peejay

JSON Parser for C++

Status

Category	Badges
Continuous Integration
Static Analysis
Dynamic Analysis
OpenSSF

Introduction

Peejay (PJ, from the English pronunciation of Parse JSON) is a header-only C++ library that provides a state-machine-driven JSON parser with the following characteristics:

• C++20 or later language standard requirement
• Single-character parsing: feed content to the parser a byte or entire document at a time
• The parser front-end has fixed and predictable overhead: it does not allocate memory and does not throw or catch exceptions
• Template-based backend system allowing DOM construction, validation-only parsing, or custom output formats
• Policy-driven configuration for customisation

Example

A configuration policy for the parser:

#include "peejay/json.hpp"

struct policy : public peejay::default_policies {
  static constexpr std::size_t max_length = 64;
  static constexpr std::size_t max_stack_depth = 8;
  static constexpr bool pos_tracking = true;

  using float_type = double;
  using integer_type = std::int64_t;
};

A class which implements a simple backend which prints the tokens as they arrive from the front-end parser:

/// A backend which prints tokens as they arrive.
class print_tokens {
public:
  using policies = policy;
  static constexpr void result() noexcept {
    // The "print_tokens" backend produces no result at all.
  }

  static std::error_code boolean_value(bool const b) noexcept { std::cout << (b ? "true" : "false") << ' '; return {}; }
  static std::error_code float_value(policy::float_type const v) { std::cout << v << ' '; return {}; }
  static std::error_code integer_value(policy::integer_type const v) { std::cout << v << ' '; return {}; }
  static std::error_code null_value() { std::cout << "null"; return {}; }
  static std::error_code string_value(std::u8string_view const &sv) { show_string(sv); std::cout << ' '; return {}; }

  static std::error_code begin_array() { std::cout << "[ "; return {}; }
  static std::error_code end_array() { std::cout << "] "; return {}; }

  static std::error_code begin_object() { std::cout << "{ "; return {}; }
  static std::error_code key(std::u8string_view const &sv) { show_string(sv); std::cout << ": "; return {}; }
  static std::error_code end_object() { std::cout << "} "; return {}; }

private:
  static void show_string(std::u8string_view const &sv) {
    std::cout << '"';
    std::ranges::copy(std::ranges::subrange{sv} | std::ranges::views::transform([] (char8_t c) { return static_cast<char>(c); }), std::ostream_iterator<char>{std::cout});
    std::cout << '"';
  }
};

A top-level driver function. Here the entire JSON document is presented to parser<>::input() at once. We could also supply it in smaller pieces down to a single byte at a time.

int main() {
  using namespace std::string_view_literals;

  int exit_code = EXIT_SUCCESS;
  peejay::parser<print_tokens> p;
  p.input(u8R"(  { "a":123, "b" : [false,"c"], "c":true }  )"sv).eof();
  if (auto const err = p.last_error()) {
    std::cerr << "Error: " << err.message() << '\n';
    exit_code = EXIT_FAILURE;
  }
  return exit_code;
}