C++ 重载 + 运算符不工作

Question

Hello guys look to my code I'm trying to make a program which asks you to enter the first value by grams And the second value is kilograms and then convert kilograms to grams by an overloaded + operator but it doesn't work why

#include <iostream>
using namespace std;
class ADD{
private:
    int Fval;
    int Sval;
public:
    ADD(){
            cout << "WELCOME TO OUR PROGRAM"<<endl<<"PLEASE ENTER THE FIRST VALUE BY GRAMS :";
            cin >> Fval;
            cout << "PLEASE ENTER THE SECOND VALUE BY KILOGRAMS :"; cin >> Sval;
        }
    ADD operator+(ADD& add){
        add.Sval *= 1000;
        return add;
    }
    int plus(){
        return Fval+Sval;
    }
};
int main(){
    ADD a1;
    cout << "THE TOTAL VALUE = " << a1.plus() << " GRAMS";
}

No Effect look to the output

WELCOME TO OUR PROGRAM
PLEASE ENTER THE FIRST VALUE BY GRAMS :2
PLEASE ENTER THE SECOND VALUE BY KILOGRAMS :3
THE TOTAL VALUE = 5 GRAMS

That means the + operator doesn't multiply 3 by 1000 Why??

Answer 1

That's because you're not calling operator + .

You're calling ADD::plus() :

int plus(){
    return Fval+Sval;
}

Fval and Sval are integers, which you're adding up. It's as simple as that.

EDIT:

Your code is fishy.

ADD operator+(ADD& add){
    add.Sval *= 1000;
    return add;
}

Multiplication inside operator + ? Really? Also, not that you're modifying the parameter, which you shouldn't. It's not intuitive. If you really must do this:

ADD operator+(const ADD& add){
    ADD ret;
    ret.Sval = add.Sval * 1000;
    return ret;
}

Answer 2

The way operator overloading works is that if you have an object of type X, like so:

class X {
    public:
        X( int v ) : x( v ) {}
        int value();
        X operator +( const X& y ) {
            return X( value() + y.value() );
        }
    private:
        int y;
    };

Now if you declare two objects of type X , say X ex and X wye , you can say

X zed = ex + wye;

and get the right result. If type X had more than just a single int field the effect would be more interesting. For example, you could implement 2D vectors and points, and then operations that add and subtract vectors, to get vectors, add and subtract vectors to/from points to get points, and subtract points to get vectors.

Hopefully this will give you enough of an idea of what's going on to be able to restructure your code to get it to do what you want. I may have a detail of syntax wrong, as I'm typing as I go.

Also, I often find the right thing is to declare a friend operator when I want a binary operator:

friend operator + ( vector2D a, vector2D b );

...

inline operator + ( vector2D a, vector 2D b ) {
    return vector2D ( a.x + b.x, a.y + b.y );
}