Add C++ support

[jbangelo]

This PR adds a few lightweight C++ classes that wrap around the existing libsbp C interface. The main item of interest here is the MessageHandler. This class automates the infrastructure for registering a callback to have a member function called upon receipt of a sbp message.

Here is an example of how it would be used

class ECEFHandler : private sbp::MessageHandler<msg_gps_time_t, msg_pos_ecef_t, msg_pos_ecef_cov_t> {
 public:
  ECEFHandler(sbp::State *state) : private sbp::MessageHandler<msg_gps_time_t, msg_pos_ecef_t, msg_pos_ecef_cov_t>(state)
  {
    /* Do other constructor stuff */
  }

 private:
  void handle_sbp_msg(uint16_t sender_id, uint8_t message_length, const msg_gps_time_t& msg)
  {
    /* handle GPS time message */
  }

  void handle_sbp_msg(uint16_t sender_id, uint8_t message_length, const msg_pos_ecef_t& msg)
  {
    /* handle pos ECEF message */
  }

  void handle_sbp_msg(uint16_t sender_id, uint8_t message_length, const msg_pos_ecef_cov_t& msg)
  {
    /* handle pos ecef cov message */
  }
};

The instantiation and registration of the sbp_msg_callbacks_node_t objects are all taken care of automatically by the sbp::MessageHandler constructor, and are unregistered in the destructor. The derived class is able to publicly or privately inherit from sbp::MessageHandler, and the handle_sbp_msg member functions can be public or private. Each handle_sbp_msg is a virtual member function so additional levels of inheritance can be employed, though we do incur the overhead of the vtable lookup at run time. The mapping of message struct to message ID is achieved via a set of type traits that are automatically generated from the specification YAML. Currently only the message ID is included in the message type traits, but additional information could be added in the future if deemed useful.

The other classes are thin wrappers around the existing C structs, and probably don't need much explanation.

[kmdade]

Amazing.

So do we draw lots for who goes back and replaces all the existing SBP one-offs? ;)

[kmdade]

Is there a reason the ctor argument isn't State&?

[jbangelo]

The State needs to be mutable, and our C++ guidelines say any non-const parameter should be passed as a pointer.

[RReichert]

Brilliant work, I see no major fault stopping me from approving.

[RReichert]

I'm wondering it the class name could be better. The class acts as a SBP processor, where you have a state internally.

[jbangelo]

The name came from the sbp_state_t which it wraps into a C++ object. I'm open to alternate suggestions though.

[RReichert]

I would actually call it SBP since this class is how I think of SBP but it might be inappropriate. I'll leave it up to you, my comment was merely a suggestion

[RReichert]

is there a CI step that validates that the committed message file is identical to that generated from the generator tool?

I'm assuming that this file was generated by the python/template file that was added

[jbangelo]

I don't think there's a double check in the CI. I think it's assumed as part of the manual review that only the relevant changes are made to the auto generated files.

[RReichert]

I'd probably say it would be worth adding that in so there is no misalignment between one source and its C++ representation

[denniszollo]

Is there a way to call the same function on multiple class types? I find this is a common pattern for me (i.e do the same thing for any ephemeris message received)

[jbangelo]

Not with how this PR currently stands. The problem with that is that all of the message types are distinct and there's little to no information relating the content of similar messages. The closest thing we could get is defining a base class that handles each of the distinct message types and converts them into a common type and then calls another member function that works on the common type.

Something like this:

class CommonEphemerisHandler : public sbp::MessageHandler<EphemerisMessage1, EphemerisMessage2> {
  public:
    struct CommonEphemerisType { ... };

    virtual void handle_common_ephemeris(const CommonEphemerisType& eph) = 0;

    void handle_sbp_msg(uint16_t sender_id, uint8_t message_length, const EphemerisMessage1& msg) {
      CommonEphemerisType eph = convertEph1(msg);
      handle_common_ephemeris(eph);
    }

    void handle_sbp_msg(uint16_t sender_id, uint8_t message_length, const EphemerisMessage2& msg) {
      CommonEphemerisType eph = convertEph2(msg);
      handle_common_ephemeris(eph);
    }
};

class CustomEphemerisHandler : public CommonEphemerisHandler {
  public:
    void handle_common_ephemeris(const CommonEphemerisType& eph) { ... }
};

class OtherEphemerisHandler : public CommonEphemerisHandler {
  public:
    void handle_common_ephemeris(const CommonEphemerisType& eph) { ... }
};