16 Library introduction [library]

16.4 Library-wide requirements [requirements]

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,

(1.1)
T denotes an object or reference type to be supplied by a C++ program instantiating a template,
(1.2)
a, b, and c denote values of type (possibly const) T,
(1.3)
s and t denote modifiable lvalues of type T,
(1.4)
u denotes an identifier,
(1.5)
rv denotes an rvalue of type T, and
(1.6)
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]

🔗 Expression	Return type	Requirement
🔗 a == b	decltype(a == b) models boolean-testable	== 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 29 — Cpp17LessThanComparable requirements [tab:cpp17.lessthancomparable]

🔗 Expression	Return type	Requirement
🔗 a < b	decltype(a < b) models boolean-testable	< is a strict weak ordering relation ([alg.sorting])

Table 30 — Cpp17DefaultConstructible requirements [tab:cpp17.defaultconstructible]

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

Table 31 — Cpp17MoveConstructible requirements [tab:cpp17.moveconstructible]

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

Table 32 — Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible) [tab:cpp17.copyconstructible]

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

Table 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
🔗 rv's state is unspecified. [Note 2: rv must still meet the requirements of the library component that is using it, whether or not t and rv refer to the same object. The operations listed in those requirements must work as specified whether rv has been moved from or not. — end note]

Table 34 — Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable) [tab:cpp17.copyassignable]

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

Table 35 — Cpp17Destructible requirements [tab:cpp17.destructible]

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

16.4.4.3 Swappable requirements [swappable.requirements]

This subclause provides definitions for swappable types and expressions.

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

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

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

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

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

[Note 1:

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

— end note]

[Note 2:

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

— end note]

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

A type X 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> // Preconditions: std::forward<T>(t) is swappable with std::forward(u). template<class T, class U> void value_swap(T&& t, U&& u) { using std::swap; swap(std::forward<T>(t), std::forward(u)); // OK, uses “swappable with'' conditions // for rvalues and lvalues } // Preconditions: T meets the Cpp17Swappable requirements. template<class T> void lv_swap(T& t1, T& t2) { using std::swap; swap(t1, t2); // OK, uses swappable conditions for lvalues of type T } namespace N { struct A { int m; }; struct Proxy { A* a; }; Proxy proxy(A& a) { return Proxy{ &a }; } void swap(A& x, Proxy p) { std::swap(x.m, p.a->m); // OK, uses context equivalent to swappable // conditions for fundamental types } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry constraint } int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }

— end example]

16.4.4.4 Cpp17NullablePointer requirements [nullablepointer.requirements]

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

A type P meets the Cpp17NullablePointer requirements if

(1.1)
P meets the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, Cpp17Swappable, and Cpp17Destructible requirements,
(1.2)
the expressions shown in Table 36 are valid and have the indicated semantics, and
(1.3)
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]

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

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

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

In Table 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

(1.1)
it is a function object type ([function.objects]),
(1.2)
it meets the Cpp17CopyConstructible (Table 32) and Cpp17Destructible (Table 35) requirements, and
(1.3)
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]

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

16.4.4.6 Cpp17Allocator requirements [allocator.requirements]

16.4.4.6.1 General [allocator.requirements.general]

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

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

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

In [allocator.requirements],

(2.1)
T, U, C denote any cv-unqualified object type ([basic.types.general]),
(2.2)
X denotes an allocator class for type T,
(2.3)
Y denotes the corresponding allocator class for type U,
(2.4)
XX denotes the type allocator_traits<X>,
(2.5)
YY denotes the type allocator_traits<Y>,
(2.6)
a, a1, a2 denote lvalues of type X,
(2.7)
u denotes the name of a variable being declared,
(2.8)
b denotes a value of type Y,
(2.9)
c denotes a pointer of type C* through which indirection is valid,
(2.10)
p denotes a value of type XX::pointer obtained by calling a1.allocate, where a1 == a,
(2.11)
q denotes a value of type XX::const_pointer obtained by conversion from a value p,
(2.12)
r denotes a value of type T& obtained by the expression *p,
(2.13)
w denotes a value of type XX::void_pointer obtained by conversion from a value p,
(2.14)
x denotes a value of type XX::const_void_pointer obtained by conversion from a value q or a value w,
(2.15)
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,
(2.16)
n denotes a value of type XX::size_type,
(2.17)
Args denotes a template parameter pack, and
(2.18)
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.

[Note 1:

There are no program-defined specializations of allocator_traits.

— end note]

🔗

typename X::pointer

Remarks: Default: T*

🔗

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>

🔗

typename X::value_type

Result: Identical to T.

🔗

typename X::size_type

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

Result: Y

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::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::other is the same type as SomeAllocator, where SomeAllocator<T>::value_type is T and SomeAllocator::value_type is U.

— end note]

🔗

*p

Result: T&

🔗

*q

Result: const T&

Postconditions: *q refers to the same object as *p.

🔗

p->m

Result: Type of T::m.

Preconditions: (*p).m is well-defined.

Effects: Equivalent to (*p).m.

🔗

q->m

Result: Type of T::m.

Preconditions: (*q).m is well-defined.

Effects: Equivalent to (*q).m.

🔗

static_cast<XX::pointer>(w)

Result: XX::pointer

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)

Result: XX::pointer

Postconditions: Same as p.

🔗

a.allocate(n)

Result: XX::pointer

Effects: Memory is allocated for an array of n T and such an object is created but array elements are not constructed.

[Example 1:

When reusing storage denoted by some pointer value p, launder(reinterpret_cast<T*>(new (p) byte[n * sizeof(T)])) can be used to implicitly create a suitable array object and obtain a pointer to it.

— end example]

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.

🔗

a.allocate(n, y)

Result: XX::pointer

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)

🔗

a.allocate_at_least(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}.

🔗

a.deallocate(p, n)

Result: (not used)

Preconditions:

(48.1)
If p is memory that was obtained by a call to a.allocate_at_least, let ret be the value returned and req be the value passed as the first argument of that call.

p is equal to ret.ptr and n is a value such that req ≤ n ≤ ret.count.
(48.2)
Otherwise, p is a pointer value obtained from allocate.

n equals the value passed as the first argument to the invocation of allocate which returned p.

p has not been invalidated by an intervening call to deallocate.

Throws: Nothing.

🔗

a.max_size()

Result: XX::size_type

Returns: The largest value n that can meaningfully be passed to a.allocate(n).

Remarks: Default: numeric_limits<size_type>::max() / sizeof(value_type)

🔗

a1 == a2

Result: bool

Returns: true only if storage allocated from each can be deallocated via the other.

Throws: Nothing.

Remarks: operator== shall be reflexive, symmetric, and transitive.

🔗

a1 != a2

Result: bool

Returns: !(a1 == a2).

🔗

a == b

Result: bool

Returns: a == YY::rebind_alloc<T>(b).

🔗

a != b

Result: bool

Returns: !(a == b).

🔗

X u(a);
X u = a;

Postconditions: u == a

Throws: Nothing.

🔗

X u(b);

Postconditions: Y(u) == b and u == X(b).

Throws: Nothing.

🔗

X u(std::move(a));
X u = std::move(a);

Postconditions: The value of a is unchanged and is equal to u.

Throws: Nothing.

🔗

X u(std::move(b));

Postconditions: u is equal to the prior value of X(b).

Throws: Nothing.

🔗

a.construct(c, args...)

Result: (not used)

Effects: Constructs an object of type C at c.

Remarks: Default: construct_at(c, std::forward<Args>(args)...)

🔗

a.destroy(c)

Result: (not used)

Effects: Destroys the object at c.

Remarks: Default: destroy_at(c)

🔗

a.select_on_container_copy_construction()

Result: X

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.

[Note 4:

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

— end note]

[Example 2:

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

— end example]

The following exposition-only concept defines the minimal requirements on an Allocator type.

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:

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