pointer_traits

Overview

The header <boost/core/pointer_traits.hpp> provides the class template boost::pointer_traits to facilitate use of pointer-like types. The C++11 standard library introduced std::pointer_traits along with an allocator model which supported pointer-like types in addition to plain raw pointers. This implementation also supports C++98.

It also provides the function template boost::to_address to obtain a raw pointer from an object of any pointer-like type.

Examples

The following example allocates storage and constructs an object in that storage using an allocator.

template<class Allocator>
void function(Allocator& a)
{
    auto p = a.allocate(1);
    std::allocator_traits<Allocator>::construct(a, boost::to_address(p));
}

Reference

namespace boost {
  template<class T> struct pointer_traits {
    typedef T pointer;
    typedef see below element_type;
    typedef see below difference_type;

    template<class U> struct rebind_to { typedef see below type; };
    template<class U> using rebind = typename rebind_to<U>::type;

    static pointer pointer_to(see below v);
  };

  template<class T> struct pointer_traits<T*> {
    typedef T* pointer;
    typedef T element_type;
    typedef std::ptrdiff_t difference_type;

    template<class U> struct rebind_to { typedef U* type; };
    template<class U> using rebind = typename rebind_to<U>::type;

    static pointer pointer_to(see below v) noexcept;
  };

  template<class T>
  constexpr T* to_address(T* v) noexcept;

  template<class T>
  auto to_address(const T& v) noexcept;
}

Member types

typedef see below element_type;: T::element_type if such a type exists; otherwise U if T is a class template instantiation of the form Pointer<U, Args>, where Args is zero or more type arguments; otherwise the specialization is ill-formed.
typedef see below difference_type;: T::difference_type if such a type exists; otherwise std::ptrdiff_t.
template<class U> struct rebind_to { typedef see below type; };: type is T::rebind<U> if such a type exists; otherwise, Pointer<V, Args> if T is a class template instantiation of the form Pointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind_to is ill-formed.

Member functions

static pointer pointer_traits::pointer_to(see below v);

Remark: If element_type is a void type, the type of v is unspecified; otherwise, it is element_type&.
Returns: A pointer to v obtained by calling T::pointer_to(v).

static pointer pointer_traits<T*>::pointer_to(see below v) noexcept;

Remark: If element_type is a void type, the type of v is unspecified; otherwise, it is element_type&.
Returns: addressof(v).

Optional members

static element_type* to_address(pointer v) noexcept;

Returns: A pointer of type element_type* that references the same location as the argument p.
Note: This function should be the inverse of pointer_to. If defined, it customizes the behavior of the non-member function to_address.

Free functions

template<class T> constexpr T* to_address(T* v) noexcept;

Returns: v.

template<class T> auto to_address(const T& v) noexcept;

Returns: pointer_traits<T>::to_address(v) if that expression is well-formed, otherwise to_address(v.operator->()).

Acknowledgments

Glen Fernandes implemented pointer_traits and to_address with reviews and guidance from Peter Dimov.

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