构造函数和析构函数的有关问题

构造函数和析构函数的问题
//pointer.h
class   A{
public:
    A();
    A(int   _x);
    ~A();
private:
    int   x;
};

class   B{
public:
B();
B(int   x,int   y,   A   a);
                  ~B();
private:
int   m_x;
int   m_y;
A   m_a;
};

//pointer.cpp
#include   "Pointer.h "
#include   <iostream>
using   namespace   std;

A::A()
{
      cout   < <   "constructor   A() "   < <endl;
}

A::A(int   _x)
{
      x   =   _x;
      cout   < <   "constructor   A(int   _x) "   < <endl;
}

A::~A()
{
      cout   < <   "unconstructor   ~A() "   < <endl;
}

B::B()
{
cout   < <   "constructor   B() "   < <endl;
}

B::B(int   x,   int   y,   A   a):m_a(a)
{
m_x   =   x;
m_y   =   y;
cout   < <   "constructor   B(int   x,int   y,   A   a):m_a(a) "   < <endl;
}

B::~B()
{
cout   < <   "m_x   =   "   < <   m_x   < <   "   m_y   =   "   < <   m_y   < <endl;
cout   < <   "unconstructor   ~B() "   < <endl;
}

//main.cpp
#include   "Pointer.h "
int   main()
{
A   aInstance;
B   bInstance(10,10,aInstance);
      return   0;
}

运行结果如下：
constructor   A()
constructor   B(int   x,int   y,   A   a):m_a(a)
unconstructor   ~A()
m_x   =   10   m_y   =   10
unconstructor   ~B()
unconstructor   ~A()
unconstructor   ~A()
请按任意键继续.   .   .

我觉得多了一个unconstructor   ~A()，为什么会这样呢？请大家帮助分析一下？

------解决方案--------------------
给你的A加一个拷贝构造函数，里面加一个打印。
------解决方案--------------------
//pointer.h
#ifndef POINTER_H
#define POINTER_H

class A{
public:
A();
A(int _x);
A(const A &a);
~A();
private:
int x;
};

class B{
public:
B();
B(int x,int y, A &a);
~B();
private:
int m_x;
int m_y;
A *m_a;
};

#endif


//pointer.cpp
#include "Pointer.h "
#include <iostream>
using namespace std;

A::A()
{
cout < < "constructor A() " < <endl;
}

A::A(int _x)
{
x = _x;
cout < < "constructor A(int _x) " < <endl;
}

A::A(const A &a)
{
x = a.x;
cout < < "constructor A(const A &a) " < <endl;
}

A::~A()
{
cout < < "unconstructor ~A() " < <endl;
}

B::B()
{
m_a = NULL;