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Clarified how to test for the presence of fields.

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Bug: 26863353
Change-Id: I227e10a5c0134dbe7fd7f59e64cd57a0752c568e
This commit is contained in:
Wouter van Oortmerssen
2016-02-29 14:31:06 -08:00
parent 9a76ce8a65
commit 19afcdc704
1 changed files with 26 additions and 1 deletions
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docs/source/Schemas.md
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@@ -122,7 +122,8 @@ default to `NULL` when not present.
You generally do not want to change default values after they're initially
defined. Fields that have the default value are not actually stored in the
serialized data but are generated in code, so when you change the default, you'd
serialized data (see also Gotchas below) but are generated in code,
so when you change the default, you'd
now get a different value than from code generated from an older version of
the schema. There are situations, however, where this may be
desirable, especially if you can ensure a simultaneous rebuild of
@@ -425,4 +426,28 @@ incompatible with the actual binary buffers, since those only ever address
fields by id/offset.
<br>
### Testing whether a field is present in a table
Most serialization formats (e.g. JSON or Protocol Buffers) make it very
explicit in the format whether a field is present in an object or not,
allowing you to use this as "extra" information.
In FlatBuffers, this also holds for everything except scalar values.
FlatBuffers by default will not write fields that are equal to the default
value (for scalars), sometimes resulting in a significant space savings.
However, this also means testing whether a field is "present" is somewhat
meaningless, since it does not tell you if the field was actually written by
calling `add_field` style calls, unless you're only interested in this
information for non-default values.
Some `FlatBufferBuilder` implementations have an option called `force_defaults`
that circumvents this behavior, and writes fields even if they are equal to
the default. You can then use `IsFieldPresent` to query this.
Another option that works in all languages is to wrap a scalar field in a
struct. This way it will return null if it is not present. The cool thing
is that structs don't take up any more space than the scalar they represent.
[Interface Definition Language]: https://en.wikipedia.org/wiki/Interface_description_language