How to force CMake to use the copy of vcpkg included as a submodule for a project?

[crgnam]

I have a personal copy of vcpkg installed on my machine however I have a project that includes vcpkg as a submodule. Because it has a different baseline, I would like CMake to find and use the vcpkg submodule. I have tried:

set(CMAKE_TOOLCHAIN_FILE "${CMAKE_CURRENT_SOURCE_DIR}/external/vcpkg/scripts/buildsystems/vcpkg.cmake" CACHE FILEPATH "Vcpkg toolchain file")

however this does not work, and upon running the CMake configuration (with Visual Studio) it shows:

1> CMake generation started for configuration: 'x64-Debug'.
1> Found and using vcpkg toolchain file (<MY_USER_DIR>/vcpkg/scripts/buildsystems/vcpkg.cmake).

I have tried deleting CMakeChache. I have noticed if I just completely delete my peronal copy of vcpkg, it correctly finds the submodule version. But whenever another copy is present on the machine, it seems to find that first.

How can I force it to default to the vcpkg submodule included in the repository? (And possibly, allow a user to override this should they want to use their copy of vcpkg)

[crgnam]

I can of course modify the CMakeSettings.json to directly use the correct CMAKE_TOOLCHAIN_FILE however this is cumberome as this file is not tracked in the repository, and I would like other users of the project to not need to change those sorts of settings for this. I am struggling to see why:

set(CMAKE_TOOLCHAIN_FILE "${CMAKE_CURRENT_SOURCE_DIR}/external/vcpkg/scripts/buildsystems/vcpkg.cmake" CACHE FILEPATH "Vcpkg toolchain file")

is not working. It seems to have no effect at all.

[dg0yt]

Cache variables are input variables. If you want to override user input, you must either use the INTERNAL type, the FORCE keyword, or a normal variable.
And of course it must come before the project command.

[crgnam]

Thanks this resolved it!

For others who may stumble upon this, here is the relevant part of my solution:

root CMakeLists.txt:

...
if("${CMAKE_TOOLCHAIN_FILE}" STREQUAL "")
    set(MY_VCPKG_TOOLCHAIN_FILE "${CMAKE_CURRENT_SOURCE_DIR}/external/vcpkg/scripts/buildsystems/vcpkg.cmake")
    set(CMAKE_TOOLCHAIN_FILE "${MY_VCPKG_TOOLCHAIN_FILE}" 
        CACHE FILEPATH "Vcpkg toolchain file" FORCE)
    ...
endif()
...
project(...

This allows it to default to the included vcpkg submodule if nothing is specified. It also allows users to either manually specify -DCMAKE_TOOLCHAIN_FILE=OFF or -DCMAKE_TOOLCHAIN_FILE=<path/to/vcpkg>/vcpkg.cmake if they so choose. I believe this would cause issues if they wanted to use a separate vcpkg toolchain file, but that is not currently a concern for me, and I believe it could be addressed later.