[json][review] JSON review

From: Harshada Wighe (harshadawighe_at_[hidden])
Date: 2020-09-21 18:03:49

We vote to reject the JSON library from Boost and our review of the library and reason for rejection are below:

DOCUMENTATION

The documentation is very good. The usage and examples and reference are easy to understand and contain all the information we are looking for.

IMPLEMENTATION

The organization of source code is very messy:

1. Some files do not include all headers at the very start of the file but have some include statements at the end. For example the json/object.hpp file has 10 include statements at the start of the file before the opening namespace and then 1 include statement at the end of the file after the closing namespace. This is very confusing and not a good practice.

2. Splitting the files into HPP and IPP is also not good because it spreads the class into more files than necessary and causes compilation to be slower.

Using the library in a project has a complication:

3. The library is complicated to use in a project because you cannot simply include headers in any file you want. You have to include a src.hpp file but this file must be included only once. Normal header libraries in Boost just let you include the headers without this strange requirement.

The library adds a lot of size to your executable:

4. Unlike other JSON libraries which are smaller, this adds several KB to the exe size. This is possibly because of use of templates or because it contains very big lookup tables.

The performance is not as fast for all use cases:

5. Compared to our in house JSON library, this library was slightly slower. We have very simple but very large JSON data.  We use objects and arrays, but only integer numbers with no fractional part and very large ASCII strings, and no booleans and no nulls. In one of our standalone conversion tools we tried using this library and found it had a 4% longer runtime on a small 170 MB file.

The parser allocates too much memory:

6. I think this is because the design is not optimized for the case when all the JSON content is already stored in memory. In our parser we point to the strings in the original buffer.

The library duplicate code that is in the C++ compiler libraries:

7. In json/config.hpp there are implementations of move and remove_cv from the C++ standard. This introduces more duplicate of this functionality into programs.

Besides these points the implementation is well written.

DESIGN

The allocator design is not standard:

1. The library has a different allocation system that is not the C++ standard system for allocators. We did not understand why storage_ptr was needed at first. It would be better for the library to just template on allocator just like the vector class in the C++ standard. The allocator system also uses virtual calls. The C++ standard vector class also does not have this problem when it uses the standard allocator system.

Other than these points the design is ok.

OVERALL

We would not use this library because it does not offer anything special for JSON that we need. It also uses some designs that are not standard practice. We use JSON heavily in our historical market quote service and most of our tools deal with JSON files and we use many Boost libraries so we are hoping for a JSON library in Boost that we could use instead of having our own.