virtual std::string serialize() const
{
std::stringstream ss;
typename std::vector<U>::const_iterator it = this->begin();
std::string tmp;
ss << _width << " " << _height << " ";
for (; it != this->end() ; ++it)
{
tmp = it->serialize();
ss << tmp.length() << " " << tmp << " ";
}
return ss.str();
}
