doc/smart_ptr/pointer_to_other.adoc

////
Copyright 2005, 2006 Ion Gaztañaga
Copyright 2005, 2006, 2017 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#pointer_to_other]
# pointer_to_other
:toc:
:toc-title:
:idprefix: pointer_to_other_

## Description

The `pointer_to_other` utility provides a way, given a source pointer type, to obtain a pointer of the same type
to another pointee type.

There is test/example code in link:../../test/pointer_to_other_test.cpp[pointer_to_other_test.cpp].

## Rationale

When building pointer independent classes, like memory managers, allocators, or containers, there is often a need to
define pointers generically, so that if a template parameter represents a pointer (for example, a raw or smart pointer
to an `int`), we can define another pointer of the same type to another pointee (a raw or smart pointer to a `float`.)

```
template <class IntPtr> class FloatPointerHolder
{
    // Let's define a pointer to a float

    typedef typename boost::pointer_to_other
        <IntPtr, float>::type float_ptr_t;

    float_ptr_t float_ptr;
};
```

## Synopsis

`pointer_to_other` is defined in `<boost/smart_ptr/pointer_to_other.hpp>`.

```
namespace boost {

  template<class T, class U> struct pointer_to_other;

  template<class T, class U,
    template <class> class Sp>
      struct pointer_to_other< Sp<T>, U >
  {
    typedef Sp<U> type;
  };

  template<class T, class T2, class U,
    template <class, class> class Sp>
      struct pointer_to_other< Sp<T, T2>, U >
  {
    typedef Sp<U, T2> type;
  };

  template<class T, class T2, class T3, class U,
    template <class, class, class> class Sp>
      struct pointer_to_other< Sp<T, T2, T3>, U >
  {
    typedef Sp<U, T2, T3> type;
  };

  template<class T, class U>
    struct pointer_to_other< T*, U > 
  {
    typedef U* type;
  };
}
```

If these definitions are not correct for a specific smart pointer, we can define a specialization of `pointer_to_other`.

## Example

```
// Let's define a memory allocator that can
// work with raw and smart pointers

#include <boost/pointer_to_other.hpp>

template <class VoidPtr>
class memory_allocator
{
    // Predefine a memory_block

    struct block;

    // Define a pointer to a memory_block from a void pointer
    // If VoidPtr is void *, block_ptr_t is block*
    // If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block>

    typedef typename boost::pointer_to_other
        <VoidPtr, block>::type block_ptr_t;

    struct block
    {
        std::size_t size;
        block_ptr_t next_block;
    };

    block_ptr_t free_blocks;
};
```

As we can see, using `pointer_to_other` we can create pointer independent code.