std::allocator<T>::allocate

Since C++20

// 2)
[[nodiscard]] constexpr T* allocate( std::size_t n );

Since C++17, Removed in C++20

// 1)
T* allocate( std::size_t n, const void* hint );
// 2)
T* allocate( std::size_t n );

Until C++17

// 1)
pointer allocate( size_type n, const void* hint = 0 );

Allocates n * sizeof(T) bytes of uninitialized storage by calling ::operator new(std::size_t) or ::operator new(std::size_t, std::align_val_t) (since C++17) but it is unspecified when and how this function is called. The pointer hint may be used to provide locality of reference: the allocator, if supported by the implementation, will attempt to allocate the new memory block as close as possible to hint.

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

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

In order to use this function in a constant expression, the allocated storage must be deallocated within the evaluation of the same expression. (since C++20)

Parameters

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

Return value

Pointer to the first element of an array of n 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. (since C++11)

Throws std::bad_alloc if allocation fails.

Notes

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() 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.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 578	C++98	hint was required to be either 0 or a pointer previously returned from allocate() and not yet passed to deallocate()	not required
LWG 3190	C++11	allocate() might allocate storage of wrong size	throws std::bad_array_new_length instead