Does perfect forwarding in C++0x make reference_wrapper deprecated?

Question

As usual, code first:

#include <functional>

using namespace std;
using namespace std::tr1;

void f(int& r) { r++; }

template<class F, class P> void g1(F f, P t) { f(t); }
template<class F, class P> void g2(F f, P&& t) { f(forward<P>(t)); }

int main()
{
    int i = 0;

    g1(f, ref(i)); // old way, ugly way
    g2(f, i); // new way, elegant way
}

In C++ 98, we don't have a nice way to pefect forward parameters through template functions. So the C++ gurus invented ref and cref to achieve that aim.

Now that we have had r-value reference and perfect forwarding, does it mean that ref and cref and the like should be deprecated?

Answer 1

Reference wrappers are still useful. This is the case when it's about storing things. For example, with reference wrappers you can make std::make_tuple and std::thread create objects which refer to some argument instead of copying them:

class abstract_job
{
public:
    virtual ~abstract_job() {}
    virtual void run() = 0;
};

class thread
{
public:
    template<class Fun, class... Args>
    thread(Fun&& f, Args&&... args)
    {
        typedef typename decay<Fun>::type fun_type;
        typedef decltype( make_tuple(forward<Args>(args)...) ) tuple_type;
        unique_ptr<abstract_job> ptr (new my_job<fun_type,tuple_type>(
            forward<Fun>(fun),
            make_tuple(forward<Args>(args)...)
        ));
        // somehow pass pointer 'ptr' to the new thread
        // which is supposed to invoke ptr->run();
    }
    ...
};

...

void foo(const int&);

int main()
{
   thread t (foo, 42); // 42 is copied and foo is invoked 
   t.join()            // with a reference to this copy
   int i = 23;
   thread z (foo, std::cref(i)); // foo will get a reference to i
   z.join();
}

Keep in mind that

make_tuple(std::ref(i))  // yields a tuple<int&>
make_tuple(         i )  // yields a tuple<int>

Cheers! s

Answer 2

That's assuming reference_wrapper was intended for that. Rather it seems to be mostly about allowing passing function objects by reference where they would be normally taken by value. - If you were to take arguments as T&& instead, wouldn't that mean that passing things by value becomes impossible?

#include <iostream>
#include <functional>
#include <algorithm>

class X: public std::unary_function<int, void>
{
    int n;
public:
    X(): n(0) {}
    void operator()(int m) {n += m;}
    int get_n() const { return n; }
};

template <class Iter, class Fun>
void for_each(Iter from, Iter to, Fun&& fun)
{
    for (; from != to; ++from)
        fun(*from);
}

int main()
{
    int a[] = {1, 2, 3};
    X x1;
    ::for_each(a, a + 3, x1);
    std::cout << x1.get_n() << '\n';  //6

    X x2;
    std::for_each(a, a + 3, x2);
    std::cout << x2.get_n() << '\n';  //0

    X x3;
    std::for_each(a, a + 3, std::ref(x3));
    std::cout << x3.get_n() << '\n';  //6
}