Usage of the jsoncpp CMake package

[ghisvail]

Dear jsoncpp developers,

I tried to use the CMake package built by jsoncpp (and provided by libjsoncpp-dev on Debian / Ubuntu) on a CMake project depending on it.

I tried the following discovery via CMake config mode:

find_package(jsoncpp REQUIRED)
include_directories(${jsoncpp_INCLUDE_DIRS})
link_libraries(${jsoncpp_LIBRARIES})

But it does not seem to work. I then ran the the following test:

cmake_minimum_required(VERSION 2.6.2)
project(jsoncpp_test)

find_package(jsoncpp REQUIRED)
message("FOUND CONFIG: ${jsoncpp_CONFIG}")
message("INCLUDE DIRS: ${jsoncpp_INCLUDE_DIRS}")
message("LIBRARIES: ${jsoncpp_LIBRARIES}")

which produced the following output:

FOUND CONFIG: /usr/lib/x86_64-linux-gnu/cmake/jsoncpp/jsoncppConfig.cmake
INCLUDE DIRS: 
LIBRARIES: 

meaning the CMake package file is found but none of the expected variables is set.

How are we expected to discover and use jsoncpp under CMake then? FYI, using pkg-config works fine, but is not what I would consider a solution.

Many thanks,
Ghis

[arrufat]

I've managed to make it work like this

find_package(PkgConfig REQUIRED)
pkg_check_modules(JSONCPP jsoncpp)
link_libraries(${JSONCPP_LIBRARIES})
add_executable(myprogram myprogram.cpp)
target_link_libraries(myprogram ${JSONCPP_LIBRARIES})

[cdunn2001]

Thanks, Max Headroom. Update the wiki too, if you want.

[pazz]

thanks @arrufat
Silly question: you would then include jsoncpp in your main file as

#include <jsoncpp/json/json.h>

right? Does that mean the code is not portable in the sense that if will only compile if run on a distri that packages the headers in this way?

[arrufat]

Yes, I had exactly this issue with Arch Linux and Ubuntu...

[pazz]

Thanks for the quick response. I've just seen that you can add the discovered include path with

include_directories("${JSONCPP_INCLUDE_DIRS}")

in CMakeLists.txt. Then you can just include as <json/json.h>...

[christian-rauch]

The jsoncppConfig.cmake defines property INTERFACE_INCLUDE_DIRECTORIES for targets jsoncpp_lib and jsoncpp_lib_static.

You need to query the target property and set it manually:

get_target_property(JSON_INC_PATH jsoncpp_lib INTERFACE_INCLUDE_DIRECTORIES)
include_directories(${JSON_INC_PATH})

Linking is done via:

target_link_libraries(${PROJECT_NAME} jsoncpp_lib)

[cdunn2001]

@christian-rauch , If you submit a PR, we'll merge it. We don't maintain the Cmake files anymore; users do that. We use only the Meson files ourselves.

[christian-rauch]

@cdunn2001 The code snippet that I mentioned has to be used on the user side, e.g. an application that wants to use jsoncpp.

If someone is interested in making the usage of jsoncpp in CMake projects easier, there is a way to export header and libraries in such a way that an application can just import the library without dealing with include directories and libraries manually: https://rix0r.nl/blog/2015/08/13/cmake-guide/

[cdunn2001]

Thanks. Updated our Wiki.

If you submit the change to simplify Cmake, we'd merge it.

[mathemaphysics]

I'm trying to use jsoncpp via add_subdirectory(). I can't seem to guess the target names right. Neither jsoncpp nor json appear to be library targets. I'm planning on reading through all the CMake files, but I thought I'd ask here just in case anyone could help.

[devkarlos]

I'm trying to use jsoncpp via add_subdirectory(). I can't seem to guess the target names right. Neither jsoncpp nor json appear to be library targets. I'm planning on reading through all the CMake files, but I thought I'd ask here just in case anyone could help.

Did you figured it out ?

[Leonkoithara]

Hi I'm using mac os x(installed jsoncpp using brew install jsoncpp) and I found ${JSONCPP_LINK_LIBRARIES} was the required library to be linked. Sample CMakeLists.txt

cmake_minimum_required(VERSION 3.22)
set(CMAKE_CXX_STANDARD 11)

find_package(PkgConfig REQUIRED)
pkg_check_modules(JSONCPP jsoncpp)

add_executable(utils main.cpp)

target_link_libraries(
    utils
    ${JSONCPP_LINK_LIBRARIES}
)

[Lithimlin]

I can't work out how to include this library when installed on Ubuntu 22.04.

I've tried the version in the wiki, the one above, and a method I found on stackoverflow.
I'm trying to include the library using #include <json/json.h>. However, when using #include <jsoncpp/json/json.h>, it works

[eli-schwartz]

Hi I'm using mac os x(installed jsoncpp using brew install jsoncpp) and I found ${JSONCPP_LINK_LIBRARIES} was the required library to be linked. Sample CMakeLists.txt

From the CMake docs: https://cmake.org/cmake/help/latest/module/FindPkgConfig.html#command:pkg_check_modules

New in version 3.6: The IMPORTED_TARGET argument will create an imported target named PkgConfig::<prefix> that can be passed directly as an argument to target_link_libraries().

Please switch this:

pkg_check_modules(JSONCPP jsoncpp)

target_link_libraries(
    utils
    ${JSONCPP_LINK_LIBRARIES}
)

for this:

pkg_check_modules(JSONCPP jsoncpp IMPORTED_TARGET GLOBAL)

target_link_libraries(
    utils
    PkgConfig::JSONCPP
)