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.

# Declarations

protected:
virtual pos_type seekpos( pos_type sp,
std::ios_base::openmode which = std::ios_base::in | std::ios_base::out );

# Parameters

sp: file position obtained by seekoff() or seekpos() called earlier on the same file
which: defines which of the input and/or output sequences to affect. It can be one or a combination of the following constants: Constant Explanation in affect the input sequence out affect the output sequence

# 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);
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 55	C++98	seekpos returned an undefinedinvalid stream position on failure	pos_type(off_type(-1))is returned on failure
LWG 171	C++98	the sequence of the operations of reposition was not clear	made clear