The Problem

I have this problem where I have this singleton `RenderingEngine` but I have added a template to `RenderingEngine` so now the singleton declaration is:

template <class ClockworkT>
using RenderingEngine = ACPPL::Singleton<Private::RenderingEngine<ClockworkT>>;

And it will be up to the user of RenderingEngine to specify ClockworkT. The problem is that there are other parts of the library that call upon the RenderingEngine singleton like so:

ClockworkCanvas::RenderingEngine<ClockworkT>::Instance();

You might be thinking at this point, why not just template anything that uses RenderingEngine? But there is a callback function that get in the way of that idea:

template <class ClockworkT>
static void framebufferResizeCallback(GLFWwindow* window, int width, int height) {
    ClockworkCanvas::RenderingEngine<ClockworkT>::Instance().frameBufferResized();
}

Throws the compiler error

error: no matches converting function 'framebufferResizeCallback' to type 'GLFWframebuffersizefun' {aka 'void (*)(struct GLFWwindow*, int, int)'

Potential Solutions

What I was wondering is whether it would be possible to set `ClockworkT` to a default type which is then overrided by the end user so that I could go back to `ClockworkCanvas::RenderingEngine::Instance();` with out the template. Also this solution would get round the potential to have multiple running RenderingEngine singletons.