initializing base class array member variable via base class constructor from drived class template value

Question

I have this code which I think should work, but it is not!

 class base
 {
     std::array<uint8_t, 8> m_ID;
 public:
     base(std::array<uint8_t, 8> id) :m_ID(id)
     {

     }
 }
 template<char ...Ts>
 class derived:base(Ts...)
 {
 }
 class MyClass: public derived<'1','2','3','4','5','6','7','8'>
 {
 }

How can I do this? The idea is that I can pass the ID value from template values.

I am getting error that MyClass is not complete. (Incomplete type is not allowed)

I find it odd that base has an array of 8 values which are populated from a parameter pack. What if the user provides fewer/more than 8 parameters? — François Andrieux
– François Andrieux, Commented Dec 12, 2017 at 15:57

Jodocus · Accepted Answer · 2017-12-13 17:12:39Z

3

You just have to call the base class constructor properly:

#include <array>
#include <cstdint>

 class base
 {
     std::array<std::uint8_t, 8> m_ID;
 public:
     base(std::array<std::uint8_t, 8> id) :m_ID(id)
     {

     }
 };

 template<char ...Ts>
 class derived: public base
 {
    public:
    derived() : base{ { Ts... } } { }
 };

class MyClass: public derived<'1','2','3','4','5','6','7','8'>
 {
 };

 int main() {
    MyClass d;
 }

Note that in the constructor initializer list, the inner pair of braces is needed to convert the single uint8_ts to an array.

edited Dec 13, 2017 at 17:12

answered Dec 12, 2017 at 15:58

Jodocus

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2 Comments

Constructor Over a year ago

You forgot semicolon after the MyClass class definition. And (void) MyClass d; is not a valid C++ line of code.

Jodocus Over a year ago

@Constructor Feel free to improve answers on SO.

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