20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.3 Requirements on types and expressions [utility.requirements]

20.5.3.5 Allocator requirements [allocator.requirements]

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.
The class template allocator_­traits supplies a uniform interface to all allocator types.
Table 30 describes the types manipulated through allocators.
Table 31 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 31 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 30 and 31, the use of move and forward always refers to std​::​move and std​::​forward, respectively.
Table 30 — 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 31 — Allocator 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.178
[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.

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
​::​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
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.
Note B: If X​::​propagate_­on_­container_­copy_­assignment​::​value is true, X shall satisfy the CopyAssignable requirements and the copy operation shall not throw exceptions.
If X​::​propagate_­on_­container_­move_­assignment​::​value is true, X shall satisfy the MoveAssignable requirements 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 satisfy the requirements of CopyConstructible.
The X​::​pointer, X​::​const_­pointer, X​::​void_­pointer, and X​::​const_­void_­pointer types shall satisfy the requirements of NullablePointer.
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.
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.
[Example
:
The following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 31:
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
]
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
]
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.

20.5.3.5.1 Allocator completeness requirements [allocator.requirements.completeness]

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: