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

From: David Abrahams (dave_at_[hidden])
Date: 2005-04-30 07:36:01

"Delfin Rojas" <drojas_at_[hidden]> writes:

> Hello all,
>
> I hope somebody can help me with this. What I want to do is the following:
>
> Let's say I have a templated class called Foo
>
> template<class U>
> class Foo
> {
> public:
> 	Foo(U * p) : m_p(p) {}
> 	~Foo() {}
> 	U* getP() const { return m_p; }
> private:
> 	U* m_p;
> };
>
> Now let's say I want to have a collection of pointers to Foo. What should I
> do? The possibilities I can think of are the following:
>
> 1) Create a class FooBase and have Foo inherit from FooBase. FooBase is
> non-templated so there should be no problem with having a collection of
> pointers to FooBase where I can put instances of Foo. The problem with this
> approach is that given a FooBase* in that collection I could not use the
> getP method because I would not know what type of Foo it holds.

Not knowing what type of Foo it holds, how would you know what to do
with the result of getP?  Its type depends on the type of the Foo.

If you are willing to just get a void*, type erasure (see
http://www.boost.org/doc/html/signals/s05.html#id574178 and
http://www.boost-consulting.com/mplbook) might help.  Otherwise...

> 2) Create a boost::variant of all the possible types of Foo<U>* I want to
> have. Then have a collection of this FooVariant. Using static visitation I
> can obtain the original type of Foo<U>* in a given FooVariant. The problem
> with this approach is that I get an ICE from VC++ 7.1 when the type of U
> gets too complex (shared_ptr to another type for example).
>
> Have I overlooked something? Is there a better technique for what I want to
> achieve? Any help would be great,

You could store them in shared_ptr<void>, or use boost::any.

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