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

From: Delfin Rojas (drojas_at_[hidden])
Date: 2005-05-02 01:40:50

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