inheritance - Specialising on two ancestor class in C++ -

i have defined abstract interface consisting of 3 classes:

class rendering_context {...} class window : public rendering_context {...} class offscreen : public rendering_context {...} 

this allows various client objects ignore type of rendering context they’re in. far good.

i defined concrete implementations:

class sdl::window : public window {...} class sdl::offscreen : public offscreen {...} 

this works expected, , client classes happy no idea whether on-screen / off-screen / sdl-based or not.

my confusion:

i have sdl-specific functionality applies in rendering contexts, i.e. not in 1 or other of sdl::window or sdl::offscreen.

this suggests class of sdl::rendering_context needs pulled out somewhere, how class hierarchy?

if make concrete classes so:

class sdl::window : public window, public sdl::rendering_context {...} class sdl::offscreen : public offscreen, public sdl::rendering_context {...} 

and assuming sdl::rendering_context inherits rendering_context haven’t inherited rendering_context twice implicitly? fact rendering_context, in case, abstract base class, no members , several pure virtual functions, make okay?

is same diamond problem? seems might be, difference there’s no ambiguity base class method call: pure virtual , concrete implementation @ bottom of hierarchy executed in practice.

[n.b. none of class declarations above, illustrate hierarchy. current code compiles , works, duplicates functionality.]

and assuming sdl::rendering_context inherits rendering_context haven’t inherited rendering_context twice implicitly?

if problem, consider virtual inheritance, in rendering_context shared base class. if wish this:

class window : public virtual rendering_context {...} class offscreen : public virtual rendering_context {...}