JSON parser in modern C++

json.cpp

#include "json.hpp"

namespace json {

Wrapper::Wrapper(Value const &value) : v{value} {}
Wrapper::Wrapper(Value &value) : v{value} {}

Wrapper::operator Value &() { return v; }

Wrapper::operator Value const &() const { return v; }

Value &
Wrapper::value() {
  return v;
}
Value const &
Wrapper::value() const {
  return v;
}

std::optional<Array>
detail::JSONParser::parseArray() {
  Array array;
  if (!accept('['))
    return {};
  if (expect(']'))
    return array;
  while (true) {
    if (auto v = parseValue(); v) {
      array.emplace_back(Wrapper(v.value()));
      if (expect(','))
        continue;
      if (expect(']'))
        return array;
      return {};
    }
    return {};
  }
}

std::optional<Object>
detail::JSONParser::parseObject() {
  Object object;
  if (!accept('{'))
    return {};
  if (expect('}'))
    return object;
  while (true) {
    if (auto s = parseString(); s) {
      if (!expect(':'))
        return {};
      if (auto v = parseValue(); v)
        object.emplace(s.value(), Wrapper(v.value()));
      else
        return {};
      if (expect(','))
        continue;
      if (expect('}'))
        return object;
      return {};
    }
    return {};
  }
}

std::optional<String>
detail::JSONParser::parseString() {
  if (!accept('\"'))
    return {};
  auto ptr = std::next(this->ptr);
  unsigned length{0};
  while (true) {
    advance();
    if (*(this->ptr - 1) != '\\' && *(this->ptr) == '"')
      return String{ptr, length};
    ++length;
  }
  return {};
}

std::optional<Number>
detail::JSONParser::parseNumber() {
  auto ptr = this->ptr;
  accept('-');
  if (!check(::isdigit))
    return {};
  while (check(::isdigit))
    advance();
  if (accept('.')) {
    if (!check(::isdigit))
      return {};
    while (check(::isdigit))
      advance();
  }
  if (accept('e') || accept('E')) {
    accept('+') || accept('-');
    if (!check(::isdigit))
      return {};
    while (check(::isdigit))
      advance();
  }
  return std::stod(
      std::string{ptr, static_cast<unsigned long>(this->ptr - ptr)});
}

std::optional<Value>
detail::JSONParser::parseValue() {
  auto matches = [](String string) {
    return [&string](char &c) { return string.compare(&c) == 0; };
  };
  if (check(matches("true"))) {
    advance(4);
    return bool{true};
  }
  if (check(matches("false"))) {
    advance(5);
    return bool{true};
  }
  if (check(matches("null"))) {
    advance(4);
    return Null{};
  }
  if (auto n = parseNumber(); n)
    return n.value();
  if (auto s = parseString(); s)
    return s.value();
  if (auto o = parseObject(); o)
    return o.value();
  if (auto a = parseArray(); a)
    return a.value();
  return {};
}

detail::JSONParser::JSONParser(std::string const &str)
    : ptr{nullptr}, data{str}, obj{parseObject()} {}

bool
detail::JSONParser::accept(char c) {
  bool result = (*ptr == c);
  ptr += result;
  return result;
}

bool
detail::JSONParser::expect(char c) {
  return (*ptr == c);
}

void
detail::JSONParser::advance(int i) {
  ptr += i;
}

detail::JSONParser::operator std::optional<Object> &() { return obj; }

detail::JSONParser::operator std::optional<Object> const &() const {
  return obj;
}

} // end namespace json

json.hpp

// WIP -- not done yet :)

#include <iomanip>
#include <iostream>

#include <optional>
#include <string>
#include <string_view>
#include <unordered_map>
#include <variant>
#include <vector>

namespace json {

struct Wrapper;
using Null   = std::nullptr_t;
using Bool   = bool;
using Number = double;
using String = std::string_view;
using Array  = std::vector<Wrapper>;
using Object = std::unordered_map<String, Wrapper>;
using Value  = std::variant<Bool, Number, String, Array, Object, Null>;

struct Wrapper {
  Wrapper()                = default;
  Wrapper(Wrapper &&)      = default;
  Wrapper(Wrapper const &) = default;
  Wrapper(Value &value);
  Wrapper(Value const &value);

  Wrapper &
  operator=(Wrapper const &) = default;

  operator Value &();
  operator const Value &() const;

  Value &
  value();

  Value const &
  value() const;

private:
  Value v{Null{}};
};

namespace detail {

struct JSONParser {
  JSONParser() = delete;
  JSONParser(std::string const &);
  JSONParser(JSONParser const &) = default;
  JSONParser(JSONParser &&)      = default;
  JSONParser &
  operator=(JSONParser const &) = default;

  std::optional<Object> &
  object();
  std::optional<Object> const &
  object() const;

  operator std::optional<Object> &();
  operator std::optional<Object> const &() const;

private:
  std::optional<Array>
  parseArray();
  std::optional<Object>
  parseObject();
  std::optional<String>
  parseString();
  std::optional<Number>
  parseNumber();
  std::optional<Value>
  parseValue();

  bool
  accept(char);
  bool
  expect(char);
  void
  advance(int i = 1);

  template <typename Fn>
  bool
  check(Fn &&fun) {
    return fun(*ptr);
  }

  std::string::pointer ptr;
  std::string data;
  std::optional<Object> obj;
};
}

using Parser = detail::JSONParser;

template <typename T>
void
print(std::optional<T> const &type) {
  std::visit(
      [](auto &&arg) {
        using Type = std::decay_t<decltype(arg)>;
        if
          constexpr(std::is_same_v<Type, Number>) {
            std::cout << std::get<Number>(arg).value();
          }
        else if
          constexpr(std::is_same_v<Type, Bool>) {
            std::cout << std::boolalpha << std::get<Bool>(arg).value();
          }
        else if
          constexpr(std::is_same_v<Type, Null>) { std::cout << "null"; }
        else if
          constexpr(std::is_same_v<Type, String>) {
            std::cout << '"' << std::quoted(std::get<String>(arg).value())
                      << '"';
          }
        else if
          constexpr(std::is_same_v<Type, Array>) {
            std::cout << '[';
            auto const &arr = std::get<Array>(arg).value();
            print(arr[0]);
            for (unsigned int i = 1; i < arr.size(); ++i) {
              std::cout << ',';
              print(arr[i]);
            }
            std::cout << ']';
          }
        else if
          constexpr(std::is_same_v<Type, Object>) {
            std::cout << '{';
            auto const &map    = std::get<Object>(arg).value();
            unsigned int index = 0;
            for (auto & kv : map) {
              if (index++ != 0) {
                std::cout << ',';
              }
              auto key = std::optional<String>(kv.first);
              print(key);
              std::cout << ':';
              print(kv.second);
            }
            std::cout << '}';
          }
        else
          static_assert("Whoops");
      },
      type.value());
}

} // end namespace json

test.cpp

#include <iostream>
#include <utility>

#include "json.hpp"

int
main() {
  // std::string num = "-123.40e-2";
  // std::string trueString = "true";
  // std::string falseString = "false";
  // std::string string = "Hello \"a quote\" world!";
  // std::string arrNums = "[1,2,3,4]";
  // std::string arrString = "[\"hello\",\"goodbye\"]";
  std::string obj = "{\"key\":1.2,\"key2\":true}";
  {
    std::string object = "{\"key\":1.2,\"key2\":true}";
    auto p             = json::Parser(object);
    json::print(p.object());
  }
}

@willkill07 Thank you for sharing your amazing work!

I tried it but it doesnt work with the following system environment : (ubuntu 18.04.3 LTS)

cpchung:~$ g++ --version
g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

#set(CMAKE_CXX_STANDARD 17) #tried this too
set(CMAKE_CXX_STANDARD 20)

I am very interested in trying this since I am having this issue here with the existing json library:

nlohmann/json#1281

I hope the complimented example there is a helpful test case for us. I basically need to parse the json inside this file :
https://github.com/smogon/pokemon-showdown/blob/master/data/moves.js