How Can I Check for the Existence of a Non-Member `operator==` in C ?

Checking for the Existence of Non-Member Operator== in C

In C , checking for the existence of a member function operator== is straightforward using the provided member function. However, determining the presence of a non-member operator== can prove more challenging.

C 03 Method:

The following technique can be employed to check for the existence of any operator:

namespace CHECK
{
  class No { bool b[2]; };
  template<typename T, typename Arg> No operator== (const T&amp;, const Arg&amp;);

  bool Check (...);
  No&amp; Check (const No&amp;);

  template <typename T, typename Arg = T>
  struct EqualExists
  {
    enum { value = (sizeof(Check(*(T*)(0) == *(Arg*)(0))) != sizeof(No)) };
  };  
}

To use this method, simply invoke the EqualExists template:

CHECK::EqualExists<A>::value;

If the EqualExists value is non-zero, the non-member operator== exists.

C 11 Method:

C 11 provides a cleaner approach using decltype and std::declval:

namespace CHECK
{
  struct No {}; 
  template<typename T, typename Arg> No operator== (const T&amp;, const Arg&amp;);

  template<typename T, typename Arg = T>
  struct EqualExists
  {
    enum { value = !std::is_same<decltype(std::declval<T>() < std::declval<Arg>()), No>::value };
  };  
}

To check for the non-member operator==, use:

CHECK::EqualExists<A>::value;

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