Is passing a reference through function safe?

Question

Is the following function safe in C++03 or C++11 or does it exhibit UB?

string const &min(string const &a, string const &b) {
    return a < b ? a : b;
}

int main() {
    cout << min("A", "B");
}

• Is it OK to return a reference to an object passed to the function by reference?

• Is it guaranteed that the temporary string object is not destroyed too soon?

• Is there any chance that the given function min could exhibit UB (if it does not in the given context)?

• Is it possible to make an equivalent, but safe function while still avoiding copying or moving?

Answer 1

Is it OK to return a reference to an object passed to the function by reference?

As long as the object isn't destroyed before you access it via that reference, yes.

Is it guaranteed that the temporary string object is not destroyed too soon?

In this case, yes. A temporary lasts until the end of the full expression which creates it, so it is not destroyed until after being streamed to cout.

Is there any chance that the given function min could exhibit UB (if it does not in the given context)?

Yes, here is an example:

auto const & r = min("A", "B"); // r is a reference to one of the temporaries
cout << r;                      // Whoops! Both temporaries have been destroyed

Is it possible to make an equivalent, but safe function while still avoiding copying or moving?

I don't think so; but this function is safe as long as you don't keep hold of a reference to its result.