As a style, always program to an interface.
The header files for those interfaces absolutely should not include headers like <string>. An interface does not require it.
Then how do you use std::string in your class declaration?
I don't think we should forward declare symbols in third party or even system libraries.
To answer OP's question, here is how I do in my code,
1, For the declarations (such as classes) in my code, I forward declare them whenever possible, in the header. Of course the source file needs to include the corresponding header, that's fine.
2, For the declarations in third party and system library, I never forward declare them. If I do, I will get a lot of trouble if the library changes the name or namespace.
3, I always try to avoid #include in the header (my point #1), but if I can't avoid, just include it (my point 2), no problem.
I do everything I can to stop includes in headers, and use forward declarations instead. But if I need the #include in the header, I won't put it in the source file, it already has it in the header so it doesn't need it. If you don't see the #include for whatever you're using, it most likely is in the header. I despise multiple includes when they're not needed, it's equivelant to the sound of Styrofoam rubbing against other Styrofoam to me.
When you include <string> in a header it's because it must be there, you have no option to forward declare. Since you know that this header will be in the cpp there is no reason to reinclude it in the cpp file, its already part of the classes declaration.
