std::allocator_traits<Alloc>::allocate_at_least

 

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std::allocator_traits

Member functions

allocator_traits::allocate (C++11)

allocator_traits::allocate_at_least (C++23)

allocator_traits::deallocate (C++11)

allocator_traits::construct (C++11)

allocator_traits::destroy (C++11)

allocator_traits::max_size (C++11)

allocator_traits::select_on_container_copy_construction (C++11)

 

[[nodiscard]] static constexpr std::allocation_result<pointer, size_type>     allocate_at_least( Alloc& a, size_type n );		(since C++23)

allocate_at_least calls a.allocate_at_least(n) and returns its result if the call is well-formed, otherwise, it is equivalent to return {a.allocate(n), n};.

allocator_at_least tries to allocate a storage for at least n value_type objects, and provides a fallback mechanism that allocates a storage for exact n objects.

Parameters

a	-	an allocator used for allocating storage
n	-	the lower bound of number of objects to allocate storage for

Return value

a.allocate_at_least(n) if it is well-formed.

Otherwise, std::allocation_result<pointer, size_type>{a.allocate(n), n}.

Exceptions

Throws what and when the selected allocation function throws.

Notes

The allocate_at_least member function of Allocator types are mainly provided for contiguous containers, e.g. std::vector and std::basic_string, in order to reduce reallocation by making their capacity match the actually allocated size when possible. Because allocate_at_least provides a fallback mechanism, it can be directly used where appropriate.

Given an allocator object a of type Alloc, let result denote the value returned from std::allocator_traits<Alloc>::allocate_at_least(a, n), the storage should be deallocated by a.deallocate(result.ptr, m) (typically called via std::allocator_traits<Alloc>::deallocate(a, result.ptr, m)) in order to avoid memory leak.

The argument m used in deallocation must be not less than n and not greater than result.count, otherwise, the behavior is undefined. Note that n is always equal to result.count if the allocator does not provide allocate_at_least, which means that m is required to be equal to n.

Feature-test macro	Value	Std	Feature
__cpp_lib_allocate_at_least	202302L	(C++23)	allocate_at_least etc.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3887	C++23	P2652R0 did not update the feature test macro	updated

See also

allocate_at_least (C++23)

allocates uninitialized storage at least as large as requested size (public member function of std::allocator<T>)