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Check for Eigen target before finding, supporting downstream FetchContent usage #332

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Check for Eigen target before finding, supporting downstream FetchContent usage #332

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@willat343 willat343 commented Jul 20, 2025

Scenario this addresses:

  • enables specific versions of Eigen3 and manif to be used in a downstream package via CMake FetchContent (eigen already supports this)

How the code changes:

  • Before finding Eigen3, we check first if the target Eigen3::Eigen is already defined. Thus if the downstream target includes a specific version of eigen and the target is available, manif will use this target instead of finding Eigen itself (which may be the system install for example).
  • We add the MANIF::manif alias to the library, so that if manif is made available via FetchContent (but not installed), it can be linked to with MANIF::manif, consistent and not conflicting with the install/export NAMESPACE.

Example fo downstream usage (providing Eigen3::Eigen and with this PR MANIF::manif targets to link to):

option(USE_SYSTEM_Eigen3 "Use system install of Eigen3" OFF)
option(USE_SYSTEM_manif "Use system install of manif" OFF)

if (NOT TARGET Eigen3::Eigen)
    set(EIGEN3_VERSION "3.3.7")
    if (${USE_SYSTEM_Eigen3})
        find_package(Eigen3 ${EIGEN3_VERSION} REQUIRED)
        message(STATUS "Found Eigen3 at ${Eigen3_DIR} with version ${Eigen3_VERSION}")
    else()
        FetchContent_Declare(
            eigen
            GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
            GIT_TAG ${EIGEN3_VERSION}
        )
        FetchContent_MakeAvailable(eigen)
        message(STATUS "Fetched eigen to ${eigen_SOURCE_DIR} with version ${EIGEN3_VERSION}")
    endif()
endif()

if (NOT TARGET MANIF::manif)
    set(MANIF_VERSION "0.0.6") # Assuming this PR makes it into the next release
    if (${USE_SYSTEM_manif})
        find_package(manif ${MANIF_VERSION} REQUIRED)
        message(STATUS "Found manif at ${manif_DIR} with version ${manif_VERSION}")
    else()
        FetchContent_Declare(
            manif
            GIT_REPOSITORY https://github.com/artivis/manif.git
            GIT_TAG ${MANIF_VERSION}
        )
        FetchContent_MakeAvailable(manif)
        message(STATUS "Fetched manif to ${manif_SOURCE_DIR} with version ${MANIF_VERSION}")
    endif()
endif()

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