You forward declare class_foo::bar (as you can see in the eror message) but you want class_bar::bar.

You need the forward declaration in the right namespace:

namespace class_bar {
     class bar;
}

And same for the other class.

You forward declare class_foo::bar (as you can see in the eror message) but you want class_bar::bar.

You need the forward declaration in the right namespace:

namespace class_bar {
     class bar;
}

And then inside foo you need to refer to it with the right namespace too: class_bar::bar (rather than just bar). And same for the other class.