在运算符重载参数列表中包含const会导致错误（C ++）

Question

I am trying out with operator overloading, for which I wrote below code

class OwnClass
{
private:
    int x,y;
public:
    OwnClass(int x, int y) { SetX(x); SetY(y); }
    int GetX() { return x; }
    void SetX(int x) { this->x = x;}
    int GetY() { return y; }
    void SetY(int y) {this->y = y;}

    OwnClass& operator + (const OwnClass &o)  // Problematic line
    {
        this->x += o.GetX();
        this->y += o.GetY();

        return *this;
    }
};

When compiled, following error is shown

fun.cpp(65): error C2662: 'OwnClass::GetX' : cannot convert 'this' pointer from 'const OwnClass' to 'OwnClass &' Conversion loses qualifiers

fun.cpp(66): error C2662: 'OwnClass::GetY' : cannot convert 'this' pointer from 'const OwnClass' to 'OwnClass &' Conversion loses qualifiers

When I modify the code as under, it compiles fine.

OwnClass& operator + (OwnClass &o)  // removed const
{
    this->x += o.GetX();
    this->y += o.GetY();

    return *this;
}

I could not understand why so ? I mean I am not able to understand the compiler error.

Answer 1

The parameter o is declared as reference to const , which can't be called with GetX and GetY , because they're non-const member function. You can (and should) change them to const member functions to solve the issue.

int GetX() const { return x; }
int GetY() const { return y; }

BTW: In general binary operator+ is not supposed to return a reference to non-const. It's better to return a new object by value.

OwnClass operator + (const OwnClass &o) const
{
    OwnClass r(GetX(), GetY());
    r.x += o.GetX();
    r.y += o.GetY();

    return r;
}

Note for this case operator+ could (and should) be declared as const member function too. And as @MM suggested, making it non-member function would be better.

Answer 2

The problem is that you are calling non-const member functions on const objects. Make getters const to fix this problem:

int GetX() const { return x; }
int GetY() const { return y; }