103 lines
3.3 KiB
Markdown
103 lines
3.3 KiB
Markdown
# CMake
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You can also use the `nlohmann_json::nlohmann_json` interface target in CMake. This target populates the appropriate usage requirements for `INTERFACE_INCLUDE_DIRECTORIES` to point to the appropriate include directories and `INTERFACE_COMPILE_FEATURES` for the necessary C++11 flags.
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## External
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To use this library from a CMake project, you can locate it directly with `find_package()` and use the namespaced imported target from the generated package configuration:
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```cmake
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# CMakeLists.txt
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find_package(nlohmann_json 3.2.0 REQUIRED)
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...
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add_library(foo ...)
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...
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target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
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```
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The package configuration file, `nlohmann_jsonConfig.cmake`, can be used either from an install tree or directly out of the build tree.
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## Embedded
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To embed the library directly into an existing CMake project, place the entire source tree in a subdirectory and call `add_subdirectory()` in your `CMakeLists.txt` file:
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```cmake
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# Typically you don't care so much for a third party library's tests to be
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# run from your own project's code.
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set(JSON_BuildTests OFF CACHE INTERNAL "")
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# If you only include this third party in PRIVATE source files, you do not
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# need to install it when your main project gets installed.
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# set(JSON_Install OFF CACHE INTERNAL "")
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# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
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# unintended consequences that will break the build. It's generally
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# discouraged (although not necessarily well documented as such) to use
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# include(...) for pulling in other CMake projects anyways.
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add_subdirectory(nlohmann_json)
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...
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add_library(foo ...)
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...
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target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
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```
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## Embedded (FetchContent)
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Since CMake v3.11,
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[FetchContent](https://cmake.org/cmake/help/v3.11/module/FetchContent.html) can
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be used to automatically download the repository as a dependency at configure type.
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Example:
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```cmake
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include(FetchContent)
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FetchContent_Declare(json
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GIT_REPOSITORY https://github.com/nlohmann/json
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GIT_TAG v3.7.3)
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FetchContent_GetProperties(json)
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if(NOT json_POPULATED)
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FetchContent_Populate(json)
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add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
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endif()
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target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
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```
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!!! Note
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The repository <https://github.com/nlohmann/json> download size is huge.
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It contains all the dataset used for the benchmarks. You might want to depend on
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a smaller repository. For instance, you might want to replace the URL above by
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<https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent>.
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## Supporting Both
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To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:
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``` cmake
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# Top level CMakeLists.txt
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project(FOO)
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...
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option(FOO_USE_EXTERNAL_JSON "Use an external JSON library" OFF)
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...
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add_subdirectory(thirdparty)
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...
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add_library(foo ...)
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...
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# Note that the namespaced target will always be available regardless of the
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# import method
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target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
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```
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```cmake
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# thirdparty/CMakeLists.txt
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...
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if(FOO_USE_EXTERNAL_JSON)
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find_package(nlohmann_json 3.2.0 REQUIRED)
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else()
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set(JSON_BuildTests OFF CACHE INTERNAL "")
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add_subdirectory(nlohmann_json)
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endif()
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...
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```
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`thirdparty/nlohmann_json` is then a complete copy of this source tree.
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