Question

I am trying to build small logger library.
I am facig some problem with c++ templates.
Here is what my class structure looks like.

class abstract_logger_t  {
public:
  typedef abstract_logger_t logger_type;
  template<typename data_t>
  abstract_logger_t& log(const data_t& data)  {
    return *this;
  }
};

class stdout_logger_t : public abstract_logger_t  {
public:
  typedef stdout_logger_t logger_type;
  template<typename data_t>
  stdout_logger_t& log(const data_t& data)  {
    cout << data << endl;
    return *this;
  }
};


template<typename logger_t, typename data_t>
void output(logger_t& logger, const data_t& data)  {
  static_cast<typename logger_t::logger_type&>(logger).log(data);
  cout << data;
}


template<typename data_t>
abstract_logger_t& operator<< (abstract_logger_t& logger, const data_t& data)  {
  output(logger, data);
  return logger;
}


stdout_logger_t logger;
logger << "Hi " << 1;

Here I am expecting stdout_logger_t::log to be called by output.
but it looks like derived type is getting lost
and abstract_logger_t::log ends up getting called.
Can someone tell me if I am doing something wrong ?

Answer 1

template<typename data_t>
abstract_logger_t& operator<< (abstract_logger_t& logger, const data_t& data)  {
  output(logger, data);
  return logger;
}

Here, whatever logger you pass in, the compiler will convert it into an abstract_logger_t&. You need to make the first argument templated too.

template<typename T, typename data_t>
T& operator<< (T& logger, const data_t& data)  {
  output(logger, data);
  return logger;
}