NAME¶

std::allocator_traits::allocate_at_least - std::allocator_traits::allocate_at_least

Synopsis¶

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

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.

Example¶

This section is incomplete
Reason: no example

See also¶

allocate_at_least allocates uninitialized storage at least as large as requested
(C++23) size
(public member function of std::allocator<T>)

Category:¶

* Todo no example