c++c++11perfect-forwardingreference-collapsing

Working of std::forward and reference collapsing


I am trying to familiarize myself with the concept of perfect forwarding. I have read this and this post here. I believe I am also comfortable with the reference collapsing concept. While reading this other post. I came up with the following question

Suppose we have the following example

Example-1
template <class T>
void foo(T &&t)
{
  bar(std::forward<T>(t));
}

and we pass it something like this

int i= 12;
foo(i);

Now I understand this i will be treated as a int& what I don't understand is why does the answered question in the above link mention that it will be treated as int& && which will collapse as int&. I am of the opinion that it will be treated as int&& & which will collapse to int& I understand the return type of both is the same but I would like to get the first part right. The reason why I think it is int&& & instead of int& && is mentioned below please correct me if I am wrong

When I pass in something like this

int i =12;
foo(i);

Then Example 1 becomes something like this

void foo(int &&t)
{
  bar(std::forward<int>(&t)); // t is a reference now ------>A
}

Now std::forward implementation is this

template<typename T>                // For lvalues (T is T&),
T&& std::forward(T&& param)         // take/return lvalue refs.
{                                   // For rvalues (T is T),
    return static_cast<T&&>(param); // take/return rvalue refs.
}

so when our solution is applied to it . It becomes

return static_cast<int&&>(&param) ; //&param since we passed &t in A

we get

int&& & and not `int& &&` please correct me if I am wrong

Solution

  • When you pass i as an argument to foo:

    int i = 12;
    foo(i);
    

    expression i has an lvalue category, thus T is deduced as int&, so example 1 becomes:

    void foo<int&>(int& && t) // `int& t` due to reference collapsing
    //         |   \ /
    //         +--> T  = int&
    {//                   V
        bar(std::forward<int&>(t));
    }
    

    Now, in std::forward, the param's type is specified explicitly to be int&, so:

    int& && std::forward<int&>(typename std::remove_reference<int&>::type& param)
    //                    ^~~+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
    //                       |
    {//                      V          
        return static_cast<int& &&>(param); // `int&` due to reference collapsing
    }
    

    which restores the lvalue category of i.


    To make it clear, this is what happens when the argument expression's value category is an rvalue:

    foo(12); // -> foo<int>(12);
    //                  |
    //        v~~~~~~~~~+
    void foo<int>(int && t) // `int&& t`
    //        |   \ /
    //        +--> T  =  int
    {//                   V
        bar(std::forward<int>(t));
    }//                   |
    //                    V 
    int && std::forward<int>(typename std::remove_reference<int>::type& param)
    //                    ^~~+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
    //                       |
    {//                      V          
        return static_cast<int &&>(param); // `int&&`
    }
    

    And when the type template parameter is specified as int&& explicitly:

    foo<int&&>(12);
    //     ^~~~+
    //         V
    void foo<int&&>(int&& && t) // `int&& t` due to reference collapsing
    //         |    \   /
    //         |     \ /
    //         +----> T = int&&
    {//                    V
        bar(std::forward<int&&>(t));
    }//                    |
    //                     V 
    int&& && std::forward<int&&>(typename std::remove_reference<int&&>::type& param)
    //                     ^~~+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
    //                        |
    {//                       V          
        return static_cast<int&& &&>(param); // `int&&` due to reference collapsing
    }