std::ofstream 对象无法正常关闭的任何原因？

Question

我注意到在我的 C++ 代码中，每当我关闭 std::ofstream 对象时，我都无法重新打开使用 std::ifstream 关闭的文件。 std::ifstream 的 open 函数总是会失败。

我可以做些什么来确保我的 std::ofstream 对象正确关闭吗？

有人可能会要求查看我的具体代码，因此为了使这篇文章保持较小规模，我将其粘贴到此处。在我的代码中，运行完情况 a 或 d 后，所有 std::ifstream open 调用都会失败。 （在发布这个问题之前，有几个人在玩我的代码，除了 std::ofstream close 因未知原因而失败之外，他们无法得出任何结论）

提前感谢收到的所有回复。

代码是

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

typedef struct Entry
{
   string Name;
   string Address;
   string Phone;   
};

int main()
{
   bool exit = false, found = false;
   char input, ch;
   string stringinput, stringoutput;
   ifstream fin, ibuffer;
   ofstream fout, obuffer;
   Entry buffer;

   while(!exit)
   {
      cout << "Welcome to the Address Book Application!" << endl << endl;
      cout << "\tSelect an option:" << endl << endl;
      cout << "\tA -- Add New Entry" << endl;
      cout << "\tD -- Delete an Entry" << endl;
      cout << "\tS -- Search for an Existing Entry" << endl;
      cout << "\tE -- Exit" << endl << endl;

      cin >> input;

      switch(input)
      {
         case 'a':
         case 'A':
         cin.ignore(255,'\n');//Apparently necessary because an extra new line carrys over in the cin stream
         system("cls");
         //Get Information from User
         cout << "Enter Phone Number: ";
         getline(cin, buffer.Phone);
         cout << endl << "Enter Name: ";
         getline(cin, buffer.Name);
         cout << endl << "Enter Address: ";
         getline(cin, buffer.Address);
         /*Copy existing database into a buffer. In other words, back it up*/
         fin.open("address.txt");
         if(!fin)
         {
            fin.close();
            fout.open("address.txt");
            fout << buffer.Phone << endl << buffer.Name << endl << buffer.Address << endl;
         }
         if(fin)
         {
            obuffer.open("buffer.txt");
            while(fin && fin.get(ch))
               obuffer.put(ch);
            fin.close();
            obuffer.close();
            /*Copy buffer to new database file*/
            ibuffer.open("buffer.txt");
            fout.open("address.txt");//This removes all of the previously existing info from database.txt
            while(ibuffer && ibuffer.get(ch))
               fout.put(ch);
            ibuffer.close();
            remove("buffer.txt");//Delete the buffer
            fout << endl << buffer.Phone << endl << buffer.Name << endl << buffer.Address << endl;
         }

         buffer.Phone.erase();
         buffer.Name.erase();
         buffer.Address.erase();
         fout.close();
         break;

         case 'd':
         case 'D':
         cin.ignore(255,'\n');//Apparently necessary because an extra new line carrys over in the cin stream
         system("cls");
         cout << "Enter the phone number of the entry to delete: ";
         cin >> stringinput;
         fin.open("address.txt");
         if(!fin)
         {
            cout << endl << "No entries exist!";
            fin.close();
            break;
         }
         obuffer.open("buffer.txt");
         /* Copy everything over into the buffer besides the account we wish to delete */
         while(!fin.eof())
         {

            fin.read(&ch, sizeof(char));

            if(ch != '\n' && ch != '\0')
            stringoutput += ch;

            if(ch == '\n' || ch == '\0')
            {
               if(stringinput.compare(stringoutput))
               {
                  stringoutput += ch;
                  obuffer << stringoutput;
                  stringoutput.erase();
               }

               if(!stringinput.compare(stringoutput))
               {
                  stringoutput += ch;
                  getline(fin, stringoutput);
                  getline(fin, stringoutput);
                  fin.read(&ch, sizeof(char));//Get rid of the extra '\n'
                  stringoutput.erase();
               }

            }

         }

         //Hack: Copy the last line over since the loop for some reason doesn't
         obuffer << stringoutput;
         stringoutput.erase();

         fin.close();
         obuffer.close();

         fout.open("address.txt");
         ibuffer.open("buffer.txt");

         while(ibuffer && ibuffer.get(ch))
            fout.put(ch);

         ibuffer.close();
         fout.close();
         remove("buffer.txt");

         cout << endl << "Entry " << stringinput << " deleted successfully!" << endl;
         stringinput.erase();
         stringoutput.erase();
         break;

         case 's':
         case 'S':
         cin.ignore(255,'\n');//Apparently necessary because an extra new line carrys over in the cin stream
         system("cls");

         found = false;

         fin.open("address.txt");
         if(!fin)
         {
            cout << "No entries currently exist!" << endl << endl;
            fin.close();
            break;
         }

         cout << "Enter the phone number to search for: ";
         cin >> stringinput;

         while(!fin.eof())
         {
            fin.read(&ch, sizeof(char));

            if(ch != '\n' && ch != '\0')
               stringoutput += ch;

            if(ch == '\n' || ch == '\0')
            {
               if(!stringinput.compare(stringoutput))
               {
                  found = true;
                  break;
               }

               stringoutput.erase();
            }

         }

         if(found)
         {
            cout << "Phone Number: " << stringinput << endl;
            getline(fin, stringoutput);
            cout << "Name: " << stringoutput << endl;
            getline(fin, stringoutput);
            cout << "Address: " << stringoutput << endl << endl;
         }

         if(!found)
         {
            stringoutput.erase();
            cout << endl << stringinput << " is not in the address book!" << endl << endl;
         }

         stringinput.erase();
         stringoutput.erase();
         fin.close();
         break;

         case 'e':
         case 'E':
         exit = true;
         break;

         default:
         system("cls");
         cout << input << " is not a valid option." << endl << endl;
         break;
      }

   }

   return 0;

}

Answer 1

确保流在此处理中的每个点都重置的最佳方法不是显式关闭它们，而是在使用时声明它们并允许它们超出范围。这是评论中提出的 RAII 观点。在您的情况下，这意味着将声明从函数顶部移动到使用它们的开关的每个臂内部。这样，您可以确保当您从特定的 case 退出时，每个文件都被彻底关闭，因为 ofstream 和 ifstream 在范围退出时销毁期间关闭文件。

我注意到的另一件事是，您在文件打开后测试了一个奇怪的事情：

 fin.open("address.txt");
 if(!fin)

这测试了一个坏流，但我认为它还没有完成 - 如果您正在测试成功打开，您还应该测试

 fin.open("address.txt");
 if (!fin.is_open())

所有文件处理循环和 open() 调用需要检查 fin.fail() 或 fout.fail() 以及 fin.eof ()，以排除文件访问错误作为未处理的情况，从而混淆您观察到的行为。

此时，我建议您纠正这些明显的误解，如果您无法使其正常工作，请回来提出更具体的问题。