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

Try to think of writing this:

file bar.h:

        #include "bar.h"

        class Foo
        {
        Bar* bar_ptr;
        }

file foo.h:

        #include "foo.h"

        class Bar
        {
            Foo* foo_ptr;
        }

This won't work, first due to infinite #include chain, then if you get rid of one of the includes, either Foo won't know what Bar is, or Bar won't know what Foo is.

Try this instead:

        class Bar;

        class Foo
        {
        Bar* bar_ptr;
        };

file foo.h:

        class Foo;

        class Bar
        {
            Foo* foo_ptr;
        };