Re: [Boost-users] [any] Converting between types

Iulian M wrote:

Hello,

I'm trying to use boost::any to store a group of types. For value types everything works as expected. The problem is that i need to store some polymorphic types also. The following code explains what i'm trying to accomplish.

#include

class A {}; //base calss class B: public A {}; // derived class

int main() { B *b = new B(); //create derived class boost::any n(b); // place inside boost::any

/* other part of code where i only know n contains a pointer to a class derived from A this will throw bad_any_cast: typeid(A*) != typeid (B*) */ A* a = boost::any_cast(n); }

Is there any way of making boost::any_cast work work ok with "convertible" types? Or is there a way of storing different types of objects and having the possibility of retrieving them as compatible objects ?

I've looked into any.hpp and found that it uses a typeid() equality test to see if the cast can be made. Can't there be a is_convertible test insted , at least for the case when is holding pointer to objects?

There's no easy way for the boost.any code to work with polymorphic types as you expect since there's no current way to know what types are convertible at runtime. (Perhaps when there's a boost.reflect) There's an easy workaround though, which is the same as you and the compiler would use if you had, for example, a "A* v". You can do the polymorphic conversion yourself, you just need to do it at injection time instead of extraction time: int main() { B *b = new B(); //create derived class boost::any n(static_cast(b)); // place inside boost::any A* a = boost::any_cast(n); } HTH. -- -- Grafik - Don't Assume Anything -- Redshift Software, Inc. - http://redshift-software.com -- rrivera/acm.org - grafik/redshift-software.com -- 102708583/icq - grafikrobot/aim - grafikrobot/yahoo