How to pass a reference through a parameter pack?

Question

I have the following code:

#include <cstdio>

template<class Fun, class... Args>
void foo(Fun f, Args... args)
{
    f(args...);
}

int main()
{
    int a = 2;
    int b = 1000;

    foo([](int &b, int a){ b = a; }, b, a);
    std::printf("%d\n", b);
}

Currently it prints 1000, that is, the new value of b gets lost somewhere. I guess that's because foo passes the parameters in the parameter pack by value. How can I fix that?

BЈовић · Accepted Answer · 2012-02-01T21:51:37.583

19

By using reference :

template<class Fun, class... Args>
void foo(Fun f, Args&&... args)
{
    f( std::forward<Args>(args)... );
}

edited Feb 01 '12 at 21:51

answered Feb 01 '12 at 20:48

BЈовић

62,405
41
173
273

3

This will fail if `a` is an rvalue, even though that should be semantically permissible. `&&` would be better I think. – ildjarn Feb 01 '12 at 21:24
Perfectly fits my needs if `&&` is used. – p12 Feb 01 '12 at 21:34
2

You should forward: `f(std::forward(args)...);` – GManNickG Feb 01 '12 at 21:37
@GMan : +1 I was going to suggest some use of `forward<>` here as well, but I didn't know the syntax for forwarding a parameter pack. :-[ – ildjarn Feb 01 '12 at 22:00

score 8 · Answer 2 · answered Feb 01 '12 at 20:48

like this:

#include <iostream>
#include <functional>

template<class Fun, class... Args>
void foo(Fun f, Args... args)
{
    f(args...);
}

int main()
{
    int a = 2;
    int b = 1000;

    foo([](int &b, int a){ b = a; }, std::ref(b), a);
    std::cout << b << std::endl;
}

How to pass a reference through a parameter pack?

2 Answers2

Linked