What is difference between these two pieces of code when we do operator overloading?-CodePudding

Code 1:

class CVector {
public:
    int x, y;
    CVector() {};
    CVector(int a, int b) :x(a), y(b) {}
};

CVector operator- (const CVector& lhs, const CVector& rhs)
{
    CVector temp;
    temp.x = lhs.x - rhs.x;
    temp.y = lhs.y - rhs.y;
    return temp;
}

int main()
{
    CVector foo(2, 3);
    CVector bar(3, 2);
    CVector result;
    result = foo - bar;
    cout << result.x << ',' << result.y << endl;
}

Code 2:

class CVector {
public:
    int x, y;
    CVector() {};
    CVector(int a, int b) :x(a), y(b) {}
};

CVector operator- (const CVector& lhs, const CVector& rhs)
{
    return CVector(lhs.x - rhs.x, lhs.y - rhs.y);
}

int main()
{
    CVector foo(2, 3);
    CVector bar(3, 2);
    CVector result;
    result = foo - bar;
    cout << result.x << ',' << result.y << endl;
}

These two pieces of code operate identically. I want to know why we can write CVector(lhs.x - rhs.x, lhs.y - rhs.y). What does it mean? What happens when we use a class without a name?

CodePudding user response：

why we can write CVector(lhs.x - rhs.x, lhs.y - rhs.y). What does it mean? What happens when we use a class without a name?

The expression CVector(lhs.x - rhs.x, lhs.y - rhs.y) uses the pameterized constructor CVector::CVector(int, int) to construct a CVector by passing the arguments lhs.x - rhs.x and lhs.y - rhs.y. Just like CVector foo(2, 3); and CVector bar(3, 2) uses the parameterized constructor, the expression CVector(lhs.x - rhs.x, lhs.y - rhs.y) also uses the parameterized ctor.

You can confirm this by adding a cout inside the parameterized constructor as shown below:

#include <iostream>

class CVector {
public:
    int x, y;
    CVector() {};
    CVector(int a, int b) :x(a), y(b) {
        std::cout<<"paremterized ctor used"<<std::endl;
        }
};

CVector operator- (const CVector& lhs, const CVector& rhs)
{
    return CVector(lhs.x - rhs.x, lhs.y - rhs.y); //this uses the parameterized ctor
}

int main()
{
    CVector foo(2, 3);
    CVector bar(3, 2);
    CVector result;
    result = foo - bar;
    std::cout << result.x << ',' << result.y << std::endl;
}

The output of the above program is:

paremterized ctor used
paremterized ctor used
paremterized ctor used
-1,1

Notice in the output shown above, the third call to the parameterized ctor is due to the return statement return CVector(lhs.x - rhs.x, lhs.y - rhs.y);