Motivation

This feature requests schema-specifiable defaults for all fields in tables. After this feature and #6014 are implemented, schema writers will have full control over the semantics of non-presence for all fields in tables :) Table fields may be optional (and non presence maps to None) or not (and non-presence maps to some default value).

As was the case with #6014, we should only do this if we have sufficient buy-in to get all major languages to add these features in a small time window, which means we won't get around to implementing this until after we're done with #6014. But there's no harm discussing this now.

High Level

Schema writers may specify:

• Empty defaults for tables and vectors, anything more complex seems challenging.
  • Default empty tables doesn't seem to add that much benefit and may break/complicate the object API if there are cycles
• Static strings for strings.
• Static variants for unions, but the default variant's table is empty.
  • We're not doing default empty tables.
• Static structs for structs. Parsing a struct in the schema will be complex, but doable.
  • Turns out parser support is kinda hard. We'll revisit this if there's interest

Generated readers must return the specified default if a field is non-present. Generated builders must leave the field non-present if asked to write a default value. The generated type must not be optional.

Example Schema:

https://github.com/google/flatbuffers/blob/master/tests/more_defaults.fbs

enum ABC: int { A, B, C }

table MoreDefaults {
  ints: [int] = [];
  floats: [float] = [     ];
  empty_string: string = "";
  some_string: string = "some";
  abcs: [ABC] = [];
  bools: [bool] = [];
}

When reading this table

• if a vector field is not present in binary, it must be read as an empty vector
• if a string field is not present in binary, it must be read as the default provided string
• when writing an empty vector or default string to binary, it should not be written at all (i.e. vtable offset set to 0)
• The accessor's return types must not be optionally typed.
  • E.g. in Rust the accessor fn ints(&self) -> &[i32] does not return an Option<&[i32]>
• the native object's field types must not be optionally typed
  • E.g. in Rust, the field some_string: String rather than Option<String>.

TODO