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 https://www.BruceEckel.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