Unable to find Eigen3 with CMake - c++

Question

I am kind of desperate:
For my studies I need to work with Eigen and CMake. I'm able to use Eigen if I copy the whole library in the directories where my compiler looks by default but as soon as I try to find it via
find_package(Eigen3 REQUIRED)
I get the following error:
CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
Could NOT find Eigen3 (missing: EIGEN3_INCLUDE_DIR EIGEN3_VERSION_OK)
(Required is at least version "2.91.0")
Call Stack (most recent call first):
/usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:388 (_FPHSA_FAILURE_MESSAGE)
FindEigen3.cmake:76 (find_package_handle_standard_args)
CMakeLists.txt:8 (find_package)
-- Configuring incomplete, errors occurred!
Now I searched for solutions but all I those I tried (also those available on stackoverflow:
Find package Eigen3 for CMake
or
CMake Can't find Eigen3 )
did not work.
My Eigen Version (according to the Macros in Core/util/Macros.h) is 3.2.5.
I keep the Eigen directory in /usr/local/include, I use the FindEigen3.cmake which comes with the Eigen library and my CMakeLists.txt looks as follows:
cmake_minimum_required(VERSION 2.8)
project(Test)
set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR})
find_package(Eigen3 REQUIRED)
include_directories(${EIGEN3_INCLUDE_DIR})
message("Found Eigen3 in: ${EIGEN3_INCLUDE_DIR}")
add_executable(main test.cpp)
Has anyone an idea what's going wrong?
Kind regards,
Julien

Answer 1

Turning my comment into an answer
The find package scripts - like FindEigen3.cmake - normally use the find_path() command to detect the package's include directory (see it's documentation for the full details).
FindEigen3.cmake uses the following code snippet:
find_path(EIGEN3_INCLUDE_DIR NAMES signature_of_eigen3_matrix_library
PATHS
${CMAKE_INSTALL_PREFIX}/include
${KDE4_INCLUDE_DIR}
PATH_SUFFIXES eigen3 eigen
)
So it looks in CMAKE_INSTALL_PREFIX which on Unix/Linux hosts is /usr/local by default.
The following has worked for me:
Go to the Eigen source directory and run the CMake and installation steps
> mkdir build
> cd build
> cmake ..
> make install
Then copy - as you have done - FindEigen3.cmake to your projects source directory.
Now your code does find Eigen (just changed to list(APPEND ...))
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}")
find_package(Eigen3 REQUIRED)
References
Eigen & CMake
CMake: Of what use is find_package() if you need to specify CMAKE_MODULE_PATH anyway?
Cmake doesn't find Boost

Answer 2

Add the path of FindEigen3.cmake before find_package(Eigen3 REQUIRED), like this:
LIST(APPEND CMAKE_MODULE_PATH "/usr/share/cmake-2.8/Modules/")
find_package(Eigen3)