How to pass an operator as default functor argument? - c++

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/

Answer 2

You can overload functions, no?
// Use the user-supplied TCompare.
template<typename T, typename TCompare>
bool Eq(const T& a, const T& b, TCompare cmp) {
return cmp(a, b);
}
// Use op== for equality otherwise.
template<typename T>
bool Eq(const T& a, const T& b) {
return a == b;
}
If the code calls Eq() with three arguments, the compiler will resolve the call to the first overload. If the code calls Eq() with only two arguments, then the compiler will have to resolve to the second overload.

Answer 3

I don't suppose something like the following would work for you:
#include <functional>
template<typename T, typename TCompare = std::equal_to<T>>
class Math {
public:
static bool Eq(const T& a, const T& b) {
TCompare cmp;
return cmp(a, b);
}
};
int _tmain(int argc, _TCHAR* argv[]) {
Math<int>::Eq(1,1);
return 0;
}
Default template parameters are only allowed on class templates (or at least that's what MSVC 10 says)