To my knowledge, there are 3 Qt based JSON parsers out there – QJson, JsonQt and this. QJson uses bison for parsing and JsonQt is hand-written. I have used QJson before and it works perfectly fine.

If you are like me, you will sense an opportunity here to write a qlalr based parser So, here it is – qjsonparser. The grammar is from RFC4627. It behaves very much like QJson – it returns a QVariant for the parsed JSON, uses QVariantMap for objects and QVariantList for arrays. Unlike the others, code is meant to be compiled in place (instead of a library). So, there’s just 3 files overall that you need to drop into your code (README).

If you haven’t used qlalr before:
1. qlalr generates reentrant parsers out of the box. With flex/bison in C mode, one needs to do all sorts of stuff to create a reentrant parser. (QJson uses bison in c++ mode, so it’s reentrant)
2. Complete control over shift/reduce. AFAIK, parsing incrementally using bison isn’t possible easily. With qlalr, you have to write the equivalent of yyparse() yourself and this gives a great deal of control over the shift/reduce steps.
3. It’s completely undocumented. With bison, you don’t really need to understand how LALR parsers work. qlalr, on the other hand, will make you pull out your compiler book. Most of the yyparse() equivalent code that I mentioned can be copy/pasted. But if you are averse to copy/pasting seemingly obfuscated/random code, you absolutely have to understand how LALR parsers work before touching qlalr. Which is why you shouldn’t believe those posts which say that you will be at home with qlalr if you have used bison before. And oh, I don’t intend to spoil the fun of using it by documenting it .
4. qlalr is used by QXmlStreamReader and the old QtScript code. I think it’s the best parser generator for Qt projects.

PS: Currently, I use a hand made lexer, but you can use lexgen which is a Qt friendly scanner. I would have used it but it would add to the number of files. lexgen is used in the Qt’s CSS scanner. The CSS parser, however, is hand made since qlalr didn’t exist then.

UPDATE: This project has moved to http://gitorious.org/qjsonparser/qjsonparser