I had a confusion about whether to return with constant reference or non-constant reference from operator=. It seemed clever and practical to return T const&, because of the following faulty reasoning:
T a,b; a = b = T(100,200,"something");
The fully bracketed form of the second line is:
a = (b = T(100,200,"something"));
So if we return T const& from T::operator=(T const& rhs) it is sufficient for that purpose. And also when we accidentally write something silly like (a = b) = T(100,200,"something") it will help us by giving a compile error.
Reasoning for non-constant return
The nice argument is that there are useful cases when we do want to modify the result of an assignment, for example:
T a,b,c; // ... std::swap(a=b,c); // #1 (b=c).Transponate(); // #2 // ...
The strong argument is that many STL containers require their type parameter(s) to fulfill the Assignable[1] concept, which defines assignment operator to return non-constant reference.
- [1] Assignable described at SGI: http://www.sgi.com/tech/stl/Assignable.html
- See also: The C++ standard (23.2.1.4, table 96 – Container requirements)
