604.cpp

C++实训教程

//程序中Book类有一个静态成员Num，记录总的对象个数。但运行的结果有点"奇怪"：在main()标记的①处，应输出："Num of Books is 1."运行正确，因为此前只有一个对象--r1。接着②处调用fn(r1)，进入fn(Book c)内创建了4个对象：形参c、对象o1、o2、o3，至此共有5个对象，但却输出："Num of Books is 2."。在③处，因为退出fn()，4个对象撤消，至此只有一个对象--r1，但输出"Num of Books is -2."。⑴原因是什么？⑵如何修改524.cpp。
//604.CPP  
#include <iostream.h>
class Book
{
		int page;
		static int Num;// private静态成员变量
	public:
		Book(int pp){page=pp;Num++;}
		//Book(Book & r){page=r.page; Num++;}//Must be here
		~Book(){Num--;}
		static int GetNum(){return Num;}
};
void fn(Book c);
int Book::Num=0;// 对静态公有成员变量进行初始化

void main()
{
	Book r1(20.0);
	cout<<"Num of Books is "<<Book::GetNum()<<".\n";//①
	fn(r1);//②
	cout<<"Num of Books is "<<Book::GetNum()<<".\n";//③
}
void fn(Book c)
{
	Book obj=c;
	cout<<"Num of Books is "<<Book::GetNum()<<".\n";
}
/*
Num of Books is 1.
Num of Books is 3.
Num of Books is 1.

*/
/* if without =overload
Num of Books is 1.
Num of Books is 1.
Num of Books is -1.

*/