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std::allocator<T>::allocate

From cppreference.com
< cpp‎ | memory‎ | allocator
 
 
Dynamic memory management
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(C++11)
(C++11)
(C++11)
(until C++17)
(C++11)
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std::allocator
Member functions
(until C++20)
allocator::allocate
(until C++20)
(until C++20)
Non-member functions
 
(1)
pointer allocate( size_type n, const void * hint = 0 );
(until C++17)
T* allocate( std::size_t n, const void * hint);
(since C++17)
(deprecated)
(removed in C++20)
(2)
T* allocate( std::size_t n );
(since C++17)
(until C++20)
[[nodiscard]] constexpr T* allocate( std::size_t n );
(since C++20)

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)

Contents

[edit] Parameters

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

[edit] Return value

Pointer to the first element of an array of n objects of type T whose elements have not been constructed yet.

[edit] 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.

[edit] 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 arithmethic of T* is well-defined within the allocated array, but the behavior is undefined if elements are accessed.

[edit] 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 3190 C++11 allocate might allocate storage of wrong size throws bad_array_new_length instead

[edit] See also

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