forward declaration error

From: Scott (scott_at_nospam.com)
Date: 05/29/04 

Date: Sat, 29 May 2004 00:28:25 GMT

Hi All,

I am trying to understand the following errors, and hope someone can help
me along. This is code not originally written by me, but that I am
trying to fix and compile. I have the following in a single file:

#include "A.h"
class foo;

class B : public A
{
public:
        foo * getFooPtr() { return fooObject; }

protected:
        foo * fooObject;
};

template <class T>
int compare( const T * first, const T * second )
{
        foo * object = first->getFoo();
        int number = foo->getNum();
}

It compiles with errors:

error: invalid use of undefined type `struct foo'
error: forward declaration of `struct foo'

I believe this is because the forward declaration of foo doesn't actually
provide any information on what functions are contained within foo, so I
need to change:

class foo;

to

#include "foo.h"

Correct?

However, when I do that, I get:

error: ISO C++ forbids declaration of `foo' with no type
error: `foo' undeclared (first use this function)

The first referring to the use of foo in class B, and the second to the
use of foo in the template function. I am confused, foo is declared in
foo.h, so why the errors? Please help!

Best,
Scott

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