Std::tuple no appropriate default constructor available

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Hello,

If I try to compile the example below i get the error

'std::tuple<long &,int>::tuple': no appropriate default constructor available

How can i make this example work in order to make tuple accept mixed lvalue ref and rValue?

C++

#include "stdafx.h"
#include <tuple>


template<class ...Arg>
class A
{
	std::tuple<Arg...> tuple_;
public:
	A(Arg&&...args)
	{
		tuple_ = std::make_tuple(std::forward<Arg>(args)...);
		
	}
};

template<class...Arg>
void f(Arg&&...args)
{
	A<Arg...> a(std::forward<Arg>(args)...);
}


int main()
{
    long t = 5;
    f(t,5);
	
    return 0;
}

What I have tried:

I tried using std::make_tuple , std::forward_as_tuple or std::tie, but nothing makes the example compile without error.

Posted 19-Jul-20 3:19am

Optimistic76

Updated 19-Jul-20 4:01am

Sandeep Mewara

v4

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CPallini · Accepted Answer · 2020-07-19T04:01:00

It works (At least on my system) this way:

C++

#include <tuple>


template<class ...Arg>
class A
{
  std::tuple<Arg...> tuple_;
public:
  A(Arg&&...args):tuple_(std::forward<Arg>(args)...)
  {
  }
};

template<class...Arg>
void f(Arg&&...args)
{
  A<Arg...> a(std::forward<Arg>(args)...);
}


int main()
{
    long t = 5;
    f(t,5);

    return 0;
}

I guess (just a wild guess though) the reason beign such tuple ctor is (conditionally) explicit, see std::tuple<types...>::tuple[^].