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std::allocator_traits<Alloc>::allocate_at_least

From cppreference.com
 
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)



 
 
[[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.

Contents

[edit] Parameters

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

[edit] Return value

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

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

[edit] Exceptions

Throws what and when the selected allocation function throws.

[edit] 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.

[edit] Example

[edit] See also

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