Structurize your JSON

json_struct is a single-header C++ library that parses JSON to structs/classes and serializes structs/classes back to JSON. With support for relaxed parsing rules, it's also excellent for configuration files and human-editable data formats.

Getting Started: Simply copy json_struct.h from the include folder into your project's include path.

Requirements: C++11 or newer. Tested on GCC, Clang, and Visual Studio 2015+.

Quick Start

json_struct automatically maps JSON to C++ structs by adding simple metadata declarations.

{
    "One" : 1,
    "Two" : "two",
    "Three" : 3.333
}

can be parsed into a structure defined like this:

struct JsonObject
{
    int One;
    std::string Two;
    double Three;

    JS_OBJ(One, Two, Three);
};

or

struct JsonObject
{
    int One;
    std::string Two;
    double Three;
};
JS_OBJ_EXT(JsonObject, One, Two, Three);

Parse JSON to struct:

JS::ParseContext context(json_data);
JsonObject obj;
context.parseTo(obj);

Serialize struct to JSON:

std::string pretty_json = JS::serializeStruct(obj);
// or
std::string compact_json = JS::serializeStruct(obj, JS::SerializerOptions(JS::SerializerOptions::Compact));

Relaxed Parsing for Config Files

json_struct supports relaxed JSON parsing rules, making it ideal for configuration files and human-editable data. Enable optional features for a more forgiving syntax:

• Comments using // syntax
• Unquoted property names and string values (supports A-Z, a-z, 0-9, _, -, ., /)
• Newlines instead of commas as delimiters
• Trailing commas in objects and arrays

Example configuration file:

{
  // Server configuration
  host: localhost
  port: 8080

  database: {
    name: myapp_db
    max_connections: 100,  // Trailing comma is OK
  }

  log_file: /var/log/app.log
}

Enable relaxed parsing:

JS::ParseContext context(config_data);
context.tokenizer.allowComments(true);
context.tokenizer.allowAsciiType(true);
context.tokenizer.allowNewLineAsTokenDelimiter(true);
context.tokenizer.allowSuperfluousComma(true);
context.parseTo(config_obj);

Dynamic JSON with Maps

When the JSON structure depends on runtime values, you can parse into a JS::Map first, inspect the data, then dispatch to the appropriate type. For example, consider JSON describing different vehicle types:

{
  "type" : "car",
  "wheels" : 4,
  "electric" : true,
...
}

or it could look like this:

{
  "type" : "sailboat",
  "sail_area_m2" : 106.5,
  "swimming_platform": true,
...
}

Parse into a map, query for the type field, then convert to the specific struct:

void handle_data(const char *data, size_t size)
{
  JS::Map map;
  JS::ParseContext parseContext(data, size, map);
  if (parseContext.error != JS::Error::NoError)
  {
    fprintf(stderr, "Failed to parse Json:\n%s\n", parseContext.makeErrorString().c_str());
    return;
  }
  VehicleType vehicleType = map.castTo<VehicleType>("type", parseContext);
  if (parseContext.error != JS::Error::NoError)
  {
    fprintf(stderr, "Failed to extract type:\n%s\n", parseContext.makeErrorString().c_str());
    return;
  }
  switch (vehicleType)
  {
  case VehicleType::car:
  {
    Car car = map.castTo<Car>(parseContext);
    if (parseContext.error != JS::Error::NoError)
    {
      //error handling 
    }
    handle_car(car);
    break;
  }
  case VehicleType::sailboat:
    Sailboat sailboat;
    map.castToType(parseContext, sailboat);
    if (parseContext.error != JS::Error::NoError)
    {
      //error handling 
    }
    handle_sailboat(sailboat);
    break;
  }
}

The JS::Map allows querying and extracting individual fields before converting the entire object. Two casting styles are available:

    Car car = map.castTo<Car>(parseContext);
    // or
    Sailboat sailboat;
    map.castToType(parseContext, sailboat);

Advanced Macro Usage

The JS_OBJ macro adds a static metadata object to your struct without affecting its size or semantics. For more control, use the verbose JS_OBJECT macro with explicit member declarations:

struct JsonObject
{
    int One;
    std::string Two;
    double Three;

    JS_OBJECT(JS_MEMBER(One)
            , JS_MEMBER(Two)
            , JS_MEMBER(Three));
};

Custom JSON keys and aliases:

struct JsonObject
{
    int One;
    std::string Two;
    double Three;

    JS_OBJECT(JS_MEMBER(One)
            , JS_MEMBER_WITH_NAME(Two, "TheTwo")
            , JS_MEMBER_ALIASES(Three, "TheThree", "the_three"));
};

• JS_MEMBER_WITH_NAME - Uses only the supplied name, ignoring the member name
• JS_MEMBER_ALIASES - Checks aliases after the primary member name

Note: Don't mix JS_MEMBER macros with JS_OBJ as it will double-apply the macro.

Custom Type Handlers

For types that don't fit the standard object model (e.g., custom serialization logic), implement a TypeHandler specialization. The JS::ParseContext looks for template specializations:

namespace JS {
    template<typename T>
    struct TypeHandler;
}

Built-in type handlers:

• std::string
• double
• float
• uint8_t
• int16_t
• uint16_t
• int32_t
• uint32_t
• int64_t
• uint64_t
• std::unique_ptr
• bool
• std::vector
• [T]

Custom type handler example:

namespace JS {
template<>
struct TypeHandler<uint32_t>
{
public:
    static inline Error to(uint32_t &to_type, ParseContext &context)
    {
        char *pointer;
        unsigned long value = strtoul(context.token.value.data, &pointer, 10);
        to_type = static_cast<unsigned int>(value);
        if (context.token.value.data == pointer)
            return Error::FailedToParseInt;
        return Error::NoError;
    }

    static void from(const uint32_t &from_type, Token &token, Serializer &serializer)
    {
        std::string buf = std::to_string(from_type);
        token.value_type = Type::Number;
        token.value.data = buf.data();
        token.value.size = buf.size();
        serializer.write(token);
    }
};
}

This gives complete control over serialization/deserialization and can represent any JSON structure (objects, arrays, primitives).

Examples and Tests

• Examples - See practical usage patterns
• Unit Tests - Comprehensive test coverage

Quality Assurance

Static Analysis:

• PVS-Studio - C, C++, C#, and Java static analyzer

Dynamic Analysis: All pull requests are tested with:

• Clang Address Sanitizer
• Clang Memory Sanitizer