Why does the copy constructor of an object that is being used for intializing another object gets in-CodePudding

class point
{
public:
    point(double x, double y) : x(x), y(y)
    {
        std::cout << "point parameterized constructor of: "
        << getThis() << std::endl;
    }

    point(const point& that) : x(that.x), y(that.y)
    {
        std::cout << "point copy constructor of: "
        << getThis() << std::endl;
    }

    ~point()
    {
        std::cout << "point destructor of: "
        << getThis() << std::endl;
    }
private:
    double x;
    double y;
    point* getThis() { return this; }
};

class line
{
public:
    line(const point& startPoint, const point& endPoint) :
    startPoint(startPoint)
    endPoint(endPoint)
    {
        std::cout << "line parameterized constructor: " << getThis() << std::endl;
    }
    ~line()
    {
        std::cout << "line destructor of: "
        << getThis() << std::endl;
    }
private:
    point startPoint;
    point endPoint;
    line* getThis() { return this; }
};

int main()
{
    point p1(3.0, 4.0);
    point p2(5.0, 6.0);

    line l1(p1, p2);

    return 0;
}

Output of the program:

point parameterized constructor of: 0x577fffc00
point parameterized constructor of: 0x577fffbe0
point copy constructor of: 0x577fffbb0
point copy constructor of: 0x577fffbc8
lineSegment parameterized constructor of: 0x577fffbb0
lineSegment destructor of: 0x577fffbb0
point destructor of: 0x577fffbc8
point destructor of: 0x577fffbb0
point destructor of: 0x577fffbe0
point destructor of: 0x577fffc00

I don't understand how point's copy constructor gets invoked 2 times (1 for each point parameter) Reason why is originally, line constructors parameters were not const references. And the compiler was giving this warning for the line constructor

Clang-Tidy: The parameter 'endPoint' is copied for each invocation but only used as a const reference; consider making it a const reference

line(point startPoint, point endPoint) : startPoint(startPoint), endPoint(endPoint) {...}

And this was the output:

point parameterized constructor: 0x8feedffa40
point parameterized constructor: 0x8feedffa20
point copy constructor: 0x8feedffa60
point copy constructor: 0x8feedffa80
point copy constructor: 0x8feedff9f0
point copy constructor: 0x8feedffa08
lineSegment parameterized constructor: 0x8feedff9f0
point destructor: 0x8feedffa80
point destructor: 0x8feedffa60
lineSegment destructor: 0x8feedff9f0
point destructor: 0x8feedffa08
point destructor: 0x8feedff9f0
point destructor: 0x8feedffa20
point destructor: 0x8feedffa40

As you can see the copy constructor of the point gets invoked 4 times(2 for each point parameter). I assumed all 4 invocations would go away when I made the parameters const point references. Instead, they halved. Why is that?

CodePudding user response：

In this constructor of the class line

line(const point& startPoint, const point& endPoint) :
startPoint(startPoint)
endPoint(endPoint)
{
    std::cout << "line parameterized constructor: " << getThis() << std::endl;
}

there is used the copy constructor of the type point for data members startPoint and endPoint in this mem-initializer list

startPoint(startPoint)
endPoint(endPoint)

In this constructor

line(point startPoint, point endPoint) : startPoint(startPoint), endPoint(endPoint) {...}

where arguments are not accepted by reference the copy constructor is called two times more to initialize the parameters.