value-initialization of a const member reference

Question

I am looking at code of the following form:

class foo
{
  public:
    foo() {}

  //...
};

class bar
{
  public:
    bar() : ref() {}

  private:
    const foo &ref;
};

Is initializing a reference using a temporary in this way correct? I know that it is possible to initialize a const reference that's a local variable with a temporary, and that doing so extends the lifetime of the temporary, e.g.

const foo &tmp = funcThatReturnsByValue(); //OK

However, one of the answers to the related initialize reference in initialization list suggests that there is a difference between "short-lived" and "long-lived" references, and that initializing ref as above is undefined behavior (even though ref is a const reference).

12.2.5 in the standard says, in part, "A temporary bound to a reference member in a constructor's ctor-initializer persists until the constructor exits." Is that describing this situation?

Answer 1

This code is ill-formed. You can't default initialize or value initialize a reference.

If you actually had an expression inside of ref(), then yes, 12.2.5 would apply and the temporary would be destroyed when the constructor exits.

Answer 2

Your example isn't creating a temporary - to do that you need to change to:

    bar() : ref(foo()) {} 

Now you're binding the reference to a temporary, and that temporary object will be destroyed at the end of the constructor. Your reference will be invalid, and that is Not A Good Thing.

Answer 3

I guess what you want to do is:

bar() : ref(foo()) {}

but don't naively think that the lifetime of a temporary is extended until there's a reference to it. No, it actually is not. So, whether const or not, you sould initialize the reference with a normal object.