1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> |
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2 | <html> |
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3 | <head> |
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4 | <title>pointer_cast.hpp</title> |
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5 | </head> |
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6 | <body> |
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7 | <h1><IMG height="86" alt="C++ Boost" src="../../boost.png" width="277" align="middle" border="0">Pointer |
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8 | cast functions</h1> |
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9 | <p>The pointer cast functions (<code>boost::static_pointer_cast</code> <code>boost::dynamic_pointer_cast</code> |
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10 | <code>boost::reinterpret_pointer_cast</code> <code>boost::const_pointer_cast</code>) |
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11 | provide a way to write generic pointer castings for raw pointers. The functions |
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12 | are defined in <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A>.</CITE></p> |
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13 | <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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14 | <h2><a name="rationale">Rationale</a></h2> |
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15 | <P>Boost smart pointers usually overload those functions to provide a mechanism to |
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16 | emulate pointers casts. For example, <code>boost::shared_ptr<...></code> implements |
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17 | a static pointer cast this way:</P> |
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18 | <pre> |
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19 | template<class T, class U> |
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20 | shared_ptr<T> static_pointer_cast(shared_ptr<U> const &r); |
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21 | </pre> |
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22 | <P>Pointer cast functions from <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A></CITE> |
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23 | are overloads of <code>boost::static_pointer_cast</code>, <code>boost::dynamic_pointer_cast</code>, |
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24 | <code>boost::reinterpret_pointer_cast</code> and <code>boost::const_pointer_cast</code> |
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25 | for raw pointers. This way when developing pointer type independent classes, |
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26 | for example, memory managers or shared memory compatible classes, the same code |
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27 | can be used for raw and smart pointers.</p> |
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28 | <H2><A name="synopsis">Synopsis</A></H2> |
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29 | <BLOCKQUOTE> |
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30 | <PRE> |
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31 | namespace boost { |
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32 | |
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33 | template<class T, class U> |
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34 | inline T* static_pointer_cast(U *ptr) |
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35 | { return static_cast<T*>(ptr); } |
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36 | |
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37 | template<class T, class U> |
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38 | inline T* dynamic_pointer_cast(U *ptr) |
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39 | { return dynamic_cast<T*>(ptr); } |
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40 | |
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41 | template<class T, class U> |
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42 | inline T* const_pointer_cast(U *ptr) |
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43 | { return const_cast<T*>(ptr); } |
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44 | |
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45 | template<class T, class U> |
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46 | inline T* reinterpret_pointer_cast(U *ptr) |
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47 | { return reinterpret_cast<T*>(ptr); } |
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48 | |
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49 | } // namespace boost |
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50 | </PRE> |
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51 | </BLOCKQUOTE> |
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52 | <P>As you can see from the above synopsis, the pointer cast functions are just |
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53 | wrappers around standard C++ cast operators.</P> |
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54 | <H2><A name="example">Example</A></H2> |
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55 | <BLOCKQUOTE> |
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56 | <PRE> |
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57 | #include <boost/pointer_cast.hpp> |
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58 | #include <boost/shared_ptr.hpp> |
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59 | |
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60 | class base |
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61 | { |
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62 | public: |
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63 | |
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64 | virtual ~base() |
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65 | { |
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66 | } |
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67 | }; |
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68 | |
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69 | class derived: public base |
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70 | { |
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71 | }; |
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72 | |
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73 | template <class BasePtr> |
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74 | void check_if_it_is_derived(const BasePtr &ptr) |
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75 | { |
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76 | assert(boost::dynamic_pointer_cast<derived>(ptr) != 0); |
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77 | } |
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78 | |
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79 | int main() |
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80 | { |
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81 | <I>// Create a raw and a shared_ptr</I> |
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82 | |
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83 | base *ptr = new derived; |
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84 | boost::shared_ptr<base> sptr(new derived); |
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85 | |
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86 | <I>// Check that base pointer points actually to derived class</I> |
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87 | |
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88 | check_if_it_is_derived(ptr); |
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89 | check_if_it_is_derived(sptr); |
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90 | |
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91 | // <EM>Ok!</EM> |
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92 | |
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93 | delete ptr; |
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94 | return 0; |
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95 | }</PRE> |
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96 | </BLOCKQUOTE> |
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97 | <P>The example demonstrates how the generic pointer casts help us create pointer |
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98 | independent code.</P> |
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99 | <hr> |
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100 | <p>Revised: $Date: 2005/12/06 13:26:13 $</p> |
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101 | <p>Copyright 2005 Ion Gaztañaga. Use, modification, and distribution are subject to |
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102 | the Boost Software License, Version 1.0. (See accompanying file <A href="../../LICENSE_1_0.txt"> |
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103 | 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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104 | </body> |
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105 | </html> |
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