CODEWITHSUNDEEP
c++templatesvariadic-templates
mans
mans

Reputation: 18168

inheriting from a templates class with variable parameters

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?

Upvotes: 3

Views: 65

Answers (2)

Jodocus
Jodocus

Reputation: 7591

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.

Upvotes: 2

John Zwinck
John Zwinck

Reputation: 249163

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.

Upvotes: 2

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