Question

I’m implementing some C++ static analysis rules, and one of them prohibits a function from returning a reference or pointer to a reference parameter of the function, i.e. the following are all non-compliant:

int *f(int& x) { return &x; } // #1
const int *g(const int& x) { return &x; } // #2
int& h(int& x) { return x; } // #3
const int& m(const int& x) { return x; } // #4

The justification given for this is that “It is implementation-defined behaviour whether the reference parameter is a temporary object or a reference to the parameter.”

I’m puzzled by this, however, because stream operators in C++ are written in this way, e.g.

std::ostream& operator<<(std::ostream& os, const X& x) {
    //...
    return os;
}

I think I’m pretty confident that stream operators in C++ do not in general exhibit implementation-defined behaviour, so what’s going on?

According to my understanding as it is at present, I would expect #1 and #3 to be well-defined, on the basis that temporaries cannot be bound to non-const references, so int& x refers to a real object that has lifetime beyond the scope of the function, hence returning a pointer or reference to that object is fine. I would expect #2 to be dodgy, because a temporary could have been bound to const int& x, in which case trying to take its address would seem a bad plan. I’m not sure about #4 – my gut feeling is that that’s also potentially dodgy, but I’m not sure. In particular, I’m not clear on what would happen in the following case:

const int& m(const int& x) { return x; }
//...
const int& r = m(23);

Answer 1

As you say, #1 and #3 are fine (though #1 is arguably bad style).

#4 is dodgy for the same reason #2 is; it allows propagating a const reference to a temporary past its lifetime.

Let’s check:

#include <iostream>

struct C {
  C() { std::cout << "C()\n"; }
  ~C() { std::cout << "~C()\n"; }
  C(const C &) { std::cout << "C(const C &)\n"; }
};

const C &foo(const C &c) { return c; }

int main() { 
   const C &c = foo(C());
   std::cout << "c in scope\n";
}

This outputs:

C()
~C()
c in scope