how to get a member from derived class through the base class except using pure virtual

Question

I've class G.

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class B : public G

class A : public B

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class D : public G

class C : public D

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class F : public G

class E : public F

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From a different class, I've an access to G (G sorry!!). I've to get some memeber of class A (for example, string m_Astr).

How can I access the A string? If I use pure virtual in class G, I'll have to implement this getStr function in E and C in addition to the implementation of A, and I need it for A only.

please help

Answer 1

If you know (somehow) that the object pointed to is actually an instance of an A, then you can use static_cast:

G* p = get_the_thing();
A* a = static_cast <A*> (p); // OK

However, if the object pointed to by p above doesn't instantiate an A, then the above code will yield undefined behavior.

A more safe approach would be to use dynamic_cast,

G* p = get_the_thing();
A* a = dynamic_cast <A*> (p); // OK

Here, a will be NULL if p isn't actually an A. However, dynamic_cast can only be used in this manner with polymorphic classes: the base class must have at least one virtual function. A virtual destructor would do nicely here.