std::allocator_traits<Alloc>::allocate

Since C++20

// 1)
[[nodiscard]] static constexpr pointer allocate( Alloc& a, size_type n );
// 2)
[[nodiscard]] static constexpr pointer allocate( Alloc& a, size_type n,
                                                 const_void_pointer hint );

Since C++11, Until C++20

// 1)
static pointer allocate( Alloc& a, size_type n );
// 2)
static pointer allocate( Alloc& a, size_type n,
                         const_void_pointer hint );

Uses the allocator a to allocate n*sizeof(Alloc::value_type) bytes of uninitialized storage. An array of type Alloc::value_type[n] is created in the storage, but none of its elements are constructed.

1. Calls a.allocate(n)
   
2. Additionally passes memory locality hint hint. Calls a.allocate(n, hint) if possible. If not possible (e.g. a has no two-argument member function allocate() ), calls a.allocate(n)

Parameters

a - allocator to use
n - the number of objects to allocate storage for
hint - pointer to a nearby memory location

Return value

The pointer returned by the call to a.allocate(n)

Notes

Alloc::allocate was not required to create array object until P0593R6, which made using non-default allocator for std::vector and some other containers not actually well-defined according to the core language specification.

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