Question

For simplicity, let’s say I want to do implement a function which takes two parameters and predicate which test for equality,

template<typename T, typename TCompare>
bool Eq(const T& a, const T& b, const TCompare& cmp) {
  return cmp(a, b);
}

but I also want that operator== is assumed if predicate is not passed.

I tried several different approaches, but all resulted in either syntax error or “operator==” not defined. I even tried replacing the Eq function with a

Nevertheless, I suppose what I want is possible since the big part of STL supports not passing the optional TCompare parameter (std::sort, std::set …).

UPDATE:
Thanks to levis501 suggestion, this is the best way I have found so far:

template<typename T, typename TCompare>
bool Eq(const T& a, const T& b, const TCompare& cmp) {
  return cmp(a, b); // or some lengthy code
}

template<typename T>
bool Eq(const T& a, const T& b) {
  static std::equal_to<T> EqCmp;
  return Eq(a, b, EqCmp);
}

So, you basically need to create a wrapper which passes std::equal_to, or any other operator-equivalent functor. For some reason, having it as a default argument does not compile when you invoke the function without it.

Answer 1

How about std::equal_to ? http://www.cplusplus.com/reference/std/functional/equal_to/