Integrating into existing CMake Project

[jeffmorr]

Hi all,

I am new to using CMake and am very confused as to how I am supposed to integrate this project into my existing CMake Project. This is what I currently have:

#cmake version to be used
cmake_minimum_required(VERSION 3.5.1)

#project name
project("Json Library")

set(JSON_BuildTests OFF CACHE INTERNAL "")

#flags
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(WINDOWS_COMPILE_FLAGS "/W3 /D_CRT_SECURE_NO_WARNINGS -D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING=1")
set(COMPILE_FLAGS "${WINDOWS_COMPILE_FLAGS}")
SET(${flags} "${COMPILE_FLAGS} ")
set(CMAKE_PREFIX_PATH "$CMAKE_SOURCE_DIR}/third-party/nlohmann;${CMAKE_PREFIX_PATH}")
find_package(nlohmann_json 3.6.1 REQUIRED)
add_library(foo ...)
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

add_executable(Driver foo ${CMAKE_SOURCE_DIR}/src/LootGeneration.cpp)

The result of this file creates this error and I'm not sure how to fix:

By not providing "Findnlohmann_json.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "nlohmann_json", but CMake did not find one.

  Could not find a package configuration file provided by "nlohmann_json"
  (requested version 3.6.1) with any of the following names:

    nlohmann_jsonConfig.cmake
    nlohmann_json-config.cmake

  Add the installation prefix of "nlohmann_json" to CMAKE_PREFIX_PATH or set
  "nlohmann_json_DIR" to a directory containing one of the above files.  If
  "nlohmann_json" provides a separate development package or SDK, be sure it
  has been installed.

Any help is appreciated! Thank you.

[nlohmann]

Here are two example projects of how to integrate the library:

• https://github.com/nlohmann/json/blob/develop/test/cmake_import/project/CMakeLists.txt
• https://github.com/nlohmann/json/blob/develop/test/cmake_add_subdirectory/project/CMakeLists.txt

[jeffmorr]

Thank you!

[coderDec]

@jeffmorr how did you fix that error?
I tried to build the project in cmake_import directory and I still get the same error.

[alex-cornejo]

Same problem here. I have installed the library through vcpkg. This is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.16)
project(test_cpp)
set(CMAKE_CXX_STANDARD 14)
find_package(nlohmann-json REQUIRED)
add_executable(test_cpp main.cpp)
target_link_libraries(test_cpp nlohmann_json)

I get the next output:

  By not providing "Findnlohmann-json.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "nlohmann-json", but CMake did not find one....

Can anybody help me about what it is wrong in my cmake file?

[nlohmann]

I'm not using vcpkg, but looking at https://vcpkg.readthedocs.io/en/latest/examples/installing-and-using-packages/ it seems you need to pass a toolchain file.

[alex-cornejo]

I'm not using vcpkg, but looking at https://vcpkg.readthedocs.io/en/latest/examples/installing-and-using-packages/ it seems you need to pass a toolchain file.

I am starting with c++ and I finished switching to conan. Now I am using the library and it is working fine. Thanks so much!!!

[CtfChan]

Hey guys I had trouble with find_package as well. I am using Ubuntu 18.04.

In the end, I used this line and the package was found

find_package(nlohmann_json REQUIRED
    CONFIG
    PATHS "/usr/lib/cmake/"
)

[brentsony]

Set you Toolchain to:

$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake

(You'll need to specify $VCPKG_ROOT yourself.)

If you're using a Remote Toolchain in an IDE, you cannot just set the toolchain, so include the above vcpkg toolchain in your CMakeLists.txt file, instead:

include($ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake)

[WillNilges]

Here are two example projects of how to integrate the library:

* https://github.com/nlohmann/json/blob/develop/test/cmake_import/project/CMakeLists.txt

* https://github.com/nlohmann/json/blob/develop/test/cmake_add_subdirectory/project/CMakeLists.txt

Looks like these tests have moved.
For the lazy:

• https://github.com/nlohmann/json/blob/develop/tests/cmake_import/project/CMakeLists.txt
• https://github.com/nlohmann/json/blob/develop/tests/cmake_add_subdirectory/project/CMakeLists.txt

[nlohmann]

See also https://json.nlohmann.me/integration/cmake/