basic_ostream::seekp

#include <ostream>

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

#include <iostream>
#include <sstream>

int main()
{
    std::ostringstream output( "to Jest tekst!" );
    output.seekp( 0 );
    output << 'T';
    output.seekp( 3 );
    output << 'j';
    output.seekp( - 1, std::ios_base::end );
    output << '.';
    std::cout << output.str();
    return 0;
}