NAME¶

std::allocator::allocate - std::allocator::allocate

Synopsis¶

pointer allocate( size_type n, const void* (until C++17)
hint = 0 );
T* allocate( std::size_t n, const void* (since C++17)
hint ); (deprecated)
(1) (removed in C++20)
T* allocate( std::size_t n ); (since C++17)
(2) (until C++20)
[[nodiscard]] constexpr T* allocate( (since C++20)
std::size_t n );

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 (since C++20)
expression.

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 (since C++11)
std::numeric_limits<std::size_t>::max() / sizeof(T) < n.

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

See also¶

allocate allocates uninitialized storage using the allocator
[static] (public static member function of std::allocator_traits<Alloc>)