Why should the assignment operator return a reference to the object?

问题

I\'m doing some revision of my C++, and I\'m dealing with operator overloading at the minute, specifically the \"=\"(assignment) operator. I was looking online and came across multiple topics discussing it. In my own notes, I have all my examples taken down as something like

class Foo
{
    public:  
        int x;  
        int y;  
        void operator=(const Foo&);  
};  
void Foo::operator=(const Foo &rhs)
{
    x = rhs.x;  
    y = rhs.y;  
}

In all the references I found online, I noticed that the operator returns a reference to the source object. Why is the correct way to return a reference to the object as opposed to the nothing at all?

回答1:

The usual form returns a reference to the target object to allow assignment chaining. Otherwise, it wouldn't be possible to do:

Foo a, b, c;
// ...
a = b = c;

Still, keep in mind that getting right the assigment operator is tougher than it might seem.

回答2:

The return type doesn't matter when you're just performing a single assignment in a statement like this:

x = y;

It starts to matter when you do this:

if ((x = y)) {

... and really matters when you do this:

x = y = z;

That's why you return the current object: to allow chaining assignments with the correct associativity. It's a good general practice.

回答3:

Your assignment operator should always do these three things:

Take a const-reference input (const MyClass &rhs) as the right hand side of the assignment. The reason for this should be obvious, since we don't want to accidentally change that value; we only want to change what's on the left hand side.
Always return a reference to the newly altered left hand side, return *this. This is to allow operator chaining, e.g. a = b = c;.

Always check for self assignment (this == &rhs). This is especially important when your class does its own memory allocation.

MyClass& MyClass::operator=(const MyClass &rhs) {
    // Check for self-assignment!
    if (this == &rhs) // Same object?
        return *this; // Yes, so skip assignment, and just return *this.

    ... // Deallocate, allocate new space, copy values, etc...

    return *this; //Return self
}

来源：https://stackoverflow.com/questions/9072169/why-should-the-assignment-operator-return-a-reference-to-the-object

标签

c++

operator-overloading

return-type

assignment-operator