UA Part 6: Mappings - 5.2.6 Structures

5.2.6 Structures

Structures are encoded as a sequence of fields in the order that they appear in the definition. The encoding for each field is determined by the built-in type for the field.

All fields specified in the structure shall be encoded. If optional fields exist in the Structure, then see 5.2.7.

Structures do not have a null value. If an encoder is written in a programming language that allows structures to have null values, then the encoder shall create a new instance with DefaultValues for all fields and serialize that. Encoders shall not generate an encoding error in this situation.

The following is an example of a structure using C/C++ syntax:

struct Type2

{

Int32 A;

Int32 B;

};

struct Type1

{

Int32 X;

Byte NoOfY;

Type2* Y;

Int32 Z;

UInt16 W[10];

Byte M[2,3,4];

};

In the C/C++ example above, the Y field is a pointer to an array with a length stored in NoOfY. When encoding an array, the length is part of the array encoding so the NoOfY field is not encoded. That said, encoders and decoders use NoOfY during encoding. W is a fixed length array with an implicitly defined length of 10. This length is always encoded with the array. M is a fixed length multidimensional array. The length of each dimension is always encoded with the array.

An instance of Type1 which contains an array of two Type2 instances would be encoded as 28-byte sequence. If the instance of Type1 was encoded in an ExtensionObject it would have an additional prefix shown in Table 23 which would make the total length 101 bytes The TypeId, Encoding and the Length are fields defined by the ExtensionObject. The encoding of the Type2 instances do not include any type identifier because it is explicitly defined in Type1.

Table 23 – Sample OPC UA Binary Encoded structure

Field	Bytes	Value
Type Id	4	The identifier for the ‘Type1’ Binary Encoding Node
Encoding	1	0x1 for ByteString
Length	4	28
X	4	The value of field ‘X’
Y.Length	4	2
Y.A	4	The value of field ‘Y[0].A’
Y.B	4	The value of field ‘Y[0].B’
Y.A	4	The value of field ‘Y[1].A’
Y.B	4	The value of field ‘Y[1].B’
Z	4	The value of field ‘Z’
W.Length	4	10
W	20	The value of field ‘W’.
M.Dimensions.Length	4	3
M.Dimensions	12	The sequence [2,3,4] encoded as a Int32.
M.Values	24	The values in ‘M’ encoded sequentially as described in 5.2.2.16.

The Value of the DataTypeDefinition Attribute for a DataType Node describing Type1 is shown in Table 24.

Table 24 – DataTypeDefinition for “Type1” from Sample

Name	Type	Description
defaultEncodingId	NodeId	NodeId of the “Type1_Encoding_DefaultBinary” Node.
baseDataType	NodeId	“i=22” [Structure]
structureType	StructureType	Structure_0 [Structure without optional fields]
fields [0]	StructureField
name	String	“X”
description	LocalizedText	Description of X
dataType	NodeId	“i=6” [Int32]
valueRank	Int32	-1 (Scalar)
arrayDimensions	UInt32[]	null
maxStringLength	UInt32	0
isOptional	Boolean	false
fields [1]	StructureField
name	String	“Y“
description	LocalizedText	Description of Y-Array
dataType	NodeId	NodeId of the Type2 DataType Node (e.g. “ns=3; s=MyType2”)
valueRank	Int32	1 (OneDimension)
arrayDimensions	UInt32[]	{ 0 }
maxStringLength	UInt32	0
isOptional	Boolean	false
fields [2]	StructureField
name	String	“Z“
description	LocalizedText	Description of Z
dataType	NodeId	“i=6” [Int32]
valueRank	Int32	-1 (Scalar)
arrayDimensions	UInt32[]	null
maxStringLength	UInt32	0
isOptional	Boolean	false
fields [3]	StructureField
name	String	“W“
description	LocalizedText	Description of W
dataType	NodeId	“i=5” [UInt16]
valueRank	Int32	1 (OneDimension)
arrayDimensions	UInt32[]	{ 10 }
maxStringLength	UInt32	0
isOptional	Boolean	false
fields [4]	StructureField
name	String	“M“
description	LocalizedText	Description of M
dataType	NodeId	“i=3” [Byte]
valueRank	Int32	3
arrayDimensions	UInt32[]	{ 2, 3, 4 }
maxStringLength	UInt32	0
isOptional	Boolean	false

The Value of the DataTypeDefinition Attribute for a DataType Node describing Type2 is shown in Table 25.

Table 25 –DataTypeDefinition for “Type2” from Sample

Name	Type	Description
defaultEncodingId	NodeId	NodeId of the “Type2_Encoding_DefaultBinary” Node.
baseDataType	NodeId	“i=22” [Structure]
structureType	StructureType	Structure_0 [Structure without optional fields]
fields [0]	StructureField
name	String	“A“
description	LocalizedText	Description of A
dataType	NodeId	“i=6” [Int32]
valueRank	Int32	-1 (Scalar)
arrayDimensions	UInt32[]	null
maxStringLength	UInt32	0
isOptional	Boolean	false
fields [1]	StructureField
name	String	“B“
description	LocalizedText	Description of B
dataType	NodeId	“i=6” [Int32]
valueRank	Int32	-1 (Scalar)
arrayDimensions	UInt32[]	null
maxStringLength	UInt32	0
isOptional	Boolean	false