Set position with seekg() in C++ language file handling
seekg()
is a function in the iostream library (part of the standard library) that allows you to seek to an arbitrary position in a file. It is used in file handling to sets the position of the next character to be extracted from the input stream from a given file. For example :
Input : "Hello World" Output : World
Syntax – There are two syntax for seekg()
in file handling :
istream&seekg(streampos position);
istream&seekg(streamoff offset, ios_base::seekdir dir);
Description –
- position : is the new position in the stream buffer.
- offset : is an integer value of type streamoff representing the offset in the stream’s buffer. It is relative to the dir parameter.
- dir : is the seeking direction. It is an object of type ios_base::seekdir that can take any of the following constant values.
There are 3 direction we use for offset value :
ios_base::beg
(offset from the beginning of the stream’s buffer).ios_base::cur
(offset from the current position in the stream’s buffer).ios_base::end
(offset from the end of the stream’s buffer).
Code –
// Code to demonstrate the seekg function in file handling #include <fstream> #include <iostream> using namespace std; int main ( int argc, char ** argv) { // Open a new file for input/output operations // discarding any current in the file (assumes // a length of zero on opening) fstream myFile( "test.txt" , ios::in | ios::out | ios::trunc); // Add the characters "Hello World" to the file myFile << "Hello World" ; // Seek to 6 characters from the beginning of the file myFile.seekg(6, ios::beg); // Read the next 5 characters from the file into a buffer char A[6]; myFile.read(A, 5); // End the buffer with a null terminating character A[5] = 0; // Output the contents read from the file and close it cout << buffer << endl; myFile.close(); } |
Output –
World
Note –
If we previously get an end of file on the stream, seekg will not reset it but will return an error in many implementations. – use the clear()
method to clear the end of file bit first. This is a relatively common mistake and if seekg()
is not performing as expected.
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