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

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

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:

1) If the file is open for writing, writes the put area and any unshift sequences required by the currently imbued locale, using overflow().

2) Repositions the file pointer, as if by calling std::fsetpos().

3) If the file is open for reading, updates the get area if necessary.

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

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

On some implementations, the get area is emptied by seekpos() and the second underflow() is necessary to observe the effects.

#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);
}

Possible output:

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.

DR	Applied to	Behavior as published	Correct behavior
LWG 55	C++98	`seekpos` returned an undefined invalid 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