adding a FindEigen.cmake module

[wjwwood]

@dirk-thomas @adolfo-rt can I get a review of this if you have time?

[adolfo-rt]

Why aren't we getting the include dirs from the pc file as well?. In my installation (libeigen3-dev 3.0.5-1), /usr/share/pkgconfig/eigen3.pc looks like this:

Name: Eigen3
Description: A C++ template library for linear algebra: vectors, matrices, and related algorithms
Requires:
Version: 3.0.5
Libs:
Cflags: -I/usr/include/eigen3

[wjwwood]

Yeah, this was copied over from catkin, I'll look at improving this to use pc for everything.

[wjwwood]

It is possible that the pc file doesn't work correctly on Windows? That is the only reason to search instead, but that seems unlikely.

[wjwwood]

Oh the other problem is that pkg-config only gives you the cflags, I would have to split and extract the include paths so that CMake can use them. I'm not sure which is better, but since the current implementation is in use already I would lean towards it.

[adolfo-rt]

I'd like to bring your attention to the fact that there currently exist multiple scripts for finding Eigen, each of which has its own set of variables and capitalization policy. On the one hand, the Eigen project already provides:

• A pkgconfig file, which gets installed.
• A FindEigen3.cmake script that is not part of the install target. It generates variables like EIGEN3_FOUND (note the 3, all caps).
• A few months back a pull request got merged with Eigen3Config.cmake and UseEigen3.cmake files that do get installed. It generates variables like EIGEN_FOUND (Note no 3, all caps). These files are in the default branch, and have not yet been released as of 3.2.

This PR proposes yet another script/ set of variables, but offers consistent naming between what gets passed to find_package(Eigen) and the variables that result (i.e., Eigen_FOUND).

[adolfo-rt]

This line can be removed, as the variable does not exist

[wjwwood]

Good catch, this was a copy-paste-edit of the TinyXML documentation.

[wjwwood]

I basically copied over the find-eigen.cmake from catkin (ros/catkin#492), which people are currently using, my intent was to provide something that worked with that, but I'll try to address the comments you brought up.

[wjwwood]

I am using Eigen 3.2.0 on my machine and I still do not have the Eigen3Config.cmake or the UseEigen3.cmake files, so I don't think we can rely on those.

I suppose we could use the FindEigen3.cmake you linked to, but I don't see any advantages over just moving the catkin one here and redoing the documentation.

Thoughts?

[adolfo-rt]

I am using Eigen 3.2.0 on my machine and I still do not have the Eigen3Config.cmake or the UseEigen3.cmake files, so I don't think we can rely on those.

The feature was added post-3.2, so we can't really use it as of now. It's only in the default branch.

I suppose we could use the FindEigen3.cmake you linked to, but I don't see any advantages over just moving the catkin one here and redoing the documentation.

I don't see the advantage either, especially because the variable names are significantly different. The script is looking good now.

[wjwwood]

@adolfo-rt thanks for the review.