cmake find_package unable to find Eigen3Config.cmake spectra Windows - cmake

Question

I am using Eigen3 with spectra (https://spectralib.org/), a library built on top of Eigen. Spectra uses find_package to find Eigen, and comes up with the error:
Could not find a package configuration file provided by "Eigen3" with any
of the following names:
Eigen3Config.cmake
eigen3-config.cmake
Looking through the directory of Eigen, I found that there were no files by those names, but rather one called Eigen3Config.cmake.in. I tried copying the file and renaming it Eigen3Config.cmake, but that gave me a different error of
find_package Error reading CMake code from "C:/Program Files
(x86)/Eigen3/cmake/Eigen3Config.cmake".
which was somewhat expected, but it does tell me that it can at least find the directory where Eigen3Config.cmake.in is. I'm assuming that either find_package is supposed to use Eigen3Config.cmake.in, or Eigen3Config.cmake.in is supposed to generate Eigen3Config.cmake, but i'm very new to cmake, so i'm not sure. How do I fix this?

Answer 1

There is no need to build/install Eigen to use it if you include it manually, as done in the getting started example (https://eigen.tuxfamily.org/dox/GettingStarted.html#title0)
But in order to be found by CMake, you will need to build / install it, as explained in the INSTALL file. https://gitlab.com/libeigen/eigen/-/blob/master/INSTALL
Usually, your error is followed by an hint asking you to set the variable Eigen3_DIR (or something similar) to point the build/install dir of the target project (Eigen3 here). It appears typically when you have built but not installed the project.
So:
Build Eigen
Install it (optional)
For spectra set the cmake var Eigen3_DIR to /path/to/Eigen/build . (if eigen not install or still not found)

Answer 2

These steps worked for me:
Install Eigen
Create a build directory for Eigen
cd into the build directory created
run "cmake ../"Your Eigen Source Directory""
After this is done, in your CMakeLists.txt of your project, you'll want to add "find_package( Eigen3 REQUIRED)" and "include_directories( ${EIGEN3_INCLUDE_DIR})".