Modify existing messages

[RobinUS2]

What is the most efficient way to edit an existing message? Basically I want to set a few fields on an already built message, but it seems that I need the builder for that (as there are no set methods on the Reader).

Is there a basic concept what is best to use this? Thanks!

[dwrensha]

You can call MessageBuilder.setRoot(), which requires a copy. We could also implement a way to construct a MessageBuilder from some existing buffers without needing to copy. Such a constructor might cause weird behavior on maliciously constructed input, but on trusted input it would be fine.

Note also that using Cap'n Proto as a mutable format requires some care. Whenever you set a pointer field to a new value you'll leave behind dead space in the message where the old value was. If you're just modifying primitive fields, this isn't a problem. Also, you can effectively "garbage collect" such dead space away by copying a message into a new MessageBuilder with setRoot().