How to keep object passed by rvalue reference alive without copy?

Question

I am trying to pass Child as r-value to Parent and save it there without copying the Child.

Parent p{ Child{} };

cout << "----Done----" << endl;

And now the implementation:

class Child
{
public:
    Child() { cout << "Child: created" << endl; }

    ~Child() { cout << "Child: destroyed" << endl; }
};

class Parent
{
    // store as r-value ref
    Child&& child;

public:
    // pass by r-value ref and move
    Parent(Child&& c) :child(move(c))
    {
        cout << "Parent: created" << endl;
    }

    ~Parent() { cout << "Parent: destroyed" << endl; }
};

The output is:

Child: created
Parent: created
Child: destroyed
------Done------
Parent: destroyed

cppref says: Rvalue references can be used to extend the lifetimes of temporary objects

Question 1: Why isn't Child destroyed in the very end (or at least after Done)?
Question 2: How to make it live longer?

P.S:

Child& child;
...
Parent(Child c) :child(c)

Gives the exact same result.

Answer 1

cppref says: Rvalue references can be used to extend the lifetimes of temporary objects

You are leaving out an important part of that quote. A more accurate quote is: "Rvalue references can be used to extend the lifetimes of temporary objects".

Notice the difference? (Don't worry too much if you don't, as it is subtle.) There is a link to an explanation of how and when the lifetime of a temporary can be extended. You've tagged this question C++11, so the bullet point that is valid until C++14 applies:

• a temporary bound to a reference member in a constructor initializer list persists only until the constructor exits, not as long as the object exists. (note: such initialization is ill-formed as of DR 1696).

You bound a temporary to a reference member in a constructor initializer list. The temporary's lifespan is not extended beyond the end of the constructor. The temporary is destroyed before your object's destructor is called.

---

How to make it live longer? That gets tricky, but if the parameter is always a temporary, you could make your member a non-reference and move the parameter to the member. If you want more flexibility, you may need to accept some copying.