Passing a temporary as a reference

Question

I am currently trying to understand the copy and swap idiom through this post. The answer posted has the following code in it

class dumb_array
{
public:
    // ...

    friend void swap(dumb_array& first, dumb_array& second) // nothrow
    {
        // enable ADL (not necessary in our case, but good practice)
        using std::swap; 

        // by swapping the members of two classes,
        // the two classes are effectively swapped
        swap(first.mSize, second.mSize); 
        swap(first.mArray, second.mArray);
    }

    // move constructor
    dumb_array(dumb_array&& other)
        : dumb_array() // initialize via default constructor, C++11 only
    {
        swap(*this, other); //<------Question about this statement
    }

    // ...
};

I noticed that the author used this statement

swap(*this, other);

other is a temporary or a rvalue which is being passed as a reference to the method swap. I was not sure if we could pass a rvalue by reference. In order to test this I tried doing this however the following does not work until i convert the parameter to a const reference

void myfunct(std::string& f)
{
    std::cout << "Hello";
}

int main() 
{
   myfunct(std::string("dsdsd"));
}

My question is how can other being a temporary be passed by reference in swap(*this, other); while myfunct(std::string("dsdsd")); cant be passed by reference.

I think you need to understand what an rvalue reference is: http://stackoverflow.com/a/5481588/4342498 — NathanOliver, Mar 31 '15 at 19:08
`swap(*this, other);` is wrong. It has to be `swap(*this, std::move(other));` (other is a named variable) — , Mar 31 '15 at 19:12
@DieterLücking in that case swap method wont work as it requires a reference and you are passing a temporary. It would only work as you suggested if it was a constant reference. Please correct me if i am wrong — Rajeshwar, Mar 31 '15 at 19:13
Please have a look at : http://en.cppreference.com/w/cpp/language/value_category — , Mar 31 '15 at 19:24
@DieterLücking Why do you think `swap` would expect an rvalue argument? — T.C., Apr 01 '15 at 07:38

score 7 · Accepted Answer · answered Mar 31 '15 at 19:09

7

The constructor takes a rvalue reference, but other is a lvalue (it has a name).

answered Mar 31 '15 at 19:09

Jarod42

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Yes that makes sense. So other basically holds an rvalue but is itself an lvalue. Am i correct ? – Rajeshwar Mar 31 '15 at 19:10
@Rajeshwar: I think it's slightly more accurate to say the reference is an lvalue that is _constructed from_ an rvalue. (references aren't truely "constructed", but you get the idea) – Mooing Duck Mar 31 '15 at 19:41

score 0 · Answer 2 · edited Mar 31 '15 at 19:38

0

In the case of:

myfunct(std::string("dsdsd"));

The value std::string("dsdsd") is a temporary within a scope that the call to myfunct() is actually outside of.

C++ explicitly specifies that binding the reference to const extends the lifetime of the temporary to the lifetime of the reference itself.

edited Mar 31 '15 at 19:38

Nilzone-

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answered Mar 31 '15 at 19:14

Adam Leggett

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Passing a temporary as a reference

2 Answers2