What are the coolest examples of metaprogramming that you've seen in C++?

Personally, I think Boost.Spirit is a pretty amazing example of meta-programming. It's a complete parser generator that lets you express grammars using C++ syntax.

The most practical use of meta programming is turning a runtime error into a compile time error.

Example: Lets call the interface IFoo. One of my programs dealt with a COM object that had multiple paths to IFoo (very complicated inheritance hierarchy). Unfortunately the underlying COM object implementation didn't realize they had multiple paths to IFoo. They assumed it was always the left most one. So inside their code, the following pattern was very common

   void SomeMethod(IFoo* pFoo) {
        CFooImpl *p = (CFooImpl)pFoo;
   }

The second IFoo though caused the resulting "p" pointer to be completely invalid (multiple inheritance is dangerous).

The long term solution was to have the COM object owner fix this issue. Short term though I needed to make sure that I always returned the correct IFoo. I could guarantee that I had the appropriate IFoo by using a QI and avoiding any implicit casts to IFoo. So I created a new CComPtr<> implementation and added the following override to the equal method.

template <typename T>
CComPtr<T>& operator=(const T* pT)  { 
// CComPTr Assign logic
}
template <>
CComPtr<IFoo> operator=<IFoo>(const IFoo* pT) {
  COMPILE_ERROR();
}

This quickly revealed every single place I implicitly casted to IFoo.

Not of practical usage (except maybe for compiler testing), but metatrace is a Whitted-Style (i.e. recursive and deterministic) ray tracer which generates images like those at compilation time:

Some more complex parts of the code can be seen in fixp.hh, which has an implementation of fixed-point sqrt using the Heron method, or sphere.hh which shows ray/sphere-intersection calculation.