子类调用父类的构造函数

          在项目中，发现很多子类调用父类构造函数的情况，现在特地来熟悉一下：

#include <iostream>
using namespace std;
 
class A
{
public:
	int a;
	int b;
	
public:
	A():a(0), b(0)
	{
		cout << "default A constructor" << endl;
	}
 
	A(int x, int y):a(x), b(y)
	{
		cout << "A constructor" << endl;
	}
};
 
class B : public A
{
public:
	int c;
 
public:
	B():A(), c(0)
	{
		cout << "default B constructor" << endl;
	}
 
	B(int x, int y, int z):A(x, y), c(z)
	{
		cout << "B constructor" << endl;
	}
};
 
int main()
{
	B b1;
	cout << b1.a << endl;	
	cout << b1.b << endl;
	cout << b1.c << endl;
 
	B b2(1, 2, 3);
	cout << b2.a << endl;	
	cout << b2.b << endl;
	cout << b2.c << endl;
 
	return 0;
}