私人类成员未完全封装？ [英] Private class members not fully encapsulated?

问题描述

这是使用C ++的实例层级封装的后续文章。 / p>

我定义了一个类，并从该类创建了两个对象。

  #include< iostream> 
 #include< ctime> 
 #include< string> 
 
 using namespace std; 
 
 class timeclass {
 private：
 string date; 
 
 time_t gmrawtime，rawtime; 
 struct tm * timeinfo; 
 char file_date [9]; 
 
 void tm_init（int）; 
 
 public：
 timeclass（int）; 
 void print_date（）; 
}; 
 
 void timeclass :: tm_init（int y）{
 timeinfo = gmtime（& rawtime）; 
 timeinfo-> tm_year = y-1900; // timeinfo-> tm_year保存自1900年以来的年数
 timeinfo-> tm_mon = 0; 
 timeinfo-> tm_mday = 1; 
 timeinfo-> tm_hour = 0; 
 timeinfo-> tm_min = 0; 
 timeinfo-> tm_sec = 0; 
} 
 
 timeclass :: timeclass（int y）{
 timeclass :: tm_init（y） 
 gmrawtime = mktime（timeinfo） -  timezone; 
} 
 
 void timeclass :: print_date（）{
 strftime（file_date，9，％Y％m％d，timeinfo）; 
 
 date = string（file_date）; 
 cout<<< date<< endl; 
} 
 
 / * ----------------------------------- ------------------------------------ * / 
 
 int main（） 
 {
 timeclass time1（1991）; 
 timeclass time2（1992）; 
 
 time1.print_date（）; //打印19920101，尽管使用1991初始化
 time2.print_date（）; // Prints 19920101，as expected 
 
 return 0; 
} 
  

这个例子是从我的主程序切片和切片的日期计数器的一部分，但它说明了我的观点。我想有一个日期计数器运行每个类的实例（time1和time2），但它看起来像一旦我构造time2对象，'timeinfo'变量，我认为被封装在time1被time2构造函数覆盖。

我知道C ++只支持类级封装，我想知道我的问题是因为同一个类的成员可以访问对方的私有成员。有没有办法解决这个问题，所以我可以实现我想做什么？谢谢
Taylor

解决方案

gmtime（） localtime（）， ctime（）和 asctime（）返回指向静态数据的指针。因此，后续调用可能会覆盖先前调用写入的信息。这也意味着这些调用不是线程安全的，虽然在这种情况下不涉及多个线程。

其他答案提供了这种限制的可能的解决方法。

This is a follow up post from Instance-level encapsulation with C++.

I've defined a class and created two objects from that class.

#include <iostream>
#include <ctime>
#include <string>

using namespace std;

class timeclass {
  private:
  string date;

  time_t gmrawtime, rawtime;
  struct tm * timeinfo;
  char file_date[9];

  void tm_init(int);

public:
  timeclass(int);
  void print_date();
};

void timeclass::tm_init(int y) {
  timeinfo = gmtime(&rawtime);
  timeinfo->tm_year = y - 1900; // timeinfo->tm_year holds number of years since 1900
  timeinfo->tm_mon = 0;
  timeinfo->tm_mday = 1;
  timeinfo->tm_hour = 0;
  timeinfo->tm_min= 0;
  timeinfo->tm_sec= 0;
}

timeclass::timeclass(int y) {
  timeclass::tm_init(y);
  gmrawtime = mktime(timeinfo) - timezone; 
}

void timeclass::print_date() {
  strftime(file_date,9,"%Y%m%d",timeinfo);

  date = string(file_date);
  cout<<date<<endl;
}

/* -----------------------------------------------------------------------*/

int main()
{
  timeclass time1(1991); 
  timeclass time2(1992); 

  time1.print_date(); // Prints 19920101, despite being initialized with 1991
  time2.print_date(); // Prints 19920101, as expected

  return 0;
}

This example is part of a date counter sliced and diced from my main program, but it illustrates my point. I want to have a date counter running for each instance of the class (time1 and time2), but it looks like once I construct the time2 object, the 'timeinfo' variable that I thought was encapsulated in time1 gets overwritten by the time2 constructor.

I am aware that C++ supports only class-level encapsulation, and am wondering if my problem is because members of the same class have access to one another's private members. Is there a way around this, so I can achieve what I want to do? Thank you, Taylor

解决方案

gmtime(), localtime(), ctime() and asctime() return a pointer to static data. So subsequent calls may overwrite information written by previous calls. This also means that these calls are not thread-safe although in this case multiple threads are not involved.

Other answers have provided possible workarounds for this limitation.

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