RE: [Boost-users] Re: How to get type given pointer to base?

...

In that case you're erasing the type information too early. You need to instantiate the function template you mention above (call it f) before you lose the type of Foo, e.g. make a vector of shared_ptr<FooHolderBase>:

<snip code>

There are various other ways to write this basic idiom (e.g. using function pointers instead of building a class with virtual functions), but the essential thing is that f<U> gets instantiated when you still know U at compile-time.

If you try to erase the type first, you will end up with what is essentially a massive type-switch:

type_info dynamic_type = typeid(*some_foo_base_ptr); if (dynamic_type == typeid(Foo<A>)) f(*dynamic_cast(some_foo_base_ptr)); else if (dynamic_type == typeid(Foo<B>)) f(*dynamic_cast(some_foo_base_ptr)); ...

which really sucks because it has to be maintained every time you write Foo<U> for some new U.

Thanks for the advice. Yes, that dynamic_cast switch was precisely what I as trying to avoid and I think I can avoid it using what you suggested (instantiating template function when the type of the object is still known).

HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com

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