Thread: Problem with classes

Say you have two classes, class A and class B. class A is defined first, but class B has a prototype. When I make a method in class A whose argument is of type B&, I get a strange error message from the compiler saying " invalid use of undefined type 'struct B' ".

Why am I getting this error, and what should I do to fix it?

Example:

Code:

class B;

class A
{
    public:
        void test(B& something)
        {
            something.x = 0;
        }
};

class B
{
    public:
        int x;
};

I get the following error:

In member function `void A::test(B&)':
(8) invalid use of undefined type `struct B'
(1) forward declaration of `struct B'

I'm using Dev-C++.

It's been a while, but I think you need to define a new class B inside of class A, just like you would do if you were using an int or something else.

something like this inside of class A:

B class_b_variable;

and then instead of using void test(B& something) you need void test(class_b_variable& something)

I could be way off...

but I think you need to define a new class B inside of class A,

no

class A is defined first, but class B has a prototype

it is enough for using reference or pointer, not enough to access members

or define class B first
or move the function body into the cpp file where you can include full B class definition

Problem is that the compiler does not know how B looks like when compiling A, so you can't use B. Therefore it is always best to put the implementation in a source file. The source file can safely include both headers and you can do what you want to do without problems.