Unabled to load lohmann/json.hpp using CMake - getting fatal error: 'nlohmann/json.hpp' fi-CodePudding

I have the following main.cpp, very simple script, trying to re-produce the problem and isolate to it's most basic.

#include<iostream>

#include<fmt/core.h>

// #include "json/json.hpp"
// #include <json/json.hpp>
// #include <nlohmann/json.hpp>
// #include "json.hpp"
// #include "nlohmann/json.hpp"

int main(){
    fmt::print("Hello, world!\n");
    return 0;
}

The commented out include statements are all of the paths I have tried to get this into my program.

My CMakeLists.txt file looks like this

cmake_minimum_required(VERSION 3.24)
project(main)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_FLAGS "-std=c  17")
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/build)

# include(FetchContent)
# FetchContent_Declare(
#   json
#   GIT_REPOSITORY https://github.com/nlohmann/json
# )
# FetchContent_MakeAvailable(json)

include(FetchContent)
FetchContent_Declare(fmt
    GIT_REPOSITORY https://github.com/fmtlib/fmt
    GIT_TAG 9.0.0
)
FetchContent_MakeAvailable(fmt)

add_executable(main main.cpp)
# target_link_libraries(main json)
target_link_libraries(main fmt)

I've commented out the json part since it's not working obviously, I can also json-src and json-build in my _deps folder so I know for a fact that it is being downloaded into my local machine.

After running cmake CMakeLists.txt and make and then running the executable I get the error in the title 'nlohmann/json.hpp' file not found

Any help would be appreciated.

Tried to reproduce the example with another popular C package which worked fine. Tried changing the include statements to see if maybe I had something wrong since I don't fully understand all the nuance with CMake and am quite new to it.

CodePudding user response：

You can either specifically use the interface target already included in the nlohmann library, which will automatically populate the correct include path for you, with:

target_link_libraries(main nlohmann_json::nlohmann_json)

Or you would need to specifically include the include path yourself:

include_directories(${json_SOURCE_DIR}/include)

With either way, you will be able to use #include <nlohmann/json.hpp> in your source.