std::uses_allocator
| Defined in header <memory>
|
||
| template< class T, class Alloc > struct uses_allocator; |
(since C++11) | |
If T has a nested type allocator_type which is convertible from Alloc, the member constant value is true. Otherwise value is false.
Helper variable template
| template< class T, class Alloc > constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value; |
(since C++17) | |
Inherited from std::integral_constant
Member constants
| value [static] |
true if T uses allocator Alloc, false otherwise (public static member constant) |
Member functions
| operator bool |
converts the object to bool, returns value (public member function) |
| operator() (C++14) |
returns value (public member function) |
Member types
| Type | Definition |
value_type
|
bool |
type
|
std::integral_constant<bool, value> |
Uses-allocator construction
There are three conventions of passing an allocator alloc to a constructor of some type T:
- If
Tdoes not use a compatible allocator (std::uses_allocator_v<T, Alloc> is false), thenallocis ignored. - Otherwise, std::uses_allocator_v<T, Alloc> is true, and
- if
Tuses the leading-allocator convention (is invocable as T(std::allocator_arg, alloc, args...)), then uses-allocator construction uses this form. - if
Tuses the trailing-allocator convention (is invocable as T(args..., alloc)), then uses-allocator construction uses this form. - Otherwise, the program is ill-formed (this means std::uses_allocator_v<T, Alloc> is true, but the type does not follow either of the two allowed conventions).
- if
- As a special case, std::pair is treated as a uses-allocator type even though
std::uses_allocatoris false for pairs (unlike e.g. std::tuple): see pair-specific overloads of std::pmr::polymorphic_allocator::construct and std::scoped_allocator_adaptor::construct(until C++20)std::uses_allocator_construction_args(since C++20).
|
The utility functions std::make_obj_using_allocator, and std::uninitialized_construct_using_allocator may be used to explicitly create an object following the above protocol, and std::uses_allocator_construction_args can be used to prepare the argument list that matches the flavor of uses-allocator construction expected by the type. |
(since C++20) |
Specializations
Given a program-defined type T that does not have a nested allocator_type, a program can specialize std::uses_allocator to derive from std::true_type for T if any of the following requirements is satisfied:
-
Thas a constructor which takes std::allocator_arg_t as the first argument, andAllocas the second argument. -
Thas a constructor which takesAllocas the last argument.
In the above, Alloc is a type that satisfies Allocator or is a pointer type convertible to std::experimental::pmr::memory_resource*(library fundamentals TS).
The following specializations are already provided by the standard library:
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| specializes the std::uses_allocator type trait (class template specialization) | |
| (C++11) (until C++17) |
specializes the std::uses_allocator type trait (class template specialization) |
| specializes the std::uses_allocator type trait (class template specialization) | |
| (C++11) (until C++17) |
specializes the std::uses_allocator type trait (class template specialization) |
Notes
This type trait is used by std::tuple, std::scoped_allocator_adaptor, and std::pmr::polymorphic_allocator. It may also be used by custom allocators or wrapper types to determine whether the object or member being constructed is itself capable of using an allocator (e.g. is a container), in which case an allocator should be passed to its constructor.
See also
| (C++11) |
a tag used to select allocator-aware constructors (tag) |
| prepares the argument list matching the flavor of uses-allocator construction required by the given type (function template) | |
| (C++20) |
creates an object of the given type by means of uses-allocator construction (function template) |
| creates an object of the given type at specified memory location by means of uses-allocator construction (function template) | |
| (C++11) |
implements multi-level allocator for multi-level containers (class template) |