terrakuh/smart_json

Header-only automagic mapping from C++ to JSON. Yes in C++!

Smart JSON

Header only automatic JSON encoding and decoding with Boost.Describe. There are adapters for the following JSON libraries:

• Boost.JSON
• Boost.PropertyTree
• nlohmann/json

The following requirements need to be fulfilled:

• C++ 17
• CMake >= 3.14
• Boost >= 1.78

Example

For the complete example look here.

BOOST_DEFINE_ENUM_CLASS(Log_level, off, debug, info, error);

struct Config
{
  std::string log_path;
  Log_level log_level;
  std::map<std::string, std::string> additional_headers;
  std::optional<std::string> save_path;

  struct Nested
  {
    int value = 0;
  };
  std::array<Nested, 3> fixed_array;
  std::vector<Nested> array;
};
BOOST_DESCRIBE_STRUCT(Config, (), (log_path, log_level, additional_headers, save_path, fixed_array, array));
BOOST_DESCRIBE_STRUCT(Config::Nested, (), (value));

int main()
{
  Config config{
    "v0.0.0",
    Log_level::info,
    { { "User-Agent", "smart_json" } },
    std::nullopt,
  };
  config.fixed_array[0].value = 1;
  config.fixed_array[1].value = 2;
  config.fixed_array[2].value = 3;
  config.array.push_back({ 61 });

  auto obj = smart_json::encode<boost::json::object>(config);
  std::cout << boost::json::serialize(obj) << "\n";

  config = smart_json::decode<Config>(obj);
}

And the output (formatted) is:

{
  "logPath": "v0.0.0",
  "logLevel": "info",
  "additionalHeaders": {
    "User-Agent": "smart_json"
  },
  "fixedArray": [
    {
      "value": 1
    },
    {
      "value": 2
    },
    {
      "value": 3
    }
  ],
  "array": [
    {
      "value": 61
    }
  ]
}

Note that the member names in C++ are in the typical snake-case format and will be automagically converted to the JSON typical camel-case. This is guaranteed to happen at compile time and inflicts no additional cost at runtime. If you want to implement a custom transformer, have a look at the smart_json::Camel_case_transformer or the Upper_case_transformer in the playgrounds example. You can also use smart_json::No_transformer if no conversion is desired.

Supported types

• Any arithmetic type (bool, integrals, floating points)
• std::string
• std::optional
• Any enum / struct / class described with Boost.Describe
• Any container (eg. std::vector, std::array, etc.) that statisfies the following conditions will be encoded as an array:
  • return an Iterator for begin() and end()
  • return unsigned integral for size()
  • is not std::string
  • for decoding any of (order of precedence):
    • push_back(value_type{}) (for dynamically sized array)
    • push_front(value_type{}) (for dynamically sized array)
    • operator[](value_type{}) (for fixed size array; bounds checking will be performed with size())
• Any associative container (eg. std::map, etc.) that statisfies the following conditions will be encoded as an object:
  • fulfills all the constraints of the container above except for the decoding
  • has a type key_type which is equal to std::string
  • has a mapped_type
  • has a value_type which can be destructured into [key, value]
  • has an insert(value_type{key, value}) method

Building

There is no compiling required. However, there are some CMake options:

Option	Default	Description
SMART_JSON_BUILD_EXAMPLES	OFF	Builds the examples in examples/
SMART_JSON_FETCH_CONTENT	ON	Automagically fetches the missing and required dependencies

Usage

find_package(smart_json 0.4 REQUIRED)
target_link_libraries(myproject PRIVATE smart_json::smart_json)

And if you want to automagically fetch Boost 1.78 just add this before find_package(smart_json):

include(FetchContent)
FetchContent_Declare(
  Boost
  URL https://boostorg.jfrog.io/artifactory/main/release/1.78.0/source/boost_1_78_0.tar.gz
  URL_HASH SHA256=94ced8b72956591c4775ae2207a9763d3600b30d9d7446562c552f0a14a63be7
)
FetchContent_MakeAvailable(Boost)
FetchContent_GetProperties(Boost SOURCE_DIR Boost_INCLUDE_DIR)
find_package(Boost 1.78 REQUIRED)