Search code examples
c++jsonrapidjson

RapidJSON : Differences between building using Document or Writer

I want to use rapidJSON to build JSON files.
I've noticed there are (at least) 2 options to do so.
The first is using rapidJSON::Writer directly :

StringBuffer s;
Writer<StringBuffer> writer(s);
writer.StartObject();
writer.String("hello");
...

The other is using rapidJSON::Document : 

Document d;
d.SetObject();
d.AddMember("hello", "world", d.GetAllocator());
...

Besides the obvious differences of ease of use on the Document side and more type control on the Writer size, are there any performance differences ? are both allocations the same ?

Solution

  • Document is a data structure for storing JSON tree (aka DOM) in memory. Writer is needed to stringify (dump/serialize) a Document into JSON:

    d.Accept(writer);

    So, if your application only needs to write JSON, and it can apply Writer easily, it is preferred over Document. It is because Document needs memory allocation and additional overheads.

    However, Document is more easily for parsing and modify JSON.

    P.S. In the current version of RapidJSON, it is prefer to use writer.Key("hello") instead of writer.String("hello") for object keys.