Is there ever a valid reason to not return *this from copy assignment operator?

Question

Let foo be a struct or class with a copy assignment operator:

struct foo {
    foo &operator=(const foo &);  // or with some other return type?
};

Is there ever a sensible reason to return anything other than *this from the operator=()? Using it for something unrelated to assignment doesn't qualify as sensible.

When you return `this*`, you're making it possible for the assignment operator to be chained like `i = j = k = ...`. You can return other stuff, but how useful that is would be based on your definitions and how you want your classes to be used — Alex, May 20 '15 at 21:38
@AlanStokes It would be nice if you elaborated on that, you know. — Marius Bancila, May 20 '15 at 21:44
interesting question, but why are you asking? Is it really a [practical, detailed question](http://stackoverflow.com/tour)? Then try to show it. — Wolf, May 20 '15 at 21:45
note: the answers to the duplicate explore various scenarios where `*this` is not returned — M.M, May 20 '15 at 22:49

score 9 · Accepted Answer · answered May 20 '15 at 21:44

9

The example from the standard is std::atomic. It returns the assigned value. If it returned reference, then reading through it might yield different result.

answered May 20 '15 at 21:44

zch

14,931
2
41
49

Do you know other examples? – Wolf May 20 '15 at 21:47
2

@Wolf: Boost::Phoenix is a library to create lazy expressions. So the `operator=()` there would return some object that you could call `operator()()` on to actually perform the assignment. – Bill Lynch May 20 '15 at 21:49
@BillLynch this seems to be what Alan Stokes mentioned? – Wolf May 20 '15 at 21:51

score 0 · Answer 2 · answered May 20 '15 at 22:44

If you want to prevent assignment chaining.

Sometimes its nice to prevent expressions like this:

x = y = z = a = b = c = d = foo{15};

So you make the assignment operator return void.

struct foo {
    void operator=(const foo &);
};

For some types chaining does not make sense. But you have to look at that on a case be case basis.

Is there ever a valid reason to not return *this from copy assignment operator?

2 Answers2