I am trying to understand folding and variadic templates. I designed a very naive Tuple class. I can create a tuple object, but I would like to print the object. It seems odd that this problem hasn’t been touched almost anywhere (at least I haven’t found any resource so far. This is the code
#include <iostream>
#include <string>
// this is the implementation of my own tuple using variadic templates
template <class... T>
class Tuple {};
template <class T, class... Args>
class Tuple<T, Args...> {
private:
T first;
Tuple<Args...> rest;
public:
// friends
friend std::ostream& operator<<(std::ostream& out, const Tuple<T, Args...>& tuply);
public:
Tuple(T&& firsty, Args&&... resty): first(firsty), rest(resty...){};
};
template <class T, class Args...>
std::ostream& operator<<(std::ostream& out, const Tuple<T, Args...>& tuply){
out << tuply.first;
((out << " - " << std::forward<Args>(tuply.rest)), ...);
}
probably Tuple<T, Args...>
is illegal, and the line where I am using folding does not make sense. I really cannot find a way to make this work. Can someone help?
CodePudding user response:
Just use recursion:
// this is the implementation of my own tuple using variadic templates
template<class... T>
class Tuple {};
template<class T, class... Args>
class Tuple<T, Args...> {
private:
T first;
Tuple<Args...> rest;
friend std::ostream& operator<<(std::ostream& out, const Tuple& tuply) {
out << tuply.first;
if constexpr (sizeof...(Args) == 0) return out;
else return out << " - " << tuply.rest;
}
public:
Tuple(T&& firsty, Args&&... resty)
: first(std::forward<T>(firsty)), rest(std::forward<Args>(resty)...) { }
};