Why doesn't the base class constructor set the values? - c++

Question

Here, values of x and y is being tried to set through base class constructor.
But, the code is unable to do so.
#include <iostream>
class Point2d {
public:
double x;
double y;
Point2d() : x(0), y(0) {
}
Point2d(double x, double y) : x(x), y(y) {
}
void Show() {
std::cout << "(" << x << "," << y << ")\n";
}
};
class Vector2d : public Point2d {
public:
Vector2d():Point2d(){
}
Vector2d(double x, double y) : Point2d(x,y) {
}
Vector2d(Vector2d const& vec) : Point2d(vec){
}
void Set(double x, double y) {
Point2d::Point2d(x, y);
}
};
int main() {
Vector2d v;
v.Set(20, -39);
v.Show(); // prints '(0,0)' instead of '(20,-39)'
}
My target is to reuse base class constructor, and, overloaded assignment operators as much as possible.

Answer 1

I'm afraid your code won't even compile at
void Set(double x, double y)
{
Point2d::Point2d(x, y);
}
A constructor of a base class should be called at the beginning of the member initializer lists of a subclass constructor, not in a member function.
What you need is probably
class Point2d {
public:
double x;
double y;
Point2d() : x(0), y(0) {
}
Point2d(double x, double y) : x(x), y(y) {
}
void Show() {
std::cout << "(" << x << "," << y << ")\n";
}
Point2d& operator=(Point2d const& rhs)
{
this->x = rhs.x;
this->y = rhs.y;
}
};
class Vector2d : public Point2d {
public:
Vector2d():Point2d(){
}
Vector2d(double x, double y) : Point2d(x,y) {
}
Vector2d(Vector2d const& vec) : Point2d(vec){
}
/* also need to be overloaded in the subclass */
Vector2d& operator=(Vector2d const& rhs)
{
Point2d::operator=(rhs);
return *this;
}
void Set(double x, double y) {
*this = Vector2d(x, y);
}
};
int main() {
Vector2d v;
v.Set(20, -39);
v.Show();
}

Answer 2

Well, you're trying to construct the base class in the setter function, but the base class will already be constructed when the derived class is constructed. I would just set the x and y values of the base class in the setter.