I am not a math-guy, but I sure enjoyed viewing the animations on this page: Bézier curves. The External Links section seems to point to possible sources of algorithms, some of which might be in code-form. I don’t know if code is what you’re asking for, Guillaume, but that jsDraw2d library might be useful for design ideas, even though it’s not C/C++.
I suppose a C/C++ library that wraps OpenGL would be what you’re looking for, optimally. Given that drawing smooth curves is such a core problem, there must be a GPL’ed library out there already.