CODEWITHSUNDEEP
c++templatesvariadic
Brian Martin
Brian Martin

Reputation: 13

Variadic template recursive constructor problems

i try to understand the use of recursive constructors with variadic templates.

In the following code i want to implement a recursive constructor with variadic passed values.

template <class T, class... T2>
    struct Tuple{
        T value;

        Tuple(T n){                    //constructor if only one value left
           value = n;
        }

        Tuple(T n, T2... re){          //constructor if more than one value left
            T value = n;
            //Tuple(rest...);          doesnt work for me
        }
    }; 

int main(){
    Tuple<int, float, int> t(2, 1.2, 9);

    std::cout << t.value << "\n";                   //2
    std::cout << t.rest.value << "\n";              //1.2
    std::cout << t.rest.rest.value << "\n";         //9
}

Upvotes: 1

Views: 368

Answers (1)

max66
max66

Reputation: 66230

You have also to inherit from Tuple<T2...>

template <class T, class... T2>
struct Tuple : public Tuple<T2...>

and initialize the inherited class in the initialization list (before value) [caution code not tested]

   Tuple(T n, T2... re) : Tuple<T2...>{re...}, value{n}
    { }

You also need a ground case specialization to stop the recursion

Something as

template <class T>
struct Tuple<T>
 {
   T value;

   Tuple (T n) : value{n}
    { }
 };

or also (maybe simpler)

template <>
struct Tuple<>
 { };

But for this second solution, you have to define Tuple as receiving zero or more types; maybe something as follows

template <typename...>
struct Tuple
 { };

template <class T, class... T2>
struct Tuple<T, T2...> : public Tuple<T2...>
 {
   // ...
 };

This way, when Tuple receive at least one type, the specialization is selected; when receive zero type, only the main version match and serve as ground case.

Upvotes: 3

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