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JSON++

A Flex/Bison JSON parser for C++11. Parses strings and files in JSON format, and builds an in-memory tree representing the JSON structure. JSON objects are mapped to std::maps, arrays to std::vectors, JSON native types are mapped onto C++ native types. The library also includes printing on streams. Classes exploit move semantics to avoid copying parsed structures around.

Usage

#include <iostream>
#include "json.hh"

using namespace std;
using namespace JSON;

int main(int argc, char** argv)
{
    // Read JSON from a string
    Value v = parse_string(<your_json_string>);
    cout << v << endl;

    // Read JSON from a file
    v = parse_file("<your_json_file>.json");
    cout << v << endl;

    // Or build the object manually
    Object obj;

    obj["foo"] = true;
    obj["bar"] = 3;

    Object o;
    o["given_name"] = "John";
    o["family_name"] = "Boags";

    obj["baz"] = o;

    Array a;
    a.push_back(true);
    a.push_back("asia");
    a.push_back("europe");
    a.push_back(55);

    obj["test"] = a;

    cout << o << endl;

    return 0;
}

How to build JSON++

Make sure you have Flex and Bison installed, since the parser and lexer will be generated on the fly. Then just run (to build the test ).

make

Flex/Bison quirks when using C++ classes

This section is for the ones who got here because they're trying to build stuff with Flex/Bison and C++. This was my first Flex/Bison parser (the main motivation behind its development being that I didn't find a parser for JSON in C++ which didn't require a number of extra libraries).

So, for the ones venturing in this world, here's a few things I wish I knew when I set off to write the parser.

  1. %union { ... } must only contain native C types or pointers to C++ classes,
  2. in case pointers to C++ classes are used in %union, classes extending std containers won't work,
  3. put a starting rule in the grammar to assign the result of the parse to a variable, e.g., json: value { $$ = $1; },
  4. as a general rule, functions requiring Flex functions, e.g., yy_scan_string, etc., should be defined in the .l file, and their prototypes put in the .y file as well, so that they can be called from the parser's functions,
  5. the JSON syntax tree classes (json_st.hh and json_st.cc) are implemented as collections of pointers for performance reasons (because of 1. the returned values from the parser must be pointers),