basic_istream::seekg

#include <istream>

namespace std
{
    template < class CharT, class Traits = std::char_traits < CharT > >
    class basic_istream
        : virtual public std::basic_ios < CharT, Traits >
    {
    public:
        basic_istream & seekg( pos_type pos );
        basic_istream & seekg( off_type off, std::ios_base::seekdir dir );
    };
}

#include <iostream>
#include <string>
#include <sstream>

int main()
{
    std::istringstream input( "To jest tekst." );
    std::string first;
    input >> first;
    input.seekg( 0 );
    std::string second;
    input >> second;
    input.seekg( - 6, std::ios_base::end );
    std::string third;
    input >> third;
    std::cout << "Pierwszy: \"" << first << "\"\n"
    << "Drugi: \"" << second << "\"\n"
    << "Trzeci: \"" << third << '\"';
    return 0;
}