"Darin Adler" <darin@bentspoon.com> a écrit dans le message de news: B586F909-89D7-11D6-9349-0003935B80A2@bentspoon.com...
On Thursday, June 27, 2002, at 06:38 AM, Jean Llorca wrote:
Here upcasting means casting a base class to a derived class, I know it may seem strange, bust boost offers a downcast which does the static_cast the language does implicitly already (check <boost/cast.hpp>).
I think you've got it backwards. Downcasting, in Boost at least, means casting a pointer to a base class part of an object to a pointer to a derived class part of the same object. The polymorphic_downcast is *not* doing a static_cast for a conversion the language already does implicitly.
template <class Target, class Source> inline Target polymorphic_downcast(Source* x BOOST_EXPLICIT_DEFAULT_TARGET) { assert( dynamic_cast<Target>(x) == x ); // detect logic error return static_cast<Target>(x); } Sorry about it, but it's here in the code. This is in the last version of the CVS. It does a static_cast. The compiler does static_cast implicitly in some cases. For instance a C++ compiler replaces the assert line with: assert( dynamic_cast<Target>(x) == static_cast<Target>(x) );
Could you give a specific complete example (a file that would compile, with all the declarations) of how one would use what you're proposing to help make this clearer?
I enclosed two files to this reply. Don't scream about chosen class names and stuff in the example ;) begin 666 test.cpp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` ` end begin 666 nqcast.h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` end