Modifying a flexbuffers Map after loading from JSON

[nathanmartz]

I would like to create a flexbuffers Map from a JSON string and then modify some of the elements of that map, including Maps contained within that Map. From what I can tell, the only way to do this is to use a builder to create a new Map that contains both the original Map data and the values I want to add/modify. I've written the code to do this, but my currently implementation is a bit more complicated than I would like, so I wanted to see if there was a simpler way to simply modify a Map.

[Advait251206]

As far as I know, FlexBuffers does not support in-place modification of an existing Map after it has been serialized and loaded.

A FlexBuffer is essentially a serialized binary structure, and the objects you access through APIs like Map, Vector, and Reference are views into that serialized data rather than mutable containers. Because of this, operations such as:

• Adding a new key/value pair,
• Removing a key,
• Renaming a key,
• Replacing a nested map,
• Growing or shrinking variable-length values,

would generally require rebuilding part or all of the buffer.

So your current approach—reading the existing data and using a Builder to construct a new FlexBuffer containing the desired modifications—is the approach I would expect.

The main challenge is that changes can affect offsets, key tables, value storage, and the overall layout of the serialized buffer, making true in-place edits difficult to support in a general way.

If you're frequently applying small updates, a couple of alternatives might be worth considering:

• Deserialize into a native mutable structure (e.g. std::unordered_map, JSON object, application-specific structs), make changes there, then reserialize.
• Keep modifications as an overlay layer and only rebuild the FlexBuffer when persisting.
• If you need schema evolution and frequent mutations, consider whether FlatBuffers or another data representation would better fit the workflow.

I'd be interested to hear if there's an API I've overlooked, but I don't believe FlexBuffers currently provides mutable map semantics after serialization.