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Ok, working with SDL_Rect, how would you answer the question is (mx, my) inside the rectangle? Where mx and my are the x and y position of the mouse.

I don't think it needs to be an SDL_Rect, your own rectangle class would do

What you probably want to do is create a class Region that's a rectangle, either using the top-left and lower-right coordinates to define the rectangular region, or the top-left and width, height....