Are you suggesting that C++ virtual functions have the same overhead as a C function pointer? I've never heard that; with the virtual call, there is the lookup + the invocation; perhaps a C app that stores function pointers in a hash map also has this sort of lookup, is that what you mean?

Vtable lookup in a typical C++ implementation is just loading a fixed offset in an array. In C terms, it's basically doing object->vtable[42]. In C code with function pointers, you're typically performing that exact same operation. You can go slightly faster if you store the function pointer in the object directly, at the cost of increased memory consumption per object if you have more than one function pointer.