Class inheritence guide

[seungyeon]

what does the lines i indicated do?

class A
{
	protected:
		int y;
	private:
		int z;
	public:
		int x;
		
		A(int a, int b, int c):x(a), y(b), z(c) // constructor but that colon and x(a), y(b). what does this do
		{
		}
		
		void display()
		{
    		cout << "x is " << x << endl;
  			cout << "y is " << y << endl;
	  		cout << "z is " << z << endl;
		}
};
class B : public A
{
	private:
		int num; 
	public:
	B(int x, int y, int z, int extra): A(x, y, z), num(extra) // same here its a constructor but what does the stuff after the colon do?
	{
	}

	void display()
	{
    	cout << "x is " << x << endl;
    	cout << "y is " << y << endl;
    }
	   
};

[TheIdeasMan]

That's a member initialization list :+)

http://en.cppreference.com/w/cpp/language/initializer_list

The x(a) is an example of direct initialization.

http://en.cppreference.com/w/cpp/language/initialization

Edit:

The A(x, y, z) calls the base class constructor

[seungyeon]

its basicaly like this right?

A::A(int a, int b, int c)
{
      x = a;
      y = b;
      z = c;
}

[TheIdeasMan]

Yes, but better.

Classes get their member variables initialised before the constructor starts (the opening brace), either by the member initializer list or default initialisation. When one has assignment statements in the constructor body, they get re-initialised - which is a little inefficient.