Resolving multiple inheritance ambiguity with enable_if

Question

I have an event source base class, which defines a function to add listeners, and I'm trying to use a template argument to resolve ambiguity. But that doesn't seem to work - it always uses the function from the first base class and then complains that template parameters don't match:

#include <type_traits>

template<class Event>
class EventSource
{
public:

  class Listener
  {
  public:
      virtual void handle( const Event& e ) = 0;
  };

  template<class EventType>
  typename std::enable_if<std::is_same<Event, EventType>::value, void>::type
  addListener( Listener* ptr );
};

class Event1 {};
class Event2 {};

class MultiListener : public EventSource<Event1>::Listener, public EventSource<Event2>::Listener
{
public:
    void handle( const Event1& e );
    void handle( const Event2& e );
};

class MultiSource : public EventSource<Event1>, public EventSource<Event2>
{
public:
    void addMultiListener( MultiListener* ptr )
    {
        addListener<Event1>( ptr );
        addListener<Event2>( ptr ); // this line causes error
    }
};

Why does this not fall under SFINAE?

I know I can explicitly specify base class, but templates look much better than EventSource<Event1>::addListener(...) .

Answer 1

Both lines cause error, as name look-up is ambiguous (before checking if overloads are viable).

You might fix issue by using using

class MultiSource : public EventSource<Event1>, public EventSource<Event2>
{
    using EventSource<Event1>::addListener;
    using EventSource<Event2>::addListener;
public:
    void addMultiListener( MultiListener* ptr )
    {
        addListener<Event1>( ptr );
        addListener<Event2>( ptr );
    }
};

Demo