Re: assignment operator implementation

I think applying the label of costly to copy-and-swap is premature.
While it is wasteful in the particular instance of assigning a string
with less characters than the assigned-to string's allocation, as
Joshua Lehrer pointed out above, implementing the copy and swap as:

c& operator=(c tmp) {
   swap(tmp);
   return *this;
}

can avoid double-creating a temporary in cases of implicit conversion.
For this, all assignments are one copy-creation of a c-object plus
three assignments. The "classic" assignment operator:

c& operator=(const c& tmp);

in case of implicit conversion is going to create that temporary
anyways, and then do something that probably takes a lot longer than a
swap.