1. Stream types usually support random access to data in streams.
You can relocate the stream to skip some data, reading the last row first, then the first row, and so on.
1.1 the standard database provides a pair of functions to seek to a given location in the stream and tell us the current location
- Seek function: relocate the tag seek to a given location;
- Tell function: tell the current position of the tag.
The standard library actually defines two pairs of seek and tell functions:
- The g version indicates that we are "getting" (reading) data
- The p version indicates that we are "placing" (writing) data
Applies only to fstream and s sstream.
operationexplaintellg()Tellp returns the current position of a tag in an input stream (tellg) or output stream (tellp).seekg(pos)seekp(pos) relocates the tag to a given absolute address in an input or output stream. pos is usually a value returned by the current teelg or tellp.seekp(off, from) seekg(off, from)In an input stream or output stream, position the tag to off characters before or after from. From can be one of the following values: beg, offset relative to the start of the stream; cur, offset relative to the current position of the stream; end, offset relative to the end of the stream.1.2 only one mark
The "put" and "get" versions of the standard seek and tell functions can be misleading.
Even though the standard library is differentiated, it maintains only a single tag in a stream - there are no separate read and write tags
Since there is only a single tag, as long as we switch between read and write operations, we must perform the seek operation to reposition the tag
1.3 relocation marks
The seek function has two versions: one is moved to the "absolute" address in the file; the other is moved to the specified offset at a given location:
//Move the marker to a fixed position seekg (new_position); / / Moves the read mark to the specified location pos_type Location of type seekp (new_position); / / Moves the write mark to the specified location pos_type Location of type / / Moves to the offset position specified before or after a given start point seekg (offset, from); / / Move read mark to space from Offset is offset Location of seekp (offset, from); / / Move write mark to space from Offset is of fset Location of
- pos_type indicates a file location
- off_type indicates an offset from the current position.
An off_type value can be positive or negative, that is, it can move forward or backward in the file.
1.4 access marking
The functions tellg and tellp return a pos_type indicating the current location of the stream. The tell function is usually used to remember a location so that it can be located later
ostringstream::writeStr;// Output stringstream ostringstream::pos_type mark = writeStr.tellp ( ); // ... if (cancelEntry) / / Go back to the position you just remembered writeStr.seekp (mark); / / Open the file in read-write mode and navigate to the end of the file fstream inOut("test_2.txt", fstream::ate|fstream::in|fstream::out); if(!inOut){ return EXIT_FAILURE; } / / inOut with ate Mode is turned on, so it is defined from the beginning to the end of its file auto end_mark = inOut.tellg();//Remember the end of the original file inOut.seekg(0,fstream::beg);/ / Relocate to the beginning of the file size_t cnt = 0;/ / Byte accumulator string line;/ / Save each line in the input while(inOut&&inOut.tellg() != end_mark&&getline(inOut,line)){/ / Conditions to continue reading: no error has been encountered and the original data is still being read/ / One line of input can also be obtained cnt += line.size()+1;/ / Plus 1 indicates a line break cout << line.size() << endl; auto mark = inOut.tellg();/ / Remember where to read inOut.seekp(0,fstream::end);II Move write mark to end of file inOut << cnt;/ / Output cumulative length / / If it is not the last line, print a separator if(mark != end_mark) inOut << " "; inOut.seekg(mark);/ / Restore read location } inOut.seekp(0,fstream::end);
Original file:
abed efg hi j
Modified file:
abed efg hi j5 9 12 14