C++编程之引用和拷贝构造函数、按值传递和返回、位拷贝与初始化等实例

对于传递和返回大的简单结构有了可使用的方法

一个类在任何时候都知道她存在多少个对象

//: c11:howmany.cpp
// from thinking in c++, 2nd edition
// available at http://www.51sjk.com/Upload/Articles/1/0/251/251209_20210626000555044.com
// (c) bruce eckel 2000
// copyright notice in copyright.txt
// a class that counts its objects
#include 
#include 
using namespace std;
ofstream out("howmany.out");

class howmany {
 static int objectcount;
public:
 howmany() { objectcount++; }
 static void print(const string& msg = "") {
  if(msg.size() != 0) out << msg << ": ";
  out << "objectcount = "
    << objectcount << endl;
 }
 ~howmany() {
  objectcount--;
  print("~howmany()");
 }
};

int howmany::objectcount = 0;

// pass and return by value:
howmany f(howmany x) {
 x.print("x argument inside f()");
 return x;
}

int main() {
 howmany h;
 howmany::print("after construction of h");
 howmany h2 = f(h);
 howmany::print("after call to f()");
} ///:~

howmany类包括一个静态变量int objectcount和一个用于报告这个变量的

静态成员函数print(),这个函数有一个可选择的消息参数

输出不是我们期望的那样

after construction of h: objectcount = 1

x argument inside f(): objectcount = 1

~howmany(): objectcount = 0

after call to f(): objectcount = 0

~howmany(): objectcount = -1

~howmany(): objectcount = -2

在h生成以后，对象数是1，这是对的

原来的对象h存在函数框架之外，同时在函数体内又增加了一个对象，这个对象

是通过传值方式传入的对象的拷贝

在对f()的调用的最后，当局部对象出了其范围时，析构函数就被调用，析构

函数使objectcount减小