# Pointer cast functions

The pointer cast functions (`boost::static_pointer_cast` `boost::dynamic_pointer_cast` `boost::reinterpret_pointer_cast` `boost::const_pointer_cast`) provide a way to write generic pointer castings for raw pointers. The functions are defined in boost/pointer_cast.hpp.

There is test/example code in pointer_cast_test.cpp.

## Rationale

Boost smart pointers usually overload those functions to provide a mechanism to emulate pointers casts. For example, `boost::shared_ptr<...>` implements a static pointer cast this way:

    template<class T, class U>
        shared_ptr<T> static_pointer_cast(shared_ptr<U> const &r);

Pointer cast functions from boost/pointer_cast.hpp are overloads of `boost::static_pointer_cast`, `boost::dynamic_pointer_cast`, `boost::reinterpret_pointer_cast` and `boost::const_pointer_cast` for raw pointers. This way when developing pointer type independent classes, for example, memory managers or shared memory compatible classes, the same code can be used for raw and smart pointers.

## Synopsis

>     namespace boost {

>     template<class T, class U>
>     inline T* static_pointer_cast(U *ptr)
>       { return static_cast<T*>(ptr); }

>     template<class T, class U>
>     inline T* dynamic_pointer_cast(U *ptr)
>       { return dynamic_cast<T*>(ptr); }

>     template<class T, class U>
>     inline T* const_pointer_cast(U *ptr)
>       { return const_cast<T*>(ptr); }

>     template<class T, class U>
>     inline T* reinterpret_pointer_cast(U *ptr)
>       { return reinterpret_cast<T*>(ptr); }

>     } // namespace boost

As you can see from the above synopsis, the pointer cast functions are just wrappers around standard C++ cast operators.

## Example

>     #include <boost/pointer_cast.hpp>
>     #include <boost/shared_ptr.hpp>

>     class base
>     {
>     public:

>        virtual ~base()
>        {
>        }
>     };

>     class derived: public base
>     {
>     };

>     template <class BasePtr>
>     void check_if_it_is_derived(const BasePtr &ptr)
>     {
>        assert(boost::dynamic_pointer_cast<derived>(ptr) != 0);
>     }

>     int main()
>     {
>        // Create a raw and a shared_ptr

>        base *ptr = new derived;
>        boost::shared_ptr<base> sptr(new derived);

>        // Check that base pointer points actually to derived class

>        check_if_it_is_derived(ptr);
>        check_if_it_is_derived(sptr);

>        // Ok!

>        delete ptr;
>        return 0;
>     }

The example demonstrates how the generic pointer casts help us create pointer independent code.

Revised: $Date: 2005/12/06 13:26:13 $

Copyright 2005 Ion Gaztaņaga. Use, modification, and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)