pointer_cast

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, std::shared_ptr and std::unique_ptr. 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, std::shared_ptr and std::unique_ptr. 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); }

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

template<class T, class U>
inline std::shared_ptr<T> dynamic_pointer_cast(std::shared_ptr<U> const& r);

template<class T, class U>
inline std::shared_ptr<T> const_pointer_cast(std::shared_ptr<U> const& r);

template<class T, class U>
inline std::shared_ptr<T> reinterpret_pointer_cast(std::shared_ptr<U> const& r);

template<class T, class U>
inline std::unique_ptr<T> static_pointer_cast(std::unique_ptr<U>&& r);

template<class T, class U>
inline std::unique_ptr<T> dynamic_pointer_cast(std::unique_ptr<U>&& r);

template<class T, class U>
inline std::unique_ptr<T> const_pointer_cast(std::unique_ptr<U>&& r);

template<class T, class U>
inline std::unique_ptr<T> reinterpret_pointer_cast(std::unique_ptr<U>&& r);
  
} // namespace boost

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

The pointer casts for std::shared_ptr are aliases of the corresponding standard functions with the same names and equivalent to the functions taking boost::shared_ptr.

The pointer casts for std::unique_ptr are documented below.

static_pointer_cast

template<class T, class U>
  unique_ptr<T> static_pointer_cast(unique_ptr<U>&& r); // never throws

Requires: The expression static_cast<T*>( (U*)0 ) must be well-formed.

Returns: unique_ptr<T>( static_cast<typename unique_ptr<T>::element_type*>(r.release()) ).

Throws: nothing.

Notes: the seemingly equivalent expression unique_ptr<T>(static_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

const_pointer_cast

template<class T, class U>
  unique_ptr<T> const_pointer_cast(unique_ptr<U>&& r); // never throws

Requires: The expression const_cast<T*>( (U*)0 ) must be well-formed.

Returns: unique_ptr<T>( const_cast<typename unique_ptr<T>::element_type*>(r.release()) ).

Throws: nothing.

dynamic_pointer_cast

template<class T, class U>
  unique_ptr<T> dynamic_pointer_cast(unique_ptr<U>&& r);

Requires: The expression dynamic_cast<T*>( (U*)0 ) must be well-formed. T must have a virtual destructor.

Returns:

• When dynamic_cast<typename unique_ptr<T>::element_type*>(r.get()) returns a nonzero value, unique_ptr<T>(dynamic_cast<typename unique_ptr<T>::element_type*>(r.release()));
• Otherwise, unique_ptr<T>().

Throws: nothing.

reinterpret_pointer_cast

template<class T, class U>
  unique_ptr<T> reinterpret_pointer_cast(unique_ptr<U>&& r); // never throws

Requires: The expression reinterpret_cast<T*>( (U*)0 ) must be well-formed.

Returns: unique_ptr<T>( reinterpret_cast<typename unique_ptr<T>::element_type*>(r.release()) ).

Throws: nothing.

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.

Copyright 2005 Ion Gaztañaga. 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>.)