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119 lines
4.5 KiB
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<title>pointer_cast</title>
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<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
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width="277" align="middle" border="0">pointer_cast</h1>
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<p>The pointer cast functions (<code>boost::static_pointer_cast</code> <code>boost::dynamic_pointer_cast</code>
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<code>boost::reinterpret_pointer_cast</code> <code>boost::const_pointer_cast</code>)
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provide a way to write generic pointer castings for raw pointers and std::shared_ptr. The functions
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are defined in <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A>.</CITE></p>
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<P>There is test/example code in <CITE><A href="test/pointer_cast_test.cpp">pointer_cast_test.cpp</A></CITE>.</p>
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<h2><a name="rationale">Rationale</a></h2>
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<P>Boost smart pointers usually overload those functions to provide a mechanism to
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emulate pointers casts. For example, <code>boost::shared_ptr<...></code> implements
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a static pointer cast this way:</P>
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<pre>
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template<class T, class U>
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shared_ptr<T> static_pointer_cast(shared_ptr<U> const &r);
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</pre>
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<P>Pointer cast functions from <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A></CITE>
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are overloads of <code>boost::static_pointer_cast</code>, <code>boost::dynamic_pointer_cast</code>,
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<code>boost::reinterpret_pointer_cast</code> and <code>boost::const_pointer_cast</code>
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for raw pointers and std::shared_ptr. This way when developing pointer type independent classes,
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for example, memory managers or shared memory compatible classes, the same code
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can be used for raw and smart pointers.</p>
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<H2><A name="synopsis">Synopsis</A></H2>
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<BLOCKQUOTE>
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<PRE>
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namespace boost {
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template<class T, class U>
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inline T* static_pointer_cast(U *ptr)
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{ return static_cast<T*>(ptr); }
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template<class T, class U>
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inline T* dynamic_pointer_cast(U *ptr)
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{ return dynamic_cast<T*>(ptr); }
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template<class T, class U>
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inline T* const_pointer_cast(U *ptr)
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{ return const_cast<T*>(ptr); }
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template<class T, class U>
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inline T* reinterpret_pointer_cast(U *ptr)
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{ return reinterpret_cast<T*>(ptr); }
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template<class T, class U>
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inline std::shared_ptr<U> static_pointer_cast(std::shared_ptr<T> ptr);
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template<class T, class U>
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inline std::shared_ptr<U> dynamic_pointer_cast(std::shared_ptr<T> ptr);
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template<class T, class U>
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inline std::shared_ptr<U> const_pointer_cast(std::shared_ptr<T> ptr);
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template<class T, class U>
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inline std::shared_ptr<U> reinterpret_pointer_cast(std::shared_ptr<T> ptr);
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} // namespace boost
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</PRE>
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</BLOCKQUOTE>
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<P>As you can see from the above synopsis, the pointer cast functions are just
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wrappers around standard C++ cast operators.</P>
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<H2><A name="example">Example</A></H2>
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<BLOCKQUOTE>
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<PRE>
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#include <boost/pointer_cast.hpp>
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#include <boost/shared_ptr.hpp>
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class base
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{
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public:
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virtual ~base()
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{
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}
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};
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class derived: public base
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{
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};
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template <class BasePtr>
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void check_if_it_is_derived(const BasePtr &ptr)
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{
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assert(boost::dynamic_pointer_cast<derived>(ptr) != 0);
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}
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int main()
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{
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<I>// Create a raw and a shared_ptr</I>
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base *ptr = new derived;
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boost::shared_ptr<base> sptr(new derived);
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<I>// Check that base pointer points actually to derived class</I>
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check_if_it_is_derived(ptr);
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check_if_it_is_derived(sptr);
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// <EM>Ok!</EM>
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delete ptr;
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return 0;
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}</PRE>
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</BLOCKQUOTE>
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<P>The example demonstrates how the generic pointer casts help us create pointer
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independent code.</P>
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<hr>
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<p>$Date$</p>
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<p>Copyright 2005 Ion Gaztañaga. Use, modification, and distribution are subject to
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the Boost Software License, Version 1.0. (See accompanying file <A href="../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</A> or a copy at <<A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>>.)</p>
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