I try to create a function. Function signature

template <typename T>
void func(int arg, T& ref1, T& ref2 = /* dummyref<T>() */ );

This function takes argument arg and assigns something to ref1 and ref2, but ref2 is an optional parameter. It is not used when default dummyref object detected. How do I implement this and what dummyref object can I use? It has to work for basic types as well (such as int).