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docs: Add additional CMake documentation #1270

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Oct 3, 2018
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59 changes: 57 additions & 2 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,8 @@

- [Design goals](#design-goals)
- [Integration](#integration)
- [CMake](#cmake)
- [Package Managers](#package-managers)
- [Examples](#examples)
- [JSON as first-class data type](#json-as-first-class-data-type)
- [Serialization / Deserialization](#serialization--deserialization)
Expand Down Expand Up @@ -69,17 +71,70 @@ to the files you want to process JSON and set the necessary switches to enable C

You can further use file [`include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/json_fwd.hpp) for forward-declarations. The installation of json_fwd.hpp (as part of cmake's install step), can be achieved by setting `-DJSON_MultipleHeaders=ON`.

If this library was built with CMake then you can consume it from another CMake project by using the namespaced imported target from the generated package configuration:
### CMake

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.

#### External

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:
```cmake
# CMakeLists.txt
find_package(nlohmann_json REQUIRED)
find_package(nlohmann_json 3.2.0 REQUIRED)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
The package configuration file, `nlohmann_jsonConfig.cmake`, can be used either from an install tree or directly out of the build tree.

#### Embedded

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:
```cmake
# Typically you don't care so much for a third party library's tests to be
# run from your own project's code.
set(JSON_BuildTests OFF CACHE INTERNAL "")
# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
# inintended consequences that will break the build. It's generally
# discouraged (although not necessarily well documented as such) to use
# include(...) for pulling in other CMake projects anyways.
add_subdirectory(nlohmann_json)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```

#### Supporting Both
To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:
``` cmake
# Top level CMakeLists.txt
project(FOO)
...
option(FOO_USE_EXTERNAL_JSON "Use an external JSON library" OFF)
...
add_subdirectory(thirdparty)
...
add_library(foo ...)
...
# Note that the namespaced target will always be available regardless of the
# import method
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
```cmake
# thirdparty/CMakeLists.txt
...
if(FOO_USE_EXTERNAL_JSON)
find_package(nlohmann_json 3.2.0 REQUIRED)
else()
set(JSON_BuildTests OFF CACHE INTERNAL "")
add_subdirectory(nlohmann_json)
endif()
...
```
`thirdparty/nlohmann_json` is then a complete copy of this source tree.

### Package Managers

Expand Down