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std::basic_stringbuf<CharT,Traits,Allocator>::seekpos

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

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

Effectively executes seekoff(off_type(sp), std::ios_base::beg, which).

Parameters

sp - stream position, such as one obtained by seekoff() or seekpos()

which

defines whether the input sequences, the output sequence, or both are affected. 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().

Example

#include <sstream>
#include <iostream>
 
struct mybuf : std::stringbuf
{
    mybuf(const std::string& str) : std::stringbuf(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::stringbuf::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);
}

Output:

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.

DR	Applied to	Behavior as published	Correct behavior
LWG 375	C++98	static constant members of `std::ios_base` were misspecified as members of `std::basic_ios`	corrected
LWG 564	C++98	if was unclear how to reposition `gptr` and/or `pptr`	they are repositioned by `seekoff()`

pubseekpos	invokes `seekpos()` (public member function of `std::basic_streambuf<CharT,Traits>`)
seekoff [virtual]	repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function)
seekpos [virtual]	repositions the file position, using absolute addressing (virtual protected member function of `std::basic_filebuf<CharT,Traits>`)
seekpos [virtual]	repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of `std::strstreambuf`)