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Structures with optional fields shall be encoded as JSON objects as shown in Table 33.

Table 33 – JSON Object Definition for a Structures with Optional Fields

Name	Description
EncodingMask	A bit mask indicating what fields are encoded in the structure (see 5.2.7) This mask is encoded as a JSON number. The bits are sequentially assigned to optional fields in the order that they are defined.
<FieldName>	The fields structure encoded according to the rules defined for their DataType.

For the non-reversible form, Structures with optional fields are encoded like Structures.

If a Structure with optional fields is subtyped, the subtypes extend the EncodingMask (e) defined for the the parent.

The following is an example of a structure with optional fields using C++ syntax:

struct TypeA

{

Int32 X;

Int32* O1;

SByte Y;

Int32* O2;

};

O1 and O2 are optional fields where a NULL indicates that the field is not present.

Assume that O1 is not specified and the value of O2 is 0.

The reversible encoding would be:

{ "EncodingMask": 2 "X": 1, "Y": 2 }

Where decoders would assign the default value of 0 to O2 since the mask bit is set, however, the field was omitted (this is the behaviour defined for the Int32 DataType). Decoders would mark O1 as ‘not specified’.