std::allocator_traits<Alloc>::allocate_at_least

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

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

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)

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 (C++23)

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