HOWTO: Clear an istream Object During Extraction

• The C Run-Time (CRT) included with: - Microsoft C/C++ version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

Testing for errors during extraction is important. For example:

   int  n = 0;
   while (n <= 100) {cin >> n;}

MORE INFORMATION

The clear() member function clears the fail bit. However, the istream object is still unusable. The sample code below clears the fail bit and extracts the unusable characters left in the streambuf object.

Sample Code

   /* No special compile options needed. */

  #include <iostream.h>

   int ClearError(istream& isIn)        // Clears istream object
   {
      streambuf*  sbpThis;
      char        szTempBuf[20];
      int         nCount, nRet = isIn.rdstate();

      if  (nRet)                        // Any errors?
      {
          isIn.clear();                 // Clear error flags
          sbpThis = isIn.rdbuf();       // Get streambuf pointer
          nCount = sbpThis->in_avail(); // Number of characters in buffer

          while (nCount)                // Extract them to szTempBuf
          {
              if  (nCount > 20)
              {
                  sbpThis->sgetn(szTempBuf, 20);
                  nCount -= 20;
              }
              else
              {
                  sbpThis->sgetn(szTempBuf, nCount);
                  nCount = 0;
              }
          }
      }

      return  nRet;
   }

   void main()
   {
      int  n = 0, nState;
      while (n <= 100)
      {
         cout << "Please enter an integer greater than 100.\n";
         cin >> n;
         nState = ClearError(cin);   // Clears any errors in cin
      }
   }

REFERENCES

iostream Class Library Reference, Chapter 1

Keywords          : CPPLngIss CRTIss kbcode
Version           : MS-DOS:7.0;WIN3X:1.0,1.5,1.51,1.52;WINNT:2.0,2.1,4.0,5.0;
Platform          : MS-DOS NT WINDOWS
Issue type        : kbhowto