Sorry, don't know what happened then, I'll continue... class A {}; class B {}; class C : public A, public B {}; int main(int argc, char* argv[]) { C c; C* pc = &c; //B* pb = boost::polymorphic_downcast( pc ); // this cast is unnecessary, the compiler <knows> that C isa B at compile time, // so it can go ahead and upcast it without asking you or the runtime type info: B* pb = pc; // C* pc2 = polymorphic_upcast( pb); // this is precisely where the boost::polymorphic_downcast is intended to be used, // ie Cast a Base (ptr or ref) to a Derived (ptr or ref) // the compiler doesn't know that a B object is definately a valid C, so it requires you to // qualify it, using some kind of cast. C* pc2 = boost::polymorphic_downcast(pb); return 0; } So thats downcasting and upcasting, there is no need for a polymorphic_upcast, as it is implicity done and can be tested for at compile time. The only other cast not covered here is cross casting, which is what boost::polymorphic_cast is designed for, but I find that almost everytime I come across a use of a cross cast it is due to a design flaw. (the same could also be said to a lesser extent about downcasting) Hope this clears things up a bit. Sam

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-----Original Message----- From: news [mailto:news@main.gmane.org]On Behalf Of Jean Llorca Sent: 27 June 2002 15:31 To: boost-users@yahoogroups.com Subject: [Boost-Users] Re: boost::polymorphic_upcast suggestion

"Darin Adler" a écrit dans le message de news: B586F909-89D7-11D6-9349-0003935B80A2@bentspoon.com...

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On Thursday, June 27, 2002, at 06:38 AM, Jean Llorca wrote:

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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

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).

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 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) );

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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 ;)