class foo; in header file

Question

Is some one able to explain why header files have something like this?

class foo; // This here?
class bar
{
   bar();

};

Do you need an in

Answer 1

this is a forward declaration of the class. In my experience, this is typically done when you have a circular dependency.. for example

in foo.h
--------
#include "bar.h"

class foo
{
public:
    foo(bar *bar);
private:
    foo *m_foo;
};

and in bar.h
------------
class foo;
class bar
{
public:
    void methodFooWillCall();
protected:
    std::list myFoos;
}