std::strstreambuf:: seekpos

Repositions std::basic_streambuf::gptr and/or std::basic_streambuf::pptr , if possible, to the position indicated by sp .

If std::ios_base::in is set in which , attempts to reposition gptr() (the next pointer in the get area). If std::ios_base::out is set in which , attempts to reposition pptr() (the next pointer in the put area). If neither bit is set in which , the operation fails.

Each next pointer is repositioned as follows:

• If the next pointer is null, the operation fails.
• Otherwise, the new offset newoff (of type off_type ) is determined by calling sp. offset ( ) . If newoff is negative, out of bounds of the buffer, or invalid, the operation fails.
• Otherwise, the next pointer is assigned as if by gptr ( ) = eback ( ) + newoff or pptr ( ) = pbase ( ) + newoff .

Parameters

Return value

The resultant offset converted to pos_type 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() .

Example

#include <cstring>
#include <iostream>
#include <strstream>
 
struct mybuf : std::strstreambuf
{
    mybuf(const char* str) : std::strstreambuf(str, std::strlen(str)) {}
 
    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::strstreambuf::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";
 
        return rc;
    }
};
 
int main()
{
    mybuf buf("12345");
    std::iostream stream(&buf);
    stream.seekg(2);
}

Before seekpos(2), size of the get area is 5 with 5 read positions available.
seekpos() returns 2.
After the call, size of the get area is 5 with 3 read positions available.

Defect reports

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

See also