5 Data encoding ToC Previous Next

5.4 OPC UA JSON ToC Previous Next

5.4.7 Structures with optional fields ToC Previous Next

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’.

Previous Next