Variadic templates with 'const' parameter overloading

Question

Reduced sample code:

#include <iostream>

template<typename T>
void func(T &x)
{
    std::cout << "non-const " << x << std::endl;
}

template<typename T>
void func(const T &x)
{
    std::cout << "const " << x << std::endl;
}

template<typename ...ARGS>
void proxy(ARGS ...args)
{
    func(args...);
}

int main()
{
    int i = 3;

    func(i);
    func(5);
    func("blah");

    proxy(i);
    proxy(5);
    proxy("blah");
}

Expected output:

non-const 3
const 5
const blah
non-const 3
const 5
const blah

Actual output:

non-const 3
const 5
const blah
non-const 3
non-const 5
non-const blah

So somehow the const qualifier of the function parameter gets lost when put through the variadic template. Why? How can I prevent this?

PS: tested with GCC 4.5.1 and SUSE 11.4

Answer 1

You just stumble upon the forwarding problem. This issue is solved using perfect forwarding.

Basically, you need to take your parameters by rvalue-reference, and rely on std::forward to correctly forward them while keeping their nature:

template<typename ...Args>
void proxy(Args&& ...args)  
{
    func(std::forward<Args>(args)...);
}