C++ templates and derived classes

Question

I am trying to understand the following code. Derived is a derived structure from T and what does "," means and then Fallback {}

template <class T>
struct has_FlowTraits<T, true>
{
  struct Fallback { bool flow; };
  struct Derived : T, Fallback { };   //What does it means ?

  template<typename C>
  static char (&f(SameType<bool Fallback::*, &C::flow>*))[1];

  template<typename C>
  static char (&f(...))[2];

public:
  static bool const value = sizeof(f<Derived>(0)) == 2;
};

Answer 1

It's an implementation of Member Detector Idiom. It uses SFINAE to check whether type T has got a member called flow.

Edit: The comma part you're asking about is multiple inheritance. Struct Derived is (publicly) inheriting from both T and Fallback.