How to add Eigen library to a cmake c++ project via FetchContent

Question

Adding Eigen via

FetchContent_Declare(
  eigen
  GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
  GIT_TAG        3.3.9
)

FetchContent_GetProperties(eigen)

if(NOT eigen_POPULATED)
    FetchContent_Populate(eigen)
    add_subdirectory(${eigen_SOURCE_DIR} ${eigen_BINARY_DIR})
endif()

find_package (Eigen3 3.3 REQUIRED NO_MODULE) 

gives me the error

CMake Error at o/b/x64-Debug/_deps/eigen-build/Eigen3Config.cmake:20 (include):
  The file

    D:/XXX/o/b/x64-Debug/_deps/eigen-build/Eigen3Targets.cmake

  was generated by the export() command.  It may not be used as the argument
  to the include() command.  Use ALIAS targets instead to refer to targets by
  alternative names.        D:XXXo/b/x64-Debug/_deps/eigen-build/Eigen3Config.cmake   20

But downloading Eigen manually and adding it works fine

add_subdirectory("${PROJECT_SOURCE_DIR}/extern/eigen")
find_package (Eigen3 3.3 REQUIRED NO_MODULE) 

Any ideas ?

question from:https://stackoverflow.com/questions/65860094/how-to-add-eigen-library-to-a-cmake-c-project-via-fetchcontent

Answer 1

In any order:

• Eigen only provide ALIAS target on master
  ref: https://gitlab.com/libeigen/eigen/-/commit/cf0b5b0344a3bfcf410e95bf22289015a2daf34b#9a2aa4db38d3115ed60da621e012c0efc0172aae_671_599

• FetchContent usage could be

include(FetchContent)
FetchContent_Declare(
  Eigen
  GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
  GIT_TAG master
  GIT_SHALLOW TRUE
  GIT_PROGRESS TRUE)
set(EIGEN_BUILD_DOC OFF)
# note: To disable eigen tests,
# you should put this code in a add_subdirectory to avoid to change
# BUILD_TESTING for your own project too since variables are directory
# scoped
set(BUILD_TESTING OFF)
set(EIGEN_BUILD_PKGCONFIG OFF)
set( OFF)
FetchContent_MakeAvailable(Eigen)
...
target_link_libraries(YourTarget PRIVATE Eigen3::Eigen)

• For find_package() and FetchContent()/add_subdirectory() please see https://gitlab.kitware.com/cmake/cmake/-/issues/17735