Operator *= overloading in C++

Question

The following code demonstrate simple operator overloading:

Class Position has 3 int field and a method to print them.

class Position
{

    private:
        int x,y,z;

    public:
        void print()
        {
            cout << x << "," << y << "," << z << endl;
        }

        Position (int a, int b, int c)
        : x(4),
        y(50),
        z(23)
        {
            x=a;
            y=b;
            z=c;
        }

        Position operator+(const Position &other);
        Position operator*=(const int &multi);
};

Operators + and *= are overloaded as such:

Position Position::operator+ (const Position &other)
{
    x = x + other.x;
    y = y + other.y;
    z = z + other.z;

    return *this;
}


Position Position::operator*= (const int &multi)
{
    x = x * multi;
    y = y * multi;
    z = z * multi;

    return *this;    
}

The code runs:

int main()
{
    Position p5( 1, 2, 3 );
    p5.print();

    Position p6( 4, 5, 6 );
    p6.print();

    Position p7 = p5 + p6;
    p7.print();

    p7 *= 2;
    p7.print();

    (p7 *= 2) *= 3;
    p7.print();
}

The result yielded are:

1,2,3
4,5,6
5,7,9
10,14,18
20,28,36

Question is why won't the last result perform as it should when it is done in nested?

Because you do not return Position by reference from operator *=. — blackbird
– blackbird, Commented May 31, 2014 at 17:49
Your binary operator+ modifies the LHS operand. This is unlikely to be the desired behaviour. — juanchopanza
– juanchopanza, Commented May 31, 2014 at 17:49
See the Binary Arithmetic Operators section of this answer: Common operators to overload — Blastfurnace
– Blastfurnace, Commented May 31, 2014 at 17:52
The correct answer has already been given, but your constructor is also strange. It first initialises your members to some hard-coded values, then assigns the real ones. Why doesn't it initialise them to the real ones? — Christian Hackl
– Christian Hackl, Commented May 31, 2014 at 17:57

legends2k · Accepted Answer · 2019-08-19 04:13:29Z

9

(p7 *= 2) *= 3;

won't work since you're assigning to a temporary, as you return by value in the operator*= function.

For the sub-expression

p7 *= 2

your operator function is called only for its side-effect, as the temporary it returns would be thrown away. To know more about temporaries, read What are C++ temporaries? and Temporary objects - when are they created, how do you recognise them in code?

When you want expression chaining, you've to return by reference like so

Position& Position::operator*= (int multi)
{
    x = x * multi;
    y = y * multi;
    z = z * multi;

    return *this;    
}

Aside: You don't have to pass built-in types by const reference(const int &multi), non-const copy (int multi) should do fine.

edited Aug 19, 2019 at 4:13

answered May 31, 2014 at 17:48

legends2k

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2 Comments

Christian Hackl Over a year ago

You probably mean "primitive types", not "POD", if by "POD" you mean structs with only public member variables.

legends2k Over a year ago

@ChristianHackl: Aah, right, thanks for bringing it up.

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