Configure and Install required packages for a project - c++

Question

Coming from other package managers I expect to be able to do the following on a project:
configure the required libraries (in a config file, e.g. CMakelists.txt or nuget's packages.config)
install/upgrade all the required libraries
This seems to not be possible with vcpkg and CMake:
I managed to set up vcpkg and CMakefile (linux):
# in ternimal
vcpkg install <package>
# CMakeLists.txt
find_package(nlohmann_json CONFIG REQUIRED)
target_link_libraries(<app> <package>)
This however requires to manually install all the required libraries (vcpkg install <package>).
How can I list the required packages and have vcpkg or CMake managed them as required (e.g. automatically install them if they are missing)?

Answer 1

I've recently spent some time working with CMake and vcpkg. I dislike the typical approach of managing vcpkg and its installed packages separately from my CMake projects, so I wrote some CMake scripts to handle vcpkg during the CMake build process.
The CMake script to install vcpkg is primarily a call to ExternalProject_Add.
# ./vcpkg_utilities.cmake (part 1)
include(ExternalProject)
function(get_vcpkg)
ExternalProject_Add(vcpkg
GIT_REPOSITORY https://github.com/microsoft/vcpkg.git
CONFIGURE_COMMAND ""
INSTALL_COMMAND ""
UPDATE_COMMAND ""
BUILD_COMMAND "<SOURCE_DIR>/bootstrap-vcpkg.sh"
)
ExternalProject_Get_Property(vcpkg SOURCE_DIR)
set(VCPKG_DIR ${SOURCE_DIR} PARENT_SCOPE)
set(VCPKG_DEPENDENCIES "vcpkg" PARENT_SCOPE)
endfunction()
The script to have vcpkg install a package requires a custom command and target.
# ./vcpkg_utilities.cmake (part 2)
function(vcpkg_install PACKAGE_NAME)
add_custom_command(
OUTPUT ${VCPKG_DIR}/packages/${PACKAGE_NAME}_x64-linux/BUILD_INFO
COMMAND ${VCPKG_DIR}/vcpkg install ${PACKAGE_NAME}:x64-linux
WORKING_DIRECTORY ${VCPKG_DIR}
DEPENDS vcpkg
)
add_custom_target(get${PACKAGE_NAME}
ALL
DEPENDS ${VCPKG_DIR}/packages/${PACKAGE_NAME}_x64-linux/BUILD_INFO
)
list(APPEND VCPKG_DEPENDENCIES "get${PACKAGE_NAME}")
set(VCPKG_DEPENDENCIES ${VCPKG_DEPENDENCIES} PARENT_SCOPE)
endfunction()
Getting the main project linked to vcpkg appropriately (via toolchain file) requires vcpkg to be available when the main project is configured, so the main project is configured as an ExternalProject.
# /CMakeLists.txt
cmake_minimum_required(VERSION 3.17)
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake")
project(top LANGUAGES CXX)
include(vcpkg_utilities)
get_vcpkg()
vcpkg_install(nlohmann_json)
include(ExternalProject)
set_property(DIRECTORY PROPERTY EP_BASE main_projectname)
ExternalProject_Add(main_projectname
DEPENDS ${VCPKG_DEPENDENCIES}
SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/src
INSTALL_COMMAND ""
CMAKE_ARGS --no-warn-unused-cli;
-DCMAKE_TOOLCHAIN_FILE=${VCPKG_DIR}/scripts/buildsystems/vcpkg.cmake
)
Using this structure, CMake will manage the installation of both vcpkg and packages maintained by it.
Note that this is a point solution. I have a more generalized template for this in my Github that covers Windows and Linux, and allows the use of a separately-maintained vcpkg instance.

Answer 2

I've done something similar in my projects using ExternalProject_Add.
Code will be similar to the below pattern (I've replaced my dependency with your)
find_package(nlohmann_json CONFIG) # make package not REQUIRED to avoid error...
if(NOT NLOHMANN_JSON_FOUND) # or whatever variable indicating _FOUND
...
# here ExternalProject_Add from https://github.com/nlohmann/json.git
# using specific tag...
...
ExternalProject_Get_Property(... INSTALL_DIR)
# create imported target here similar to nlohmann_json, as if we found it
...
else()
# We've found lib already installed... will use it...
...
endif()
Another option can be using FetchContent, but I haven't tried it yet.