Lightweight and fast JSON parser for C++
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vivkin Merge pull request #26 from Nolan-O/master
Fix g++ compiler error
Latest commit 7aee524 Oct 17, 2016


gason is new version of vjson parser. It's still very fast and have very simple interface. Completly new api, different internal representation and using new C++ standard features explains why parser get a new name.


  • No dependencies
  • Small codebase (~450 loc)
  • Small memory footprint (16-24B per value)

gason is not strict parser:

  • Source buffer can contain more than one value (first will be parsed; pointer to the rest returns)
  • Single number, string or identifier will be succesfully parsed
  • Trailing , before closing ] or } is not an error

gason is destructive parser, i.e. you source buffer will be modified! Strings stored as pointers to source buffer, where closing " (or any other symbol, if string have escape sequences) replaced with '\0'. Arrays and objects are represented as single linked list (without random access).


  1. Download latest gason.h and gason.cpp
  2. Compile with C++11 support (-std=c++11 flag for gcc/clang)
  3. ...
  4. PROFIT!



#include "gason.h"
char *source = get_useless_facebook_response(); // or read file, whatever
// do not forget terminate source string with 0
char *endptr;
JsonValue value;
JsonAllocator allocator;
int status = jsonParse(source, &endptr, &value, allocator);
if (status != JSON_OK) {
	fprintf(stderr, "%s at %zd\n", jsonStrError(status), endptr - source);

All values will become invalid when allocator be destroyed. For print verbose error message see printError function in pretty-print.cpp.


double sum_and_print(JsonValue o) {
    double sum = 0;
    switch (o.getTag()) {
    case JSON_NUMBER:
        printf("%g\n", o.toNumber());
        sum += o.toNumber();
    case JSON_STRING:
        printf("\"%s\"\n", o.toString());
    case JSON_ARRAY:
        for (auto i : o) {
            sum += sum_and_print(i->value);
    case JSON_OBJECT:
        for (auto i : o) {
            printf("%s = ", i->key);
            sum += sum_and_print(i->value);
    case JSON_TRUE:
        fprintf(stdout, "true");
    case JSON_FALSE:
        fprintf(stdout, "false");
    case JSON_NULL:
    return sum;
double sum = sum_and_print(value);
printf("sum of all numbers: %g\n", sum);

Arrays and Objects use the same JsonNode struct, but for arrays valid only next and value fields!



gason stores values using NaN-boxing technique. By IEEE-754 standard we have 2^52-1 variants for encoding double's NaN. So let's use this to store value type and payload:

 |  exponent
 |  |
                 |   |
                 tag |

48 bits payload enough for store any pointer on x64. Numbers use zero tag, so infinity and nan are accessible.

Memory management

JsonAllocator allocates big blocks of memory and use pointer bumping inside theese blocks for smaller allocations. Size of block can be tuned by JSON_ZONE_SIZE constant (default 4 KiB).

Parser internals

[05.11.13, 2:52:33] Олег Литвин: о нихуя там свитч кейс на стеройдах!

Internally in jsonParse function nested arrays/objects stored in array of circulary linked list of JsonNode. Size of that array can be tuned by JSON_STACK_SIZE constant (default 32).


For build parser shootout:

  1. (need mercurial, git, curl, nodejs)

Test files downloads from random repos on github:

  • big.json - just big with big count escape sequences
  • monster.json - 3d model, lot of numbers
  • data.json - many objects

First column - parse time in microseconds, second - traverse and sum all numbers.

Intel Core i7 2.3 GHz, OSX 10.9, clang-500.2.79:

shootout/big.json: length 6072200
     gason      20092us         71us 	(5520251617769.000000)
     vjson      19723us        156us 	(5520251617769.000000)
    sajson      25226us        128us 	(5520251617769.000000)
 rapidjson      18093us         96us 	(5520251617769.000000)
shootout/data.json: length 17333
     gason         75us          4us 	(3754.333493)
     vjson         80us          5us 	(3754.333471)
    sajson        117us          8us 	(3754.333493)
 rapidjson         91us          7us 	(3754.333493)
shootout/monster.json: length 196473
     gason        924us        162us 	(34474757.667613)
     vjson       2218us        396us 	(34474757.667621)
    sajson       1898us        380us 	(34474757.667613)
 rapidjson       2210us        446us 	(34474757.667613)

Samsung Galaxy Note II (GT-N7100), Android 4.3, gcc 4.8.2:

I/ruberoid( 8944): /sdcard/Download/shootout/big.json: length 6072200
I/ruberoid( 8944):      gason      66269us        561us 	(5520251617769.000000)
I/ruberoid( 8944):      vjson      59052us       1058us 	(5520251617769.000000)
I/ruberoid( 8944):     sajson      75240us       1573us 	(5520251617769.000000)
I/ruberoid( 8944):  rapidjson      82948us        808us 	(5520251617769.000000)
I/ruberoid( 8944): /sdcard/Download/shootout/data.json: length 17333
I/ruberoid( 8944):      gason        328us         32us 	(3754.333493)
I/ruberoid( 8944):      vjson        316us         54us 	(3754.333471)
I/ruberoid( 8944):     sajson        291us         55us 	(3754.333493)
I/ruberoid( 8944):  rapidjson        386us         48us 	(3754.333493)
I/ruberoid( 8944): /sdcard/Download/shootout/monster.json: length 196473
I/ruberoid( 8944):      gason       6330us       1342us 	(34474757.667613)
I/ruberoid( 8944):      vjson       9481us       2236us 	(34474757.667621)
I/ruberoid( 8944):     sajson       6400us       1971us 	(34474757.667613)
I/ruberoid( 8944):  rapidjson       7464us       1745us 	(34474757.667613)


Distributed under the MIT license. Copyright (c) 2013-2015, Ivan Vashchaev