Os and compiler? Did you try using debugger to see if the code executes?
The fact that adding line to the loop fixes it makes me think you have your compiler settings wrong and it optimized out entire loop for some reason. I'm probably wrong but that'd be
really cool.
So? You can do all the 3D magic with OpenGL and use SFML just as context creator and optional audio output.
But then you'd not use clear(), especially not in sf::Window
Where's the pure C when he's using SFML, a C++ library? (I probably didn't get that joke...)
Knowing c++ isn't prerequisite for knowing OpenGL, he might be experienced C coder, migrating to c++. Joke: Carmack had around 10 years of experience in OpenGL and C and none in c++ at the point of beginning id tech 4(Doom 3, Quake 4, everything before was just C) engine that was his (company) first real c++ project.
If you want to reply to that send me a pm, enough goofing in this thread.