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(iostream-295.info)Backing Up

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Backing up
==========

   The GNU iostream library allows you to ask a `streambuf' to remember
the current position.  This allows you to go back to this position
later, after reading further.  You can back up arbitrary amounts, even
on unbuffered files or multiple buffers' worth, as long as you tell the
library in advance.  This unbounded backup is very useful for scanning
and parsing applications.  This example shows a typical scenario:

     // Read either "dog", "hound", or "hounddog".
     // If "dog" is found, return 1.
     // If "hound" is found, return 2.
     // If "hounddog" is found, return 3.
     // If none of these are found, return -1.
     int my_scan(streambuf* sb)
     {
         streammarker fence(sb);
         char buffer[20];
         // Try reading "hounddog":
         if (sb->sgetn(buffer, 8) == 8
             && strncmp(buffer, "hounddog", 8) == 0)
           return 3;
         // No, no "hounddog":  Back up to 'fence'
         sb->seekmark(fence); //
         // ... and try reading "dog":
         if (sb->sgetn(buffer, 3) == 3
             && strncmp(buffer, "dog", 3) == 0)
           return 1;
         // No, no "dog" either:  Back up to 'fence'
         sb->seekmark(fence); //
         // ... and try reading "hound":
         if (sb->sgetn(buffer, 5) == 5
             && strncmp(buffer, "hound", 5) == 0)
           return 2;
         // No, no "hound" either:  Back up and signal failure.
         sb->seekmark(fence); // Backup to 'fence'
         return -1;
     }

 - Constructor:  streammarker::streammarker (streambuf* SBUF)
     Create a `streammarker' associated with SBUF that remembers the
     current position of the get pointer.

 - Method: int streammarker::delta (streammarker& MARK2)
     Return the difference between the get positions corresponding to
     `*this' and MARK2 (which must point into the same `streambuffer'
     as `this').

 - Method: int streammarker::delta ()
     Return the position relative to the streambuffer's current get
     position.

 - Method: int streambuf::seekmark (streammarker& MARK)
     Move the get pointer to where it (logically) was when MARK was
     constructed.