return value of operator overloading in C++_问答_开发者

I have a question about the return value of operator overloading in C++. Generally, I found two cases, one is return-by-value, and one is return-by-reference. So what's the underneath rule of that? Especially at the c开发者_如何学运维ase when you can use the operator continuously, such as cout<<x<<y.

For example, when implementing a + operation "string + (string)". how would you return the return value, by ref or by val.

Some operators return by value, some by reference. In general, an operator whose result is a new value (such as +, -, etc) must return the new value by value, and an operator whose result is an existing value, but modified (such as <<, >>, +=, -=, etc), should return a reference to the modified value.

For example, cout is a std::ostream, and inserting data into the stream is a modifying operation, so to implement the << operator to insert into an ostream, the operator is defined like this:

std::ostream& operator<< (std::ostream& lhs, const MyType& rhs)
{
  // Do whatever to put the contents of the rhs object into the lhs stream
  return lhs;
}

This way, when you have a compound statement like cout << x << y, the sub-expression cout << x is evaluated first, and then the expression [result of cout << x ] << y is evaluated. Since the operator << on x returns a reference to cout, the expression [result of cout << x ] << y is equivalent to cout << y, as expected.

Conversely, for "string + string", the result is a new string (both original strings are unchanged), so it must return by value (otherwise you would be returning a reference to a temporary, which is undefined behavior).

To attempt an answer to your question regarding strings, the operator+() for strings is almost always implemented as a free (non-member) function so that implicit conversions can be performed on either parameter. That is so you can say things like:

string s1 = "bar";
string s2 = "foo" + s1;

Given that, and that we can see that neither parameter can be changed, it must be declared as:

RETURN_TYPE operator +( const string & a, const string & b );

We ignore the RETURN_TYPE for the moment. As we cannot return either parameter (because we can't change them), the implementation must create a new, concatenated value:

RETURN_TYPE operator +( const string & a, const string & b ) {
    string newval = a;
    newval += b;    // a common implementation
    return newval;
}

Now if we make RETURN_TYPE a reference, we will be returning a reference to a local object, which is a well-known no-no as the local object don't exist outside the function. So our only choice is to return a value, i.e. a copy:

string operator +( const string & a, const string & b ) {
    string newval = a;
    newval += b;    // a common implementation
    return newval;
}

If you want your operator overload to behave like the built-in operator, then the rule is pretty simple; the standard defines exactly how the built-in operators behave and will indicate if the result of a built-in is an rvalue or an lvalue.

The rule you should use is:

if the built-in operator returns an rvalue then your overload should return a reference
if the built-in returns an lvalue then your overload should return a value

However, your overload isn't required to return the same kind of result as the built-in, though that's what you should do unless you have a good reason to do otherwise.

For example, KennyTM noted in a comment to another answer that the stream overloads for the << and >> operators return a reference to the left operand, which is not how the built-ins work. But the designers of the stream interface did this so stream I/O could be chained.

Depending on the operator you may have to return by value.

When both can be used though, like in operator+= you could consider the following:

If your objects are immutable it's probably better to return by value.
If your objects are mutable it's probably better to return by reference.

Usually you return by reference in an operation that changes the value of the things it's operating on, like = or +=. All other operations are return by value.

This is more a rule of thumb, though. You can design your operator either way.