16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.4 Requirements on types and expressions [utility.requirements]

16.4.4.6 Cpp17Allocator requirements [allocator.requirements]

16.4.4.6.1 General [allocator.requirements.general]

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

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

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

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

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

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

Table 35 describes the types manipulated through allocators.

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

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

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

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

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

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

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

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

[Note 2:

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

— end note]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The allocator also may silently ignore the requested alignment.

[Note 3:

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

— end note]

[Example 2:

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

— end example]

176)

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

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

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