Re: Why the copy assignment operator is written to return non-const reference to this?

From:

Andrey Tarasevich <andreytarasevich@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 23 Nov 2008 18:12:40 -0800

Message-ID:

<ggd2if$9hi$1@aioe.org>

Daniel T. wrote:

"Andrew Koenig" <ark@acm.org> wrote:

...
    class Mine {
    // ...
    public:
    const Mine& operator=(const Mine&);
    };

    std::vector<Mine> v; // Undefined behavior -- Mine does not meet
the requirements for std::vector

Now I'm confused. As I understand it, the language doesn't require op=
to return any particular type. Stroustrup in "TC++PL" often declares it
to return void...

It is not about language in general. It is about standard containers
specifically. Standard containers library has its own requirements.
Container elements are required to be 'Assignable'. And part of the
'Assignable' requirement is that assignment expression must return a
non-constant reference (23.1/4).

If you are not planning to use your class as a container element, you
can declare your copy-assignment operator to return whatever you please,
including 'void'. But with standard containers your hands are tied.

--
Best regards,
Andrey Tarasevich