How to unpack template arguments in function call?

Question

I have DefineEvent class template that I use to ease the definition of new event classes. It's looks like this (pretty hairy, I know):

template<class EventClass, class... Parents>
class DefineEvent : public virtual Event, public Parents...
{
   public:
     DefineEvent()
     {
         static const EventTypeInfo& info = TypeInfoParentSetter<EventClass>
             ::SetOnce<Parents...>(TypeInfoHolder<EventClass>::Instance());
     }
};

The TypeInfoParentSetter class I used there looks like this:

template<class EventClass>
class TypeInfoParentSetter
{
  public:
    template<class... Parents>
    static const EventTypeInfo& SetOnce(TypeInfoHolder<EventClass>& holder)
    {
        // ...
    }
};

I get a compilation error pointing to the ::SetOnce<Parents...> line in DefineEvent(), telling me that the compiler "expected primary-expression before '...' token". How do I fix this?

You can view the code in context here, but take note that it's pretty ugly.

Answer 1

You need to include one template keyword to denote nested name is template :

DefineEvent()
     {
         static const EventTypeInfo& info = TypeInfoParentSetter<EventClass>
             :: template SetOnce<Parents...>(TypeInfoHolder<EventClass>::Instance());
     }

and you forgot to make member function TypeInfoParentSetter<EventClass>::Set static :

 template<class... Parents>
    static TypeInfoParentSetter<EventClass> Set(TypeInfoHolder<EventClass>& holder)
    {
        std::cout << "ParentSetter()\n";
        return TypeInfoParentSetter<EventClass>();
    }

check it: http://ideone.com/sNHMX

Answer 2

May be you must use "template" keyword before SetOnce:

template<class EventClass, class... Parents>
class DefineEvent : public virtual EventClass, public Parents...
{
   public:
     DefineEvent()
     {
         static const EventTypeInfo& info =
            TypeInfoParentSetter<EventClass>::template SetOnce<Parents ...>(TypeInfoHolder<EventClass>::Instance());
     }
};