How to unpack type-defined types?

Question

For example

struct A { typedef int Type; }
struct B { typedef float Type; }

template<class... Ts>
struct C
{
    typedef tuple<Ts::Type...> TupleType; // comilation error: parameter pack 
                                          // expects a type template argument
};

possible duplicate of [Where and why do I have to put the "template" and "typename" keywords?](http://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords) — Constructor, May 19 '14 at 16:37

score 6 · Accepted Answer · answered May 19 '14 at 05:40

You need typename.

typedef tuple<typename Ts::Type...> TupleType;

Note that this has nothing to do with the fact that you're dealing with a parameter pack. You need typename here for the same reason as usual. In fact, if you had for instance

template<class T>
struct D {
    typedef vector<typename T::type> VectorType;
};

the typename would be necessary here too.

How to unpack type-defined types?

1 Answers1