c++ template using declaration from base class

Question

Why does the following code not work (using VS2013)?

class Test1
{
public:
  template<typename T>
  using my_type = T;
};

template<typename T, typename V>
class My : public T
{
public:
  using t_type = T;
  using my_type2 = t_type::my_type<V>;
};

int main()
{
  const My<Test1, double>::my_type2 x = 5.5;

  return 0;
}

Test1::my_type would have a more complex type. My would be a policy based class where Test1 would be one of the policies. my_type2 should become a specific type based on the policy. And Test1 can't be a template directly taking the type of V. Thanks.

kudos for creating a MCVE. don't worry about the dupe. It was not obvious. — bolov, Apr 20 '15 at 12:46

score 2 · Accepted Answer · answered Apr 20 '15 at 12:38

2

Welcome to a dark corner of C++

Change this

using my_type2 = t_type::my_type<V>;

to this:

using my_type2 =  typename t_type::template my_type<V>;

answered Apr 20 '15 at 12:38

bolov

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Thanks a lot, I almost had it, and I knew that it may be in this direction, but I put in typename instead of template after t_type::. And sorry for the duplicate, I seem not to have search with the right arguments. – blackmav Apr 20 '15 at 12:40
gcc actually tells in the error message: typename needed and then template needed – bolov Apr 20 '15 at 12:44

c++ template using declaration from base class

1 Answers1