NAME¶

std::allocator::allocate_at_least - std::allocator::allocate_at_least

Synopsis¶

[[nodiscard]] constexpr std::allocation_result<T*, std::size_t> (since C++23)
allocate_at_least( std::size_t n );

Allocates count * sizeof(T) bytes of uninitialized storage, where count is an
unspecified integer value not less than n, by calling ::operator new (an additional
std::align_val_t argument might be provided), but it is unspecified when and how
this function is called.

Then, this function creates an array of type T[count] in the storage and starts its
lifetime, but does not start lifetime of any of its elements.

In order to use this function in a constant expression, the allocated storage must
be deallocated within the evaluation of the same expression.

Use of this function is ill-formed if T is an incomplete type.

Parameters¶

n - the lower bound of number of objects to allocate storage for

Return value¶

std::allocation_result<T*>{p, count}, where p points to the first element of an
array of count objects of type T whose elements have not been constructed yet.

Exceptions¶

Throws std::bad_array_new_length if std::numeric_limits<std::size_t>::max() /
sizeof(T) < n, or std::bad_alloc if allocation fails.

Notes¶

allocate_at_least is 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.

The "unspecified when and how" wording makes it possible to combine or optimize away
heap allocations made by the standard library containers, even though such
optimizations are disallowed for direct calls to ::operator new. For example, this
is implemented by libc++ ([1] and [2]).

After calling allocate_at_least and before construction of elements, pointer
arithmetic of T* is well-defined within the allocated array, but the behavior is
undefined if elements are accessed.

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

Example¶

// Run this code

#include <memory>
#include <print>

int main()
{
const std::size_t count{69};
std::allocator<int> alloc;
std::allocation_result res{alloc.allocate_at_least(count)};
std::print("count: {}\n"
"res.ptr: {}\n"
"res.count: {}\n", count, res.ptr, res.count);

/* construct, use, then destroy elements */

alloc.deallocate(res.ptr, res.count);
}

Possible output:¶

count: 69
res.ptr: 0x555a486a0960
res.count: 96

See also¶

allocation_result records the address and the actual size of storage allocated by
(C++23) allocate_at_least
(class template)
allocates storage at least as large as the requested size via an
allocate_at_least allocator
[static] (C++23) (public static member function of std::allocator_traits<Alloc>)