TITLE: argument for return type of operator =

(Newsgroups: comp.lang.c++, 2 Jan 2 97)

[ This picks up a discussion about whether the return type of
  operator=() should be void or T&. The usage of this operator
  for built-in types follows the latter. -adc ]


KOENIG: ark@research.att.com (Andrew Koenig)

[snip]

> >	b) a class whose assignment operator returns void
> >	   fails to meet the requirements for elements of some
> >	   kinds of standard library containers.

[snip]

HELLER: heller@utdallas.edu (Steve Heller)

>   Could you give me an example of this problem?

KOENIG:

Sure.

Suppose you define a class T, with an eye toward being able to
create a vector<T> object (where vector is as defined in the
STL and, presumabely, in the eventual standard C++ library).
Then the question is: What operations do you have to define for
your class in order to guarantee that vector<T> can be created?

The C++ draft answers this question by publishing some tables of
requirements, and by saying that your class T must meet particular
subsets of those requirements in order to be able to do particular
things with a vector<T>.

Among those requirements is the so-called Assignable requirement,
which says that if t and u are objects of type T, then the expression
t=u must yield a T& and afterwards, t must be a copy of u.

So, if you define a class T with an assignment operator that returns
void, the standard library is not required to implement vector<T> correctly.