Question

I was playing with variadic template parameters using gcc 4.6.1. The following code compiles as expected:

template<typename RetType, typename... ArgTypes>
class Event;

template<typename RetType, typename... ArgTypes>
class Event<RetType(ArgTypes...)>
{
public:
    typedef function<RetType(ArgTypes...)> CallbackType;

    void emit(ArgTypes...args)
    {
        for (CallbackType callback : callbacks)
        {
            callback(args...);
        }
    }

private:
    vector<CallbackType> callbacks;
};

But to my suprise the following “normal” version that has only one “Argument Type” doesn’t compile:

template<typename RetType, typename ArgType>
class Event;

template<typename RetType, typename ArgType>
class Event<RetType(ArgType)> // <- error: wrong number of template arguments (1, should be 2)
{};

g++ 4.6.1 gives error as in the comment.

Anybody knows why it causes the error and how to make it work? Also, am I right in thinking the above code is a form of “template partial specialisation”?

Answer 1

template<typename RetType, typename ArgType>
class Event;

Expects 2 template arguments, RetType and ArgType, you only give it one RetType(ArgType).

template<typename RetType, typename... ArgType>
class Event;

Expects 1 or more template arguments, RetType and optional ArgTypes.