C++ 编程 : Asking user if they would like to overwrite a file if it already exists

Question

我是一名计算机科学专业的大一新生，正在为我的编程原理 1 课布置作业。其中一小部分涉及询问用户是否要覆盖已存在的文件。我正在使用 Code::Blocks，请记住我只是在上第一堂编程课。这是我为此编写的代码:

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
   char outputFileName[81];
   ifstream tempInputFile; // to check if file already exists
   char overwrite;
   ofstream outputFile;

   system( "cls" );
   cout << endl << endl;

   cout << "Please enter the file for data to be written to: ";
   cin >> outputFileName;
   cout << endl << endl;
   tempInputFile.open( outputFileName );
   while ( tempInputFile )
   {
      cout << "This file already exists. Would you like to overwrite?  (Y/N):";
      cin >> overwrite;
      cout << endl << endl;
      if ( overwrite == 'Y' || overwrite == 'y' )
         tempInputFile.close();
           // I also tried a block if statement here with .clear() before the .close()
      else
      {
         tempInputFile.close();
         cout << "Please enter the file for data to be written to: ";
         cin >> outputFileName;
         cout << endl << endl;
         tempInputFile.open( outputFileName );
      } // end else
   } // end while

   tempInputFile.close();
   outputFile.open( outputFileName );

   cout << "The file is ready to be written to..." << endl << endl;

   cout << endl << endl;
   system( "pause" );
   return 0;
} // end main()

问题是如果输入一个现有文件，然后用户回答“Y”覆盖该文件，程序将再次询问他们是否要覆盖该文件。第二次回答“Y”会继续，但我不明白为什么要第二次问。我终于写了一些正确处理它的代码(使用 do...while 循环)，但我想知道为什么其他代码没有按照我认为应该做的去做。有人可以向我解释一下吗？这是我编写的代码，它正在执行我想要的操作:

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
   char outputFileName[81];
   ifstream tempInputFile;
   char overwrite;
   ofstream outputFile;

   system( "cls" );
   cout << endl << endl;

   cout << "Please enter the file for data to be written to: ";
   cin >> outputFileName;
   cout << endl << endl;

   tempInputFile.open( outputFileName );
   if ( tempInputFile )
      do
      {
         cout << "This file already exists. Would you like to overwrite?  (Y/N):";
         cin >> overwrite;
         cout << endl << endl;
         if ( overwrite == 'N' || overwrite == 'n' )
         {
            tempInputFile.close();
            cout << "Please enter the file for data to be written to: ";
            cin >> outputFileName;
            cout << endl << endl;
            tempInputFile.open( outputFileName );
            if ( !tempInputFile )
               overwrite = 'Y';
         } // end if
      } while ( overwrite == 'N' || overwrite == 'n' );

   cout << "The file is ready to be written to..." << endl << endl;

   tempInputFile.close();
   outputFile.open( outputFileName );

   cout << endl << endl;
   system( "pause" );
   return 0;
} // end main()

Answer 1

让我们关注这部分代码:

tempInputFile.open( outputFileName );
while ( tempInputFile )
{
  cout << "This file already exists. Would you like to overwrite?  (Y/N):";
  cin >> overwrite;
  cout << endl << endl;
  if ( overwrite == 'Y' || overwrite == 'y' )
     tempInputFile.close();       
  else
  {
      // not important
  } // end else
} // end while

当 while 在成功 open 后第一次被调用时，你询问用户是否要覆盖文件，如果答案是 Y 您关闭文件，while 循环再次检查条件。

问题是 while (tempInputFile) 表达式检查是否有 errors .由于关闭文件通常不会造成任何问题，条件为 true 并且您再次进入 while 的主体。然后您再次尝试关闭文件，但由于文件已经关闭，这里失败了。因此，当第 3 次调用 while ( tempInputFile ) 时，它以 false 退出。