If std::max() returns by reference (as it must), might that lead to a dangling reference?

Question

Consider the paradigmatic max template function, std::max():

// From the STL

    // TEMPLATE FUNCTION _Debug_lt
template

        

Answer 1

(answer removed) See Herb Sutters GOTW article on binding const references to temporaries.

Edit: I removed the answer since it was actually wrong (thanks for making that clear to me), so all I leave is the link to a somewhat related article. But beware: The feature described in the article does not apply here!

Answer 2

Yes, it's a dangling reference.

Yes, it's a gotcha.

A version that returns by value might be useful provided that you use it by default, and switch to by-reference in the cases where you need it. If you use the by-reference one by default and only switch to the by-value when you need it then you'll still fall foul of the gotcha, because realizing that you need it is the same thing as realizing you should have written const A a_= ::max(A(), A());.

Unfortunately the by-value one introduces a new gotcha:

A a, b, c;
mymax(a, b) = c; // assigns to temporary, not to a or b

(Actually, looking at this code I reckon if you call it max_by_val then you won't write this).

You could return by const value, and some people used to recommend that operator overloads should return by const value. But that introduces a performance gotcha in C++11:

A a, b, c;
c = constmymax(a, b); // copies from const rvalue instead of moving.

and even in C++03 it prevents the equivalent explicit optimization swap(c, constmymax(a,b)).