C++ 直接从 operator< 返回

Question

Is it allowed to directly return while we are sending to ostream ?

For example, instead of writing:

ostream& operator<<(ostream& os, Foo foo) {
    os << foo.a << foo.b;
    return os;
}

Instead, can I directly write:

ostream& operator<<(ostream& os, Foo foo) {
    return os << foo.a << foo.b;
}

I tried it, and it seems to work but I don't know if it's the right thing to do. Is there any problem with doing the second version?

Answer 1

The two code snippets you post are entirely equivalent.

Why? Because (I'm assuming here, for the sake of argument, that foo.a and foo.b are simple data types like int ) the << operator for std::ostream returns a reference to itself (see the "Return Value" section in the linked document).

Thus, in your first code snippet, the return os; line returns a reference to the os object referenced by the first argument. In your second snippet, the two << operations each evaluate as a reference to that same object ... so you are returning a reference to the exact same object.

To make the second version a bit clearer, you may like to enclose the returned expression in parentheses:

ostream& operator<<(ostream& os, Foo foo) {
    return (os << foo.a << foo.b);
}

Answer 2

Is it allowed to directly return while we are sending to ostream?

Yes, it is allowed.