How to prevent fetching content on each cmake?

[alexkutsan]

I found this cmake module incredible and easy to use.
That is an example on how I use it in my my small project:

# https://alandefreitas.github.io/FetchBoostContent/
include(cmake/FetchBoostContent.cmake)
FetchBoostContent_Declare(
        boost_geometry
        GIT_REPOSITORY https://github.com/boostorg/geometry
        GIT_TAG        master
)
FetchBoostContent_MakeAvailable(boost_geometry)
FetchBoostContent_Populate(boost_geometry)
add_executable(main main.cpp)
target_link_libraries(main 
                      PUBLIC 
                        Boost::headers)

Meanwhile, every time when I type make it hands for a while on fetching content from boostorg.
Maybe I can wrap FetchBoostContent_Declare with some if condition to avoid fetching it in case if it is already fetched?

[alexkutsan]

Looks like there are a mechanism to prevent t second fetch based on cmake properties.

I am not a big cmake guy - but looks like it does not work that way.
I have tried this code

get_property(alreadyDefined GLOBAL PROPERTY "BLABLA" DEFINED)
if (alreadyDefined)
    message(STATUS "alreadyDefined = 1")
else()
    message(STATUS "alreadyDefined = 0")
    define_property(GLOBAL PROPERTY "BLABLA")
    set_property(GLOBAL PROPERTY "BLABLA" "Arguments")
    message(STATUS "define alreadyDefined")
    get_property(newalreadyDefined GLOBAL PROPERTY "BLABLA" DEFINED)
    message(STATUS "newalreadyDefined = ${newalreadyDefined}")
endif()

And it gives to me:

-- alreadyDefined = 0
-- define alreadyDefined
-- newalreadyDefined = 1

every time when I run cmake,

[alandefreitas]

Maybe I can wrap FetchBoostContent_Declare with some if condition to avoid fetching it in case if it is already fetched?

CMake is already supposed to handle that. We would have to debug this script to find out where the problem is.

However, if you need a specific version of boost or need very high level libraries, you can just bypass this cmake module and fetch the transitive dependencies directly (since they are stable in a specific version) or just fetch all boost libraries (that's faster since you're already including all of them).

For instance, let's say you just want these modules from boost 1.82: mp11 assert config core static_assert throw_exception pool. Then you can just fetch all of them and skip the process of FetchBoostContent analyzing the dependency graph:

set(FUTURES_BOOST_VERSION 1.82.0)
    set(FUTURES_BOOST_MODULES mp11 assert config core static_assert throw_exception pool)
    set(FUTURES_BOOST_TRANSITIVE_MODULES integer type_traits winapi)
    set(FUTURES_BOOST_ALL_MODULES ${FUTURES_BOOST_MODULES} ${FUTURES_BOOST_TRANSITIVE_MODULES})
    add_library(boost_headers INTERFACE)
    add_library(Boost::headers ALIAS boost_headers)
    foreach (MODULE ${FUTURES_BOOST_MODULES})
        FetchContent_Declare(
                boost_${MODULE}
                URL https://github.com/boostorg/${MODULE}/archive/refs/tags/boost-${FUTURES_BOOST_VERSION}.tar.gz)
        FetchContent_GetProperties(boost_${MODULE})
        if (NOT boost_${MODULE}_POPULATED)
            FetchContent_Populate(boost_${MODULE})
        endif ()
        target_include_directories(boost_headers INTERFACE $<BUILD_INTERFACE:${boost_${MODULE}_SOURCE_DIR}/include>)
    endforeach ()
    target_include_directories(boost_headers INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)

You can find the modules for FUTURES_BOOST_TRANSITIVE_MODULES in https://pdimov.github.io/boostdep-report/boost-1.82.0/module-overview.html. That is the first case.

In your case, if you are depending on Boost.Geometry, you are effectively depending on almost all boost libraries: https://alandefreitas.github.io/boostdep_graph/libs/geometry.html.
So downloading the whole release package for cmake is going to be faster than downloading each library separately because it's a single process and because it's a single archive. For instance,

FetchContent_Declare(
                boost
                URL https://github.com/boostorg/boost/releases/download/boost-1.82.0/boost-1.82.0.tar.xz)