How to use FetchContent_Populate with Eigen? - cmake

Question

I want to use FetchContent to automatically manage the dependency to Eigen for my project, which works in general. However, when using the recommended method of FetchContent_Declare() and FetchContent_MakeAvailable() a subsequent call to install also installs all Eigen headers and documentation which is not necessary in my case.
To circumvent this behavior, I tried the method suggested in this answer: Disable install for FetchContent
FetchConten_Populate() however fails to fill the variables ${Eigen_SOURCE_DIR} and ${Eigen_BIN_DIR} (which the documentation told me should happen) so that I cannot call add_subdirectory().
Here is a minimal CMakeLists.txt:
cmake_minimum_required (VERSION 3.12)
project (FetchContentExample)
include (FetchContent)
FetchContent_Declare(
Eigen
GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
GIT_TAG 3.4.0
)
FetchContent_GetProperties(Eigen)
if(NOT Eigen_POPULATED)
FetchContent_Populate(Eigen)
message("SRC; ${Eigen_SOURCE_DIR}") # Apparently empty?
message("BIN: ${Eigen_BINARY_DIR}") # Apparently empty?
add_subdirectory(${Eigen_SOURCE_DIR} ${Eigen_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()
add_executable(FetchContentExample
main.cpp
)
target_link_libraries (FetchContentExample
PRIVATE
Eigen3::Eigen
)
install(
TARGETS FetchContentExample
DESTINATION ${CMAKE_INSTALL_BINDIR}
COMPONENT Runtime
)
The same setup works fine when I use e.g.
FetchContent_Declare(
fmt
GIT_REPOSITORY https://github.com/fmtlib/fmt.git
GIT_TAG 5.3.0
)
instead of Eigen.
What specifically am I doing wrong when it comes to Eigen?

Answer 1

FetchContent_Populate() however fails to fill the variables ${Eigen_SOURCE_DIR} and ${Eigen_BINARY_DIR} (which the documentation told me should happen).
Actually, FetchContent fills variables ${eigen_SOURCE_DIR} and ${eigen_BINARY_DIR} which names are constructed from the lowercase variant of the project's name. This is written in the documentation:
FetchContent_Populate() will set three variables in the scope of the caller:
<lowercaseName>_POPULATED
This will always be set to TRUE by the call.
<lowercaseName>_SOURCE_DIR
The location where the populated content can be found upon return.
<lowercaseName>_BINARY_DIR
A directory intended for use as a corresponding build directory.
So the correct sequence of commands for EXCLUDE_FROM_ALL inclusion of Eigen would be:
FetchContent_GetProperties(Eigen)
if(NOT eigen_POPULATED)
FetchContent_Populate(Eigen)
add_subdirectory(${eigen_SOURCE_DIR} ${eigen_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()