STRUCT <> JSON Convertor

Summary:

FOR converting data between BINARY STRUCT and JSON. It always be used for parse data between web application and socket protocols. Such as convert between MQTT and modbus(TCP or 485).

The core is C++, it included the library json.hpp

It can be running on server-end as a service, or edge compute.

Author: Ren Kun Refer to: https://bot-man-jl.github.io/articles/2018/Cpp-Struct-Field-Reflection

How to use:

1. Define "struct" in the C++ head file for JSON compiles

Please refer to the file json2struct.h and struct2json.h

#pragma pack(push,1)  

struct SampleStruct {
  long timestamp;
  bool active;
  char name[7];
  int value;
};

struct Schema {
  int id;
  SampleStruct data;
};
#pragma pack(pop)  

void to_json(nlohmann::json& j, const SampleStruct& value) {
  j["timestamp"] = value.timestamp;
  j["active"] = value.active;
  j["name"] = std::string(value.name);
  j["value"] = value.value;
}

void to_json(nlohmann::json& j, const Schema& value) {
  j["id"] = value.id;
  j["data"] = value.data;
}

2. Compile to static library for calling

Use g++ with C++14 to compile the 2 source files to .so file

$ g++ -fPIC -shared json2struct.cc -std=c++14  -o libs/json2struct.so
$ g++ -fPIC -shared struct2json.cc -std=c++14  -o libs/struct2json.so

3. Load the libraries with any language

You can load those librarries in any languages such as Python, PHP, Java or any other language which support stardard "C" library. Below are a simple in python

import ctypes
struct2json = ctypes.CDLL('./libs/struct2json.so')

struct2json.test.restype = ctypes.c_char_p
struct2json.parse.restype = ctypes.c_char_p

print(struct2json.test(b'\x41\x42\x43\x44\x45\x46\x47'))
# print(struct2json.test(b"abc"))
print(struct2json.parse(b'\x12\x27\x00\x00\x68\xAB\x4A\x5E\x00\x00\x00\x00\x01\x41\x42\x43\x44\x45\x46\x00\x0A\x00\x00\x00'))

There are 3 samples in folder /python

Compile to execute bin file

Please refer to the files in folder samples

!!!NOTES!!!

1. struct "Schema" will be used as the default root struct
2. #pragma pack(push,1) is used for make sure the struct align with byte, it can be changed as the final usage. Also #pragma pack(pop) should be used after the struct is defined

Running Result

$ python3 python/json2struct.py 
c++ output: Hi ABCDE
b'Hi ABCDE'
0x12 0x27 0x00 0x00 0x68 0xab 0x4a 0x5e 0x00 0x00 0x00 0x00 0x01 0x41 0x42 0x43 0x44 0x45 0x46 0x47 0x0a 0x00 0x00 0x00

$ python3 python/struct2json.py 
b'Hi ABCDEFG'
b'{"data":{"active":true,"name":"ABCDEF","timestamp":1581951848,"value":10},"id":10002}'

$ python3 python/mix.py 
0x12 0x27 0x00 0x00 0x68 0xab 0x4a 0x5e 0x00 0x00 0x00 0x00 0x01 0x41 0x42 0x43 0x44 0x45 0x46 0x47 0x0a 0x00 0x00 0x00
b'{"data":{"active":true,"name":"ABCDEFG\\n","timestamp":1581951848,"value":10},"id":10002}'

$ cd samples
$ clang++ json2struct.cc -std=c++14 -Wall -o ../bin/json2struct && ../bin/json2struct
0x37 0x00 0x00 0x00 
0x68 0xAB 0x4A 0x5E 
0x00 0x00 0x00 0x00 
0x01 0x41 0x42 0x43 
0x44 0x45 0x46 0x00 
0x0A 0x00 0x00 0x00 

length:24
55
1581951848
1
ABCDEF
10

$ clang++ struct2json.cc -std=c++14 -Wall -o ../bin/struct2json && ../bin/struct2json
{
  "data": {
    "active": true,
    "name": "ABCDEF",
    "timestamp": 1581951848,
    "value": 10
  },
  "id": 55
}