boost/smart_cast.hpp
#ifndef BOOST_SMART_CAST_HPP #define BOOST_SMART_CAST_HPP // MS compatible compilers support #pragma once #if defined(_MSC_VER) && (_MSC_VER >= 1020) # pragma once #endif /////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8 // smart_cast.hpp: // (C) Copyright 2002 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) // See http://www.boost.org for updates, documentation, and revision history. // casting of pointers and references. // In casting between different C++ classes, there are a number of // rules that have to be kept in mind in deciding whether to use // static_cast or dynamic_cast. // a) dynamic casting can only be applied when one of the types is polymorphic // Otherwise static_cast must be used. // b) only dynamic casting can do runtime error checking // use of static_cast is generally un checked even when compiled for debug // c) static_cast would be considered faster than dynamic_cast. // If casting is applied to a template parameter, there is no apriori way // to know which of the two casting methods will be permitted or convenient. // smart_cast uses C++ type_traits, and program debug mode to select the // most convenient cast to use. #include <exception> #include <typeinfo> #include <boost/config.hpp> #include <boost/static_assert.hpp> #include <boost/type_traits/is_base_and_derived.hpp> #include <boost/type_traits/is_polymorphic.hpp> #include <boost/type_traits/is_pointer.hpp> #include <boost/type_traits/is_reference.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/type_traits/remove_pointer.hpp> #include <boost/type_traits/remove_reference.hpp> #include <boost/mpl/eval_if.hpp> #include <boost/mpl/if.hpp> #include <boost/mpl/or.hpp> #include <boost/mpl/and.hpp> #include <boost/mpl/not.hpp> #include <boost/mpl/identity.hpp> namespace boost { namespace smart_cast_impl { template<class T> struct reference { struct polymorphic { struct linear { template<class U> static T cast(U & u){ return static_cast<T>(u); } }; struct cross { template<class U> static T cast(U & u){ return dynamic_cast<T>(u); } }; template<class U> static T cast(U & u){ // if we're in debug mode #if ! defined(NDEBUG) \ || defined(__BORLANDC__) && (__BORLANDC__ <= 0x560) \ || defined(__MWERKS__) // do a checked dynamic cast return cross::cast(u); #else // borland 5.51 chokes here so we can't use it // note: if remove_reference isn't function for these types // cross casting will be selected this will work but will // not be the most efficient method. This will conflict with // the original smart_cast motivation. typedef BOOST_DEDUCED_TYPENAME mpl::eval_if< BOOST_DEDUCED_TYPENAME mpl::and_< mpl::not_<is_base_and_derived< BOOST_DEDUCED_TYPENAME remove_reference<T>::type, U > >, mpl::not_<is_base_and_derived< U, BOOST_DEDUCED_TYPENAME remove_reference<T>::type > > >, // borland chokes w/o full qualification here mpl::identity<cross>, mpl::identity<linear> >::type typex; // typex works around gcc 2.95 issue return typex::cast(u); #endif } }; struct non_polymorphic { template<class U> static T cast(U & u){ return static_cast<T>(u); } }; template<class U> static T cast(U & u){ #if defined(__BORLANDC__) return mpl::eval_if< boost::is_polymorphic<U>, mpl::identity<polymorphic>, mpl::identity<non_polymorphic> >::type::cast(u); #else typedef BOOST_DEDUCED_TYPENAME mpl::eval_if< boost::is_polymorphic<U>, mpl::identity<polymorphic>, mpl::identity<non_polymorphic> >::type typex; return typex::cast(u); #endif } }; template<class T> struct pointer { struct polymorphic { // unfortunately, this below fails to work for virtual base // classes. need has_virtual_base to do this. // Subject for further study #if 0 struct linear { template<class U> static T cast(U * u){ return static_cast<T>(u); } }; struct cross { template<class U> static T cast(U * u){ T tmp = dynamic_cast<T>(u); #ifndef NDEBUG if ( tmp == 0 ) throw std::bad_cast(); #endif return tmp; } }; template<class U> static T cast(U * u){ // if we're in debug mode #if ! defined(NDEBUG) || defined(__BORLANDC__) && (__BORLANDC__ <= 0x560) // do a checked dynamic cast return cross::cast(u); #else // borland 5.51 chokes here so we can't use it // note: if remove_pointer isn't function for these types // cross casting will be selected this will work but will // not be the most efficient method. This will conflict with // the original smart_cast motivation. typedef BOOST_DEDUCED_TYPENAME mpl::eval_if< BOOST_DEDUCED_TYPENAME mpl::and_< mpl::not_<is_base_and_derived< BOOST_DEDUCED_TYPENAME remove_pointer<T>::type, U > >, mpl::not_<is_base_and_derived< U, BOOST_DEDUCED_TYPENAME remove_pointer<T>::type > > >, // borland chokes w/o full qualification here mpl::identity<cross>, mpl::identity<linear> >::type typex; return typex::cast(u); #endif } #else template<class U> static T cast(U * u){ T tmp = dynamic_cast<T>(u); #ifndef NDEBUG if ( tmp == 0 ) throw std::bad_cast(); #endif return tmp; } #endif }; struct non_polymorphic { template<class U> static T cast(U * u){ return static_cast<T>(u); } }; template<class U> static T cast(U * u){ #if defined(__BORLANDC__) return mpl::eval_if< boost::is_polymorphic<U>, mpl::identity<polymorphic>, mpl::identity<non_polymorphic> >::type::cast(u); #else typedef BOOST_DEDUCED_TYPENAME mpl::eval_if< boost::is_polymorphic<U>, mpl::identity<polymorphic>, mpl::identity<non_polymorphic> >::type typex; return typex::cast(u); #endif } }; template<class TPtr> struct void_pointer { template<class UPtr> static TPtr cast(UPtr uptr){ return static_cast<TPtr>(uptr); } }; template<class T> struct error { // if we get here, its because we are using one argument in the // cast on a system which doesn't support partial template // specialization template<class U> static T cast(U u){ BOOST_STATIC_ASSERT(sizeof(T)==0); return * static_cast<T *>(NULL); } }; } // smart_cast_impl // this implements: // smart_cast<Target *, Source *>(Source * s) // smart_cast<Target &, Source &>(s) // note that it will fail with // smart_cast<Target &>(s) template<class T, class U> T smart_cast(U u) { typedef BOOST_DEDUCED_TYPENAME mpl::eval_if< BOOST_DEDUCED_TYPENAME mpl::or_< boost::is_same<void *, U>, boost::is_same<void *, T>, boost::is_same<const void *, U>, boost::is_same<const void *, T> >, mpl::identity<smart_cast_impl::void_pointer<T> >, // else BOOST_DEDUCED_TYPENAME mpl::eval_if<boost::is_pointer<U>, mpl::identity<smart_cast_impl::pointer<T> >, // else BOOST_DEDUCED_TYPENAME mpl::eval_if<boost::is_reference<U>, mpl::identity<smart_cast_impl::reference<T> >, // else mpl::identity<smart_cast_impl::error<T> > > > >::type typex; return typex::cast(u); } // this implements: // smart_cast_reference<Target &>(Source & s) template<class T, class U> T smart_cast_reference(U & u) { return smart_cast_impl::reference<T>::cast(u); } } // namespace boost #endif // BOOST_SMART_CAST_HPP