Alex Rivera | Logout

Overloading assignment operator in C++

Asked 2010-03-15T14:08:18.793
20

As I've understand, when overloading operator=, the return value should should be a non-const reference.


A& A::operator=( const A& )
{
    // check for self-assignment, do assignment

    return *this;
}

It is non-const to allow non-const member functions to be called in cases like:


( a = b ).f();

But why should it return a reference? In what instance will it give a problem if the return value is not declared a reference, let's say return by value?

It's assumed that copy constructor is implemented correctly.

c++ operator-overloading assignment-operator
Edit
Report

2 Answers

1

This is Item 10 of Scott Meyers' excellent book, Effective C++. Returning a reference from operator= is only a convention, but it's a good one.

This is only a convention; code that doesn't follow it will compile. However, the convention is followed by all the built-in types as well as by all the types in the standard library. Unless you have a good reason for doing things differently, don't.

answered 2010-03-15T15:18:12.190
0

If it returned a copy, it would require you to implement the copy constructor for almost all non-trivial objects.

Also it would cause problems if you declare the copy constructor private but leave the assignment operator public... you would get a compile error if you tried to use the assignment operator outside of the class or its instances.

Not to mention the more serious problems already mentioned. You don't want it to be a copy of the object, you really do want it to refer to the same object. Changes to one should be visible to both, and that doesn't work if you return a copy.

answered 2010-03-15T14:23:57.093

Your Answer