meet the requirements for a Cpp17RandomAccessIterator ([random.access.iterators]) and the additional requirement that, when p and (p + n) are dereferenceable pointer values for some integral value n, addressof(*(p + n)) == addressof(*p) + n is true.
Let x1 and x2 denote objects of (possibly different) types XX::void_pointer, XX::const_void_pointer, XX::pointer, or XX::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 XX::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 XX::void_pointer.
Then for the expressions w1 == w2 w1 != w2 either or both objects may be replaced by an equivalently-valued object of type XX::const_void_pointer with no change in semantics.
Let p1 and p2 denote objects of type XX::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 XX::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.
[Example 2:
The following is an allocator class template supporting the minimal interface that meets the requirements of [allocator.requirements.general]: template<class T> struct SimpleAllocator { using value_type = T; SimpleAllocator(ctor args); template<class U> SimpleAllocator(const SimpleAllocator<U>& other); T* allocate(std::size_t n); void deallocate(T* p, std::size_t n); template<class U> bool operator==(const SimpleAllocator<U>& rhs) const; };
— end example]
The following exposition-only concept defines the minimal requirements on an Allocator type.
namespace std { template<class Alloc> concept simple-allocator = requires(Alloc alloc, size_t n) { { *alloc.allocate(n) } -> same_as<typename Alloc::value_type&>; { alloc.deallocate(alloc.allocate(n), n) }; } && copy_constructible<Alloc> && equality_comparable<Alloc>; }
A type Alloc models simple-allocator if it meets the requirements of [allocator.requirements.general].
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:
- X is a complete type, and
- 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
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
- an explicit specialization of any member function of a standard library class template, or
- an explicit specialization of any member function template of a standard library class or class template, or
- an explicit or partial specialization of any member class template of a standard library class or class template, or
- 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
- depends on the name of at least one program-defined type, and
- 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.
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.
Let F denote a standard library function or function template.
Unless F is designated an addressable function, it is unspecified if or how a reflection value designating the associated entity can be formed.
Let C denote a standard library class or class template specialization.
It is unspecified if or how a reflection value can be formed to any private member of C, or what the names of such members may be.
A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).
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/IEEE 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.
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:
- macros
- global names
- 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 names shown in Table 38 are reserved for previous standardization:
Table 38 — Zombie names in namespace std [tab:zombie.names.std]
The names shown in Table 39 are reserved as members for previous standardization, and may not be used as a name for object-like macros in portable code:
The names shown in Table 40 are reserved as member functions for previous standardization, and may not be used as a name for function-like macros in portable code:
The header names shown in Table 41 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.
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,144 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.145
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,146 or as a name of namespace scope in the global namespace.
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 ([over.literal]) that do not start with an underscore are reserved for future standardization.
Literal suffix identifiers that contain a double underscore __ are reserved for use by C++ implementations.
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]
If a function defined in [support] through [exec] and [depr] is specified as replaceable ([dcl.fct.def.replace]), the description of function semantics apply to both the default version defined by the C++ standard library and the replacement function defined by the program.
16.4.5.7 Handler functions [handler.functions]
The C++ standard library provides a default version of the following handler function ([support]):
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.
A C++ program can get a pointer to the current handler function by calling the following functions:
Calling the set_* and get_* functions shall not incur a data race ([intro.races]).
A call to any of the set_* functions synchronizes 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 behavior is undefined in the following cases:
For replacement functions ([replacement.functions]), if the installed replacement function does not implement the semantics of the applicable Required behavior: paragraph.
For handler functions ([new.handler], [terminate.handler]), if the installed handler function does not implement the semantics of the applicable Required behavior: paragraph.
Operations on such types can report a failure by throwing an exception unless otherwise specified.
If any replacement function or handler function or destructor operation exits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.
If an incomplete type ([basic.types.general]) 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.
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.
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.
If a function argument is bound to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument, except that the argument passed to a move assignment operator may be a reference to *this ([lib.types.movedfrom]).
[Note 1:
If the type of a parameter is a forwarding reference ([temp.deduct.call]) that is deduced to an lvalue reference type, then the argument is not bound to an rvalue reference.
— 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 would possibly 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].
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.
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],
- 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],
- exceptions in [res.on.exception.handling], and
- contract assertions in [res.contract.assertions].
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 [exec] and [depr] shall behave as if the implementation declared no additional non-member function signatures.147
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]
16.4.6.5 Member functions [member.functions]
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.
16.4.6.6 Friend functions [hidden.friends]
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 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:
For the sort algorithms the relative order of equivalent elements is preserved.
For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
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 doing so can 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.
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 Properties of library classes [library.class.props]
Unless explicitly stated otherwise, it is unspecified whether any class described in [support] through [exec] and [depr] is a trivially copyable class, a standard-layout class, or an implicit-lifetime class ([class.prop]).
16.4.6.12 Protection within classes [protection.within.classes]
It is unspecified whether any function signature or class described in [support] through [exec] and [depr] is a friend of another class in the C++ standard library.
16.4.6.13 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:
- Every base class described as virtual shall be virtual;
- Every base class not specified as virtual shall not be virtual;
- Unless explicitly stated otherwise, types with distinct names shall be distinct types.
All types specified in the C++ standard library shall be non-final types unless otherwise specified.
16.4.6.14 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 a handler ([except.handle]) for the base type.
Functions from the C standard library shall not throw exceptions148 except when such a function calls a program-supplied function that throws an exception.149
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 ([except.spec]).
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.150
An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.
16.4.6.15 Contract assertions [res.contract.assertions]
Unless specified otherwise, an implementation may check the specified preconditions and postconditions of a function in the C++ standard library using contract assertions ([basic.contract], [structure.specifications]).
16.4.6.16 Value of error codes [value.error.codes]
Certain functions in the C++ standard library report errors via a error_code ([syserr.errcode.overview]) object.
That object's category() member shall return 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]
16.4.6.17 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 ([defns.valid]).
An object of a type defined in the C++ standard library may be move-assigned ([class.copy.assign]) to itself.
Unless otherwise specified, such an assignment places the object in a valid but unspecified state.