boost_core/doc/allocator_access.qbk

[/
Copyright 2020 Glen Joseph Fernandes
(glenjofe@gmail.com)

Distributed under the Boost Software License, Version 1.0.
(http://www.boost.org/LICENSE_1_0.txt)
]

[section Allocator Access]

[simplesect Authors]

* Glen Fernandes

[endsimplesect]

[section Overview]

The header `<boost/core/allocator_access.hpp>` provides the class and function
templates to simplify allocator use. It provides the same functionality as the
C++ standard library `std::allocator_traits` but with individual templates for
each allocator feature.

These facilities also simplify existing libraries by avoiding having to check
for `BOOST_NO_CXX11_ALLOCATOR` and conditionally use `std::allocator_traits`.

[endsect]

[section Examples]

The following example shows these utilities used in the definition of
an allocator-aware container class:

```
template<class T, class A = boost::default_allocator<T> >
class container
    : boost::empty_value<typename boost::allocator_rebind<A, T>::type> {
public:
    typedef T value_type;
    typedef A allocator_type;
    typedef typename boost::allocator_size_type<A>::type size_type;
    typedef typename boost::allocator_difference_type<A>::type difference_type;
    typedef value_type& reference;
    typedef const value_type& const_reference;
    typedef typename boost::allocator_pointer<A>::type pointer;
    typedef typename boost::allocator_const_pointer<A>::type const_pointer;
    // ...
};
```

In C++11 or above, aliases such as `boost::allocator_pointer_t<A>` can be used
instead of `typename boost::allocator_pointer<A>::type`.

[endsect]

[section Reference]

```
namespace boost {

template<class A>
struct allocator_value_type;

template<class A>
using allocator_value_type_t = typename allocator_value_type<A>::type;

template<class A>
struct allocator_pointer;

template<class A>
using allocator_pointer_t = typename allocator_pointer<A>::type;

template<class A>
struct allocator_const_pointer;

template<class A>
using allocator_const_pointer_t = typename allocator_const_pointer<A>::type;

template<class A>
struct allocator_void_pointer;

template<class A>
using allocator_void_pointer_t = typename allocator_void_pointer<A>::type;

template<class A>
struct allocator_const_void_pointer;

template<class A>
using allocator_const_void_pointer_t =
    typename allocator_const_void_pointer<A>::type;

template<class A>
struct allocator_difference_type;

template<class A>
using allocator_difference_type_t =
    typename allocator_difference_type<A>::type;

template<class A>
struct allocator_size_type;

template<class A>
using allocator_size_type_t = typename allocator_size_type<A>::type;

template<class A>
struct allocator_propagate_on_container_copy_assignment;

template<class A>
using allocator_propagate_on_container_copy_assignment_t =
    typename allocator_propagate_on_container_copy_assignment<A>::type;

template<class A>
struct allocator_propagate_on_container_move_assignment;

template<class A>
using allocator_propagate_on_container_move_assignment_t =
    typename allocator_propagate_on_container_move_assignment<A>::type;

template<class A>
struct allocator_propagate_on_container_swap;

template<class A>
using allocator_propagate_on_container_swap_t =
    typename allocator_propagate_on_container_swap<A>::type;

template<class A>
struct allocator_is_always_equal;

template<class A>
using allocator_is_always_equal_t =
    typename allocator_is_always_equal<A>::type;

template<class A, class T>
struct allocator_rebind;

template<class A, class T>
using allocator_rebind_t = typename allocator_rebind<A, T>::type;

template<class A>
allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n);

template<class A>
allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n,
    allocator_const_void_pointer_t<A> h);

template<class A>
void allocator_deallocate(A& a, allocator_pointer_t<A> p,
    allocator_size_type_t<A> n);

template<class A, class T, class... Args>
void allocator_construct(A& a, T* p, Args&&... args);

template<class A, class T>
void allocator_destroy(A& a, T* p);

template<class A>
allocator_size_type_t<A> allocator_max_size(const A& a);

template<class A>
A allocator_select_on_container_copy_construction(const A& a);

} // boost
```

[section Types]

[variablelist
[[`template<class A> struct allocator_value_type;`]
[The member `type` is `A::value_type`.]]
[[`template<class A> struct allocator_pointer;`]
[The member `type` is `A::pointer` if valid, otherwise `A::value_type*`.]]
[[`template<class A> struct allocator_const_pointer;`]
[The member `type` is `A::const_pointer` if valid, otherwise
`pointer_traits<allocator_pointer_t<A> >::rebind<const
allocator_value_type_t<A> >`.]]
[[`template<class A> struct allocator_void_pointer;`]
[The member `type` is `A::void_pointer` if valid, otherwise
`pointer_traits<allocator_pointer_t<A> >::rebind<void>`.]]
[[`template<class A> struct allocator_const_void_pointer;`]
[The member `type` is `A::const_void_pointer` if valid, otherwise
`pointer_traits<allocator_pointer_t<A> >::rebind<const void>`.]]
[[`template<class A> struct allocator_difference_type;`]
[The member `type` is `A::difference_type` if valid, otherwise
`pointer_traits<allocator_pointer_t<A> >::difference_type`.]]
[[`template<class A> struct allocator_size_type;`]
[The member `type` is `A::size_type` if valid, otherwise
`std::make_unsigned_t<allocator_difference_type_t<A> >`.]]
[[`template<class A> struct allocator_propagate_on_container_copy_assignment;`]
[The member `type` is `A::propagate_on_container_copy_assignment` if valid,
otherwise `std::false_type`.]]
[[`template<class A> struct allocator_propagate_on_container_move_assignment;`]
[The member `type` is `A::propagate_on_container_move_assignment` if valid,
otherwise `std::false_type`.]]
[[`template<class A> struct allocator_propagate_on_container_swap;`]
[The member `type` is `A::propagate_on_container_swap` if valid, otherwise
`std::false_type`.]]
[[`template<class A> struct allocator_is_always_equal;`]
[The member `type` is `A::is_always_equal` if valid, otherwise
`std::is_empty<A>::type`.]]
[[`template<class A, class T> struct allocator_rebind;`]
[The member `type` is `A::rebind<T>::other` if valid, otherwise `A<T, Args>`
if this `A` is `A<U, Args>`.]]]

[endsect]

[section Functions]

[variablelist
[[`template<class A>
allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n);`]
[Calls `a.allocate(n)`.]]
[[`template<class A> allocator_pointer_t<A> allocator_allocate(A& a,
allocator_size_type_t<A> n, allocator_const_void_pointer_t<A> hint);`]
[Calls `a.allocate(n, hint)` if valid, otherwise calls `a.allocate(n)`.]]
[[`template<class A> void allocator_deallocate(A& a, allocator_pointer_t<A> p,
allocator_size_type_t<A> n);`]
[Calls `a.deallocate(p, n)`.]]
[[`template<class A, class T, class... Args>
void allocator_construct(A& a, T*p, Args&&... args);`]
[Calls `a.construct(p, std::forward<Args>(args)...)` if valid, otherwise calls
`::new(static_cast<void*>(p)) T(std::forward<Args>(args)...)`.]]
[[`template<class A, class T> void allocator_destroy(A& a, T* p);`]
[Calls `a.destroy(p)` if valid, otherwise calls `p->~T()`.]]
[[`template<class A> allocator_size_type_t<A> allocator_max_size(const A& a);`]
[Returns `a.max_size()` if valid, otherwise returns
`std::numeric_limits<allocator_size_type_t<A> >::max() /
sizeof(A::value_type)`.]]
[[`template<class A> A allocator_select_on_container_copy_construction(const
A& a);`]
[Returns `a.select_on_container_copy_construction()` if valid, otherwise
returns `a`.]]]

[endsect]

[endsect]

[section Acknowledgments]

Glen Fernandes implemented the allocator access utilities.

[endsect]

[endsect]