Why do some class methods return "*this" (object reference of self)?

Question

There are plenty of codes for explanation on the internet (also specifically here, on stackoverflow) that return *this.

For example from post Copy constructor and = operator overload in C++: is a common function possible? :

MyClass& MyClass::operator=(const MyClass& other)
{
    MyClass tmp(other);
    swap(tmp);
    return *this;
}

When I write swap as:

void MyClass::swap( MyClass &tmp )
{
  // some code modifying *this i.e. copying some array of tmp into array of *this
}

Isn't enough setting return value of operator = to void and avoid returning *this?

Answer 1

This idiom exists to enable chaining of function calls:

int a, b, c;
a = b = c = 0;

This works well for ints, so there's no point in making it not work for user defined types :)

Similarly for stream operators:

std::cout << "Hello, " << name << std::endl;

works the same as

std::cout << "Hello, ";
std::cout << name;
std::cout << std::endl;

Due to the return *this idiom, it is possible to chain like the first example.

Answer 2

One of reasons why *this is returned to allow assignment chains like a = b = c; which is equivalent to b = c; a = b;. In general, result of assignment could be used anywhere, e.g. when invoking functions (f(a = b)) or in expressions (a = (b = c * 5) * 10). Although, in most cases, it just makes code more complicated.