【题目】阅读程序，先分析程序的执行结果，在上机时运行程序进行对照，再通过单步执行跟踪程序的运行，达到理解基类、派生类中构造函数、析构函数执行过程的目的。
程序如下：
#include <iostream>
using namespace std;
class Part //部件类
{
public:
Part();
Part(int i);
~Part();
private:
int val;
};
class Whole: public Part
{
public:
Whole();
Whole(int,int,int,int);
~Whole();
private:
Part one;
Part two;
int data;
};
Part::Part()
{
val=0;
cout<<"The default constructor of part was called "<<val<<endl;
}
Part::Part(int i)
{
val=i;
cout<<"The constructor of part was called "<<val<<endl;
}
Part::~Part()
{
cout<<"The destructor of part was called "<<val<<endl;
}
Whole::Whole()
{
data=0;
cout<<"The default constructor of whole was called "<<data<<endl;
}
Whole::Whole(int p, int i,int j,int k):Part(p),two(i),one(j),data(k)
{
cout<<"The constructor of whole was called "<<data<<endl;
}
Whole::~Whole()
{
cout<<"The destructor of whole was called "<<data<<endl;
}
void f()
{
Whole w1;
Whole w2(1,2,3,4);
}
int main()
{
f();
system("pause");
return 0;
}
运行结果及解释：
| 运行结果 | 对运行结果的说明 |
| The default constructor of part was called 0 The default constructor of part was called 0 The default constructor of part was called 0 The default constructor of whole was called 0 |
程序中第60行定义对象w1时，执行构造函数的结果。 对于基类part、派生类中的part对象成员执行的都是默认构造函数； 最后执行派生类whole的默认构造函数的函数体中的语句。 |
| The constructor of part was called 1 The constructor of part was called 3 The constructor of part was called 2 The constructor of whole was called 4 |
程序中第61行定义对象w2时，执行构造函数的结果。 调用构造函数的过程也是先基类、再派生类中的对象成员，最后执行派生类构造函数的函数体。 此处执行的均是带参数的构造函数。 注意到在创建对象w2(1,2,3,4)时，第48行函数调用的实参为：Part(1),two(2),one(3),data(4)。 可能让人意外的是，给出的数字是1 3 2 4，而不是1 2 3 4。 这告诉我们，w2中对象成员one的构造函数one(3)执行在前，对象成员two的构造函数two(2)执行在后。 对象成员的构造顺序依其在对象中定义 的顺序（见第20和21行），而不是构造函数中的书写顺序。 为什么？因为在一个whole对象中，各数据成员是顺序存储的，分配空间，one在前，two在后。 C++是人工语言。大多数问题是有依据的，多想想有好处，但初学时常想不到。 所以，一旦有想不通的，将观察得到的现象有大脑中留有映像，随着学习的深入就明白了。 |
| The destructor of whole was called 4 The destructor of part was called 2 The destructor of part was called 3 The destructor of part was called 1 |
退出f()函数的调用时，结束局部对象w2的生命周期，执行析构函数。 要调用的析构函数的顺序正好与前构造函数的顺序相反：先构造的后析构，后构造的先析构。 |
| The destructor of whole was called 0 The destructor of part was called 0 The destructor of part was called 0 The destructor of part was called 0 |
退出f()函数的调用时，结束局部对象w1的生命周期，执行析构函数。 w1比w2先定义，而析构函数的执行却在后。 这个结果是由系统自动决定的，程序员需要明白其中的游戏规则。 |
