inheriting from a templates class with variable parameters

Question

I have this code:

  template<char ...T>
  class base
  {
       std::array<uint8_t, ID_SIZE> m_ID = { Ts... };
  }

  template<char ...T>
  class derived: public base<T>
  {
       // this class doesn;t need to know anything about T
  }

when I compile this code, I am getting this error:

  'T': parameter pack must be expanded in this context

What is this error and how i can fix it?

You have to unpack the pack: class derived: public base<T...> — Jodocus
– Jodocus, Commented Dec 12, 2017 at 12:42
How do you expand the parameter pack in the base class (or in the derived class for that matter)? Have you tried something like that? — Some programmer dude
– Some programmer dude, Commented Dec 12, 2017 at 12:42
@Someprogrammerdude add code that shows how data was unpacked. — mans
– mans, Commented Dec 12, 2017 at 12:47

Jodocus · Accepted Answer · 2017-12-12 12:47:36Z

2

Multiple template parameters (type or non-type) cannot be passed as packs but have to be unpacked each time:

template<char ...T>
class base { }

template<char ...T>
class derived: public base<T...> // unpack
{      
}

Inside of base<> the parameters will then be re-packed in the context of T.

answered Dec 12, 2017 at 12:47

Jodocus

7,6211 gold badge34 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Zwinck · Accepted Answer · 2017-12-12 12:47:24Z

2

T is not one type, it is the name of a "parameter pack."

base<T> is nonsensical, because base requires a list of types, not a pack of types. base<T...> will unpack the types and work as you expect.

answered Dec 12, 2017 at 12:47

John Zwinck

252k44 gold badges346 silver badges459 bronze badges

Collectives™ on Stack Overflow

inheriting from a templates class with variable parameters

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related