17.5.14 Optional Fields

Sometimes a field in a struct is optional, i.e. it exists or not depending on some criteria. A good example of this is the “extended header” in a MP3 id3v2 tag. From the specification:

The second bit (bit 6) indicates whether or not the header is followed
by an extended header. The extended header is described in section
3.2. A set bit indicates the presence of an extended header.

In order to express this in a Poke struct type, we could of course use an union, like:

type ID3V2_Tag =
  struct
  {
    ID3V2_Hdr hdr;
    union
    {
      ID3V2_Ext_Hdr hdr if hdr.extended_hdr_present;
      struct {};
    } ext;
    …
  }

That’s it, we use an union with an empty alternative. However, this is a bit cumbersome. Therefore Poke provides a more convenient way to denote this:

type ID3V2_Tag =
  struct
  {
     ID3V2_Hdr hdr;
     IDV2_Ext_Hdr ext_hdr if hdr.extended_hdr_present;
     …
  }

If both a constraint and an optional field expression are specified, the second should follow the first.

For obvious reasons, optional fields are not allowed in unions.