c++ - Why operator= returns reference not const reference

Question

The original question is related to overloading operator= and I like to share my findings as it was nontrivial for me to find them. I cannot imagine reasonable example to use (a=b) as lvalue. With the help of IRC and google I've found the next article: http://msdn.microsoft.com/en-us/magazine/cc301415.aspx

it provides two examples.

  (a=b)=c

  f(T& );
  f(a=b)

but both a bit not good, and I believe that it is bad practice. The second one give me the same feeling. Could you provide more good examples why it should be non constant?

Answer 1

One good reason is that one of the requirements in the standard for a class X to be useable in the standard containers is that the expression a = b must have type X& (where a is an lvalue of type X and b is an rvalue of type X).