Why is the superclass's default constructor being called?

[johnhoffman]

I am teaching myself the basics of inheritance with the code snippet below.

#include <iostream>
using namespace std;

class Polygon {
  protected:
    int x;
    int y;
  public:
    Polygon(int, int);
};

Polygon::Polygon(int x, int y) {
  this->x = x;
  this->y = y;
}

class Square : public Polygon {
  public:
    Square(int);
    int area();
};

Square::Square(int x) {
  Polygon::Polygon(x, x);
}

int Square::area() {
  return this->x * this->y;
}

int main() {
  Square s(5);
  cout << s.area() << endl;
}

As you see, the Square class inherits properties from the Polygon class. However, I am getting the following compilation errors.

basicInheritance.cpp: In constructor ‘Square::Square(int)’:
basicInheritance.cpp:24: error: no matching function for call to ‘Polygon::Polygon()’
basicInheritance.cpp:13: note: candidates are: Polygon::Polygon(int, int)
basicInheritance.cpp:5: note:                 Polygon::Polygon(const Polygon&)

Why is the default constructor for Polygon being called? I thought that I had explicitly called a custom constructor for Polygon in the constructor for Square.

[Fransje]

In the constructor of a derived class you should call the constructor of the base class in the initialization list, which looks like this:

Square::Square( int x ) : Polygon( x, x ) {
    // constructor body
}

Explanation: http://www.cprogramming.com/tutorial/initialization-lists-c++.html