C ++类对象函数

Question

I'm trying to understand how this type of function works and how you would use it.

I assume there's some class we'll call it test.

class Test {

}

I saw this type of function in a header file and I'm trying to figure out what it does. And how to use it properly.

Test& testFunction();

Appreciate the help.

Answer 1

What's in front of the function name is the type that the function will return. In this case, testFunction is returning a Test object. The & in this case (reference) means that it will return a Test -reference. This is important if you wish to modify the returned object when you call the function. Or use it in some way not possible with "return-by-value".

Your code doesn't tell us much about what you're going to be doing with the return value, but here is a good example that's used quite commonly:

Test & T::f() {
    // do something

    return *this;
}

*this here is the actual object on which its method .f is being called. What's special here is that since we are returning a reference, we can chain calls while maintaining the original object. *this will be the same *this every time. With return-by-value we aren't able to do this. For example:

By reference:

Test & T::f() {

    this->x++;

    return *this;

}


int main() {

    Test t;

    t.x = 5;

    t.f().f();

    std::cout << t.x; // 7 as we expect

}

By value:

Test T::f() { ... } // notice the omission of &

int main() {

    Test t;

    t.x = 5;

    t.f().f();

    std::cout << t.x; // 6

}

t is changed only once because the object is lost on the next call.