| <!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- |
| (C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com . |
| Use, modification and distribution is subject to 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) |
| --> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
| <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
| <link rel="stylesheet" type="text/css" href="style.css"> |
| <title>Serialization - BOOST_STATIC_WARNING</title> |
| </head> |
| <body link="#0000ff" vlink="#800080"> |
| <table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header"> |
| <tr> |
| <td valign="top" width="300"> |
| <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3> |
| </td> |
| <td valign="top"> |
| <h1 align="center">Serialization</h1> |
| <h2 align="center"><code>smart_cast</code></h2> |
| </td> |
| </tr> |
| </table> |
| <hr> |
| <h3>Motivation</h3> |
| To cast from one type to another related type, C++ provides the following |
| operators: |
| |
| <dl> |
| <dt><code>static_cast<T *<>(U *)<br>static_cast<T &<>(U &)</code></dt> |
| <dd> |
| <ul> |
| <li>required if neither T nor U are not polymorphic |
| <li>permitted in other cases. |
| <li>fails to detect erroneas casts of polymophic pointers/references at runtime. |
| <li>does not permit "cross casting" |
| <li>inline function calls can be optimized away during compile time. |
| </ul> |
| </dd> |
| <p> |
| <dt><code>dynamic_cast<T *<>(U *)<br>dynamic_cast<T &<>(U &)</code></dt> |
| <dd> |
| <ul> |
| <li>permitted if either T or U are polymorphic |
| <li>prohibited in other cases. |
| <li>throws exception on detecting erroneas casts of polymorphic pointers/references |
| at runtime. |
| <li>permits "cross casting" |
| <li>cannot optimise inline virtual functions at compile time. |
| </ul> |
| </dd> |
| </dl> |
| |
| These rules can make it difficult to use casting with a function template argument. |
| Consider the following example: |
| |
| <pre><code> |
| #include <boost/serialization/smart_cast.hpp> |
| |
| struct top { |
| }; |
| |
| struct base1 : public top { |
| bool is_storable() const { |
| return true; |
| } |
| virtual ~base1(); |
| }; |
| |
| struct base2 { |
| virtual ~base2(); |
| }; |
| |
| struct derived1 : |
| public base1 |
| { |
| derived1(); |
| }; |
| |
| struct derived2 : |
| public base1, |
| public base2 |
| { |
| derived2(); |
| }; |
| |
| template<class T> |
| bool is_storable(T &t){ |
| // what type of cast to use here? |
| |
| // this fails at compiler time when T == base2 |
| // return static_cast<base1 &>(t).is_storable(); |
| |
| // this fails at compiler time when T == top |
| // otherwise it works but cannot optimize inline function call |
| // return dynamic_cast<base1 &>(t).is_storable(); |
| |
| // this always works - and is guarenteed to generate the fastest code ! |
| return (boost::smart_cast_reference<base1 &>(t)).is_storable(); |
| } |
| |
| int main(){ |
| derived1 d1; |
| top & t1 = d1; |
| derived2 d2; |
| base2 & b2 = d2; |
| |
| bool result; |
| result = is_storable(d1); |
| result = is_storable(d2); |
| result = is_storable(b2); |
| result = is_storable(b2); |
| result = is_storable(t1); |
| return 0; |
| } |
| </code></pre> |
| The serialization library includes a mix of classes which use |
| both static polymorphism (<strong>CRTP</strong>) and dynamic |
| polymorphism via virtual functions. <code style="white-space: normal">smart_cast</code> |
| was written to address the more problematic manifestations of the |
| situation exmplified above. |
| |
| <h3>Usage</h3> |
| The following syntax is supported: |
| <pre><code> |
| smart_cast<Target *, Source *>(Source * s); |
| smart_cast<Target *>(Source * s); |
| smart_cast<Target &, Source &>(Source & s); |
| </code></pre> |
| Note that the above syntax doesn't include |
| <pre><code> |
| smart_cast<Target & >(Source & s) |
| </code></pre> |
| but the same functionality is supported the the following special syntax |
| <pre><code> |
| smart_cast_reference<Target &>(Source & s) |
| </code></pre> |
| |
| <h3>Requirements</h3> |
| <code style="white-space: normal">smart_cast</code> can be used only on compilers that support partial |
| template specialization or on types for which have be specified with the |
| macro <code style="white-space: normal"> |
| BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(<type>)</code> |
| has been applied. |
| |
| <hr> |
| <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
| 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) |
| </i></p> |
| </body> |
| </html> |