Template and OO

templ.cpp

 struct string_builder {
  /...
  template<typename T>
  string_builder &add(T v) {
  internal::add<T>(_s, v, _size);
  return *this;
}

namespace internal {
  //General implementation
  template <typename T>
  struct add {
    add(std::string &buffer, T val, int size_inc) {
    std::string s = std::to_string(val);
    _check_size(buffer, s.size(), size_inc);
    buffer += s;
  }
};