The header role is to define the interface of the module, that's why I believe the leaner the header is the better. When I'm using a module I do not care what t

The header role is to define the interface of the module, that's why I believe the leaner the header is the better. When I'm using a module I do not care what the implementation is using, the only thing I care about is what functions are exposed and what data structures are required by the functions. All other details are irrelevant. If any of the functions are using any types from the external header that are passed by value (for pointers forward declaration is the way to go), then that headers also become part of the interface and have to be #included in header.