简单语句解释

basic_istream& seekg( off_type off, std::ios_base::seekdir dir);

off     -     relative position to set the input position indicator to.
dir     -     defines base position to apply the relative offset to. It can be one of the following constants:
        beg     the beginning of a stream
        end     the ending of a stream
        cur     the current position of stream position indicator