std::basic_filebuf<CharT,Traits>:: seekpos

Repositions the file pointer, if possible, to the position indicated by sp . If the associated file is not open ( is_open ( ) == false ), fails immediately.

Reposition performs as follows:

If sp was not obtained by calling seekoff() or seekpos() on the same file, the behavior is undefined.

Parameters

Return value

sp on success or pos_type ( off_type ( - 1 ) ) on failure.

Notes

seekpos() is called by std::basic_streambuf::pubseekpos() , which is called by the single-argument versions of std::basic_istream::seekg() and std::basic_ostream::seekp() .

Many implementations do not update the get area in seekpos() , delegating to underflow() that is called by the next sgetc() .

Example

#include <fstream>
#include <iostream>
 
struct mybuf : std::filebuf
{
    pos_type seekpos(pos_type sp, std::ios_base::openmode which)
    {
        std::cout << "Before seekpos(" << sp << "), size of the get area is "
                  << egptr() - eback() << " with "
                  << egptr() - gptr() << " read positions available.\n";
 
        pos_type rc = std::filebuf::seekpos(sp, which);
 
        std::cout << "seekpos() returns " << rc << ".\nAfter the call, "
                  << "size of the get area is "
                  << egptr() - eback() << " with "
                  << egptr() - gptr() << " read positions available.\n";
// uncomment if get area is emptied by seekpos()
//        std::filebuf::underflow();
//        std::cout << "after forced underflow(), size of the get area is "
//                  << egptr() - eback() << " with "
//                  << egptr() - gptr() << " read positions available.\n";
 
        return rc;
    }
};
 
int main()
{
    mybuf buf;
    buf.open("test.txt", std::ios_base::in);
    std::istream stream(&buf);
    stream.get(); // read one char to force underflow()
    stream.seekg(2);
}

Before seekpos(2), size of the get area is 110 with 109 read positions available.
seekpos() returns 2.
After the call, size of the get area is 110 with 108 read positions available.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also