Re: How to convert Pointer between structs?

"Alf P. Steinbach" <>
Sun, 22 Feb 2009 20:39:32 +0100
* Immortal Nephi:

I am curious. How can you overcome Compiler's error? You define b
and c inside main(). You may want to convert from C::F_C() to B::F_B
() on C::*pC. Can reinterpret_cast keyword be used? If not, can you
replace C::*pC to global scope? It may be easier that B::F_B() can be
invoked inside global scope. Please do not mention static keyword.
Let me know if it is possible.

struct B
    void F_B() {}
    int _B;

The name _B is invalid (it's reserved for the implementation) because it starts
with an underscore followed by uppercase.


struct C
    void F_C() {}
    void (C::*pC)();
    int _C;

int main(void)
    B b;
    C c;

    c.pC = &B::F_B; // Error

    return 0;

First of all, as a novice *don't* use

   - goto
   - member pointers
   - all uppercase names for non-macro things
   - macro names that are not all uppercase
   - raw pointers where they can be avoided

It seems that you want some code to treat instances of the two classes B and C
in the same way, while they're slightly different on the inside.

This is known as polymorphism.

And C++ offers two main ways to do that: compile time polymorphism (templates),
and run time polymorphism (virtual member functions).

As an example of the latter:

   class IntHolder
       int c_;
       IntHolder( int v = 0 ): c_( v ) {}
       virtual void f() = 0;

   class B: public IntHolder
       B( int v = 0 ): IntHolder( v ) {}
       virtual void f() { ... }

   class C: public IntHolder
       C( int v = 0 ): IntHolder( v ) {}
       virtual void f() { ... }

   void foo( IntHolder& o ) { o.f(); }

   int main()
       B b;
       C c;

       foo( b ); foo( c );

By the way, which book are you using that doesn't discuss this?

It can be helpful to others to know about that book, to avoid it.

Cheers & hth.,

- Alf

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