派生类基础问题

[angelcm51]

B是A的一个派生类，声明bb1的时候会先进入基类A中，显示A's default constructor called.
可是接下来为什么是A's default constructor called.
B's default constructor called.
最后析构的时候也是，进入基类中
B's destructor called.
A's destructor called.
A's destructor called.
在A中析构了2次，为什么？

#include "iostream"
using namespace std;
class A
{
public:
	A(){a=0;cout<<"A's default constructor called.\n";}
	A(int i){a=1;cout<<"A's constructor called.\n";}
	~A(){cout<<"A's destructor called.\n";}
	void print() const{cout<<a<<",";}
	int geta(){return a;}
private:
	int a;
};
class B:public A
{
public:
	B(){b=0,cout<<"B's default constructor called.\n";}
	B(int i,int j,int k);
	~B(){cout<<"B's destructor called.\n";}
	void print();
private:
	int b;
	A aa;
};
B::B(int i,int j,int k):A(i),aa(k)
{
	b=j;
	cout<<"B's constructor called.\n";
}
void B::print()
{
	A::print();
	cout<<b<<","<<aa.geta()<<endl;
}
void main()
{
	B bb1;
	B bb2(1,2,3);
	bb1.print();
	bb2.print();
}

结果是：
A's default constructor called
A's default constructor called
B's default constructor called
A's constructor called.
A's constructor called.
B's constructor called.
0，0，0
1，2，1
B's destructor called.
A's destructor called.
A's destructor called.
B's destructor called.
A's destructor called.
A's destructor called.

[nicky_zs]

每个子类对象都会隐式地包含一个基类对象。LZ的class B中既隐式地包含了一个A类对象，又显式地包含了一个。所以在B的constructor被调用之前，当然会有两次A的constructor.

[brookmill]

B从A派生，另外B里面还有一个A类的成员，所以各自调用一次A的构造函数。
这种设计有点奇怪……

[angelcm51]

为什么在第一次回到基类中调用了A's default constructor called
到达bb2(1,2,3)的时候又有一次
A's default constructor called
B's default constructor called
不明白，自学C++迷茫啊。。。

[angelcm51]

谢谢1楼的。