Do rvalue references allow dangling references?

Question

Consider the below.

#include <string>
using std::string;

string middle_name () {
    return "Jaan";
}

int main()
{
    string&& danger = middle_name();   // ?!
    return 0;
}

This doesn't compute anything, but it compiles without error and demonstrates something that I find confusing: danger is a dangling reference, isn't it?

Answer 1

danger is a dangling reference, isn't it?

Not any more than if you had used a const &: danger takes ownership of the rvalue.

Answer 2

I'm not sure what you're doing with &&, but your compiler really ought to produce a warning in that case. You should only store a reference to a temporary if the reference is const. The article sellibitze linked (This one) says as much. Storing a non-const reference to a temporary is officially undefined.

Answer 3

rvalue references bind to rvalues. An rvalue is either a prvalue or an xvalue [explanation]. Binding to the former never creates a dangling reference, binding to the latter might. That's why it's generally a bad idea to choose T&& as the return type of a function. std::move is an exception to this rule.

T&  lvalue();
T   prvalue();
T&& xvalue();

T&& does_not_compile = lvalue();
T&& well_behaved = prvalue();
T&& problematic = xvalue();