[geometry_msgs] Workaround for #430

[gleichdick]

Workaround for #430. Snippet now compiles (with convert.h included last)

#include <tf2/LinearMath/Quaternion.h>
#include <Eigen/Geometry>
#include <tf2_geometry_msgs/tf2_geometry_msgs.h>
#include <tf2_eigen/tf2_eigen.h>
#include <tf2/convert.h>
#include <iostream>

int main() {
  Eigen::Quaterniond eq;

  const tf2::Quaternion tq(0,1,0,1);
  tf2::convert(tq, eq);
  std::cout << eq.toRotationMatrix();
}

Note that it still depends on the include order, if convert.h is included first it does not work (compiler picks forward declaration of templated fromMsg which is not resolved later on).

[v4hn]

C++ includes should be independent of the include order.
Is it difficult to resolve the problem altogether?

[gleichdick]

TL;DR Please a look at my branch. toMsg is redefined (so, client code which uses toMsg breaks) but tf2::convert should be fine. Include order does not matter anymore.

I converted all toMsg overloads to template specialisations with two parameters (because the return value does not count for template parameter deduction). All overloads (without template<>) are gone:

template<class A, class B> B& toMsg(const A&, B&);

For converting two non-Message types (lets say tf2::Vector3 and Eigen::Vector3d), I'm writing a python script which generates type mapping (both have conversions to/from geometry_msgs::Point in common), like

template<>
struct commonMsgType<tf2::Vector3, Eigen::Vector3> {
    using type = geometry_msgs::Point;
};

This struct is then read by tf2::convert with some SFINAE Magic which then selects the correct fromMsg / toMsg methods.

Please note that this branch is more or less a proof of concept, so documentation and tests are missing. I'm happy to contine, but please let me know whether this solution fits your needs.

[gleichdick]

replaced by #491