Unable to compile with preinstalled nlohmann json library

[elnoir]

My build environment is the folowing:

• Windows, Visual Studio 2019 - 64 bit
• nlohmann::json library (version: 3.11.2) compiled as a single header and installed with CMake

Reprosteps:

cd json_validator.git
mkdir build
cd build
cmake -S .. -B . -G "Visual Studio 16 2019" -Dnlohmann_json_DIR:PATH="../where/the/library/was/installed/nlohmann/share/cmake/nlohmann_json/"
cmake --build .

Result:

C:\work\git\json_validator.git\src\json-validator.cpp(878,1): error C2440: 'initializing': cannot convert from 'const nlohmann::json_abi_v3_11_2::json' to 'T' [C:\work\git\json_validator.git\build\nlohmann_json_schema_validator.vcxproj]
C:\work\git\json_validator.git\src\json-patch.cpp(68,14): error C3688: invalid literal suffix '_json'; literal operator or literal operator template 'operator ""_json' not found [C:\work\git\json_validator.git\build\nlohmann_json_schema_validator.vcxproj]

The first issue seems in incorrect casting: I beleive it should be

T value = instance.get<T>();

The second error seems to be regarding the move of the json literal:see here The following code did the trick.

using namespace nlohmann::literals;

After this modification the library compiles, but most of test cases are still broken due to the missing literals.

[LecrisUT]

What CMake version are you using? It should be handled by: https://github.com/pboettch/json-schema-validator/blob/08d8a52a8a1353b6bc953b63f7c6763e74e6047b/CMakeLists.txt#L88-L107

Normally if it doesn't find it automatically, it will still give you a version via FetchContent. Can you look into the build directory for _deps. If it's not there, or you see in the log that find_package succeeded, than there might be an issue with how nlohmann::json was packaged and installed?

PS: nlohmann_json_ROOT is preferred over nlohmann_json_DIR

[elnoir]

Okay, it seems it was an issue with the installation of the nlohmann json library. It was built from the develop branch instead of the version tag.

[LecrisUT]

Thanks, that's also useful to know that it will have to be addressed for the future version