allocator Class

Article
07/29/2015

The template class describes an object that manages storage allocation and freeing for arrays of objects of type Type. An object of class allocator is the default allocator object specified in the constructors for several container template classes in the Standard C++ Library.

template <class Type> 
class allocator

Parameters

Type
The type of object for which storage is being allocated or deallocated.

Remarks

All the Standard Template Library containers have a template parameter that defaults to allocator. Template class allocator supplies several type definitions that are rather pedestrian. They hardly seem worth defining. But another class with the same members might choose more interesting alternatives. Constructing a container with an allocator object of such a class gives individual control over allocation and freeing of elements controlled by that container.

For example, an allocator object might allocate storage on a private heap. It might allocate storage on a far heap, requiring nonstandard pointers to access the allocated objects. It might also specify, through the type definitions it supplies, that elements be accessed through special accessor objects that manage shared memory, or perform automatic garbage collection. Hence, a class that allocates storage using an allocator object should use these types for declaring pointer and reference objects, as the containers in the Standard C++ Library do.

When you derive from allocator class, you have to provide a rebind struct, whose _Other typedef references your newly-derived class.

Thus, an allocator defines the following types:

pointer behaves like a pointer to Type.
const_pointer behaves like a const pointer to Type.
reference behaves like a reference to Type.
const_reference behaves like a const reference to Type.

These Types specify the form that pointers and references must take for allocated elements. (allocator::pointer is not necessarily the same as Type* for all allocator objects, even though it has this obvious definition for class allocator.)

Members

Constructors

allocator

Constructors used to create allocator objects.

Typedefs

const_pointer	A type that provides a constant pointer to the type of object managed by the allocator.
const_reference	A type that provides a constant reference to type of object managed by the allocator.
difference_type	A signed integral type that can represent the difference between values of pointers to the type of object managed by the allocator.
pointer	A type that provides a pointer to the type of object managed by the allocator.
reference	A type that provides a reference to the type of object managed by the allocator.
size_type	An unsigned integral type that can represent the length of any sequence that an object of template class allocator can allocate.
value_type	A type that is managed by the allocator.

Member Functions

address	Finds the address of an object whose value is specified.
allocate	Allocates a block of memory large enough to store at least some specified number of elements.
construct	Constructs a specific type of object at a specified address that is initialized with a specified value.
deallocate	Frees a specified number of objects from storage beginning at a specified position.
destroy	Calls an objects destructor without deallocating the memory where the object was stored.
max_size	Returns the number of elements of type Type that could be allocated by an object of class allocator before the free memory is used up.
rebind	A structure that enables an allocator for objects of one type to allocate storage for objects of another type.

Operators

operator=

Assigns one allocator object to another allocator object.

Requirements

Header: <memory>

Namespace: std

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