7.1. Accessing the data

The types you use to write or read data may include nested structs, sequences and arrays of primitive types or structs, etc.

These types are defined in XML following the format of RTI’s XML-Based Application Creation feature.

To access the data, Instance() and SampleIterator() provide setters and getters that expect a fieldName string, used to identify specific fields within the type. This section describes the format of this string.

We will use the following XML type definition of MyType:

<types>
    <enum name="Color">
        <enumerator name="RED"/>
        <enumerator name="GREEN"/>
        <enumerator name="BLUE"/>
    </enum>
    <struct name= "Point">
        <member name="x" type="int32"/>
        <member name="y" type="int32"/>
    </struct>
    <union name="MyUnion">
        <discriminator type="nonBasic" nonBasicTypeName="Color"/>
        <case>
          <caseDiscriminator value="RED"/>
          <member name="point" type="nonBasic"  nonBasicTypeName= "Point"/>
        </case>
        <case>
          <caseDiscriminator value="GREEN"/>
          <member name="my_long" type="int32"/>
        </case>
    </union>
    <struct name= "MyType">
        <member name="my_long" type="int32"/>
        <member name="my_double" type="float64"/>
        <member name="my_enum" type="nonBasic"  nonBasicTypeName= "Color" default="GREEN"/>
        <member name="my_boolean" type="boolean" />
        <member name="my_point" type="nonBasic"  nonBasicTypeName= "Point"/>
        <member name="my_union" type="nonBasic"  nonBasicTypeName= "MyUnion"/>
        <member name="my_int_sequence" sequenceMaxLength="10" type="int32"/>
        <member name="my_point_sequence" sequenceMaxLength="10" type="nonBasic"  nonBasicTypeName= "Point"/>
        <member name="my_point_array" type="nonBasic"  nonBasicTypeName= "Point" arrayDimensions="3"/>
        <member name="my_optional_point" type="nonBasic"  nonBasicTypeName= "Point" optional="true"/>
        <member name="my_optional_long" type="int32" optional="true"/>
    </struct>
</types>

Which corresponds to the following IDL definition:

enum Color {
    RED,
    GREEN,
    BLUE
};

struct Point {
    long x;
    long y;
};

union MyUnion switch(Color) {
    case RED: Point point;
    case GREEN: string<512> my_string;
};

struct MyType {
    long my_long;
    double my_double;
    Color my_enum;
    boolean my_boolean;
    string<512> my_string;
    Point my_point;
    MyUnion my_union;
    sequence<long, 10> my_int_sequence;
    sequence<Point, 10> my_point_sequence;
    Point my_point_array[3];
    @optional Point my_optional_point;
    @optional long my_optional_long;
};

Hint

You can get the XML definition of an IDL file with rtiddsgen -convertToXml MyType.idl.

We will refer to an Output named output and Input named input such that input.samples.length > 0.

7.1.1. Using JSON objects vs accessing individual members

On an Input or an Output, you can access the data all at once by using a JSON object, or member-by-member. Using a JSON object is usually more efficient if you intend to access most or all of the data members of a large type.

On an Output, Instance.setFromJson() receives a JSON object with all, or some, of the Output type members, and in an Input, SampleIterator.getJson() retrieves all of the members.

It is also possible to provide a memberName to SampleIterator.getJson() to obtain a JSON object containing the fields of that nested member only.

On the other hand, the methods described in the following section receive a fieldName argument to get or set a specific member.

7.1.2. Accessing basic members (numbers, strings and booleans)

To set a field in an Output(), use the appropriate setter.

To set any numeric type, including enumerations:

output.instance.setNumber('my_long', 2)
output.instance.setNumber('my_double', 2.14)
output.instance.setNumber('my_enum', 2)

Warning

The range of values for a numeric field is determined by the type used to define that field in the configuration file. However, setNumber and getNumber can’t handle 64-bit integers (int64 and uint64) whose absolute values are larger than 2^53. This is a Connector limitation due to the use of double as an intermediate representation.

When setNumber or getNumber detect this situation, they will raise an DDSError(). getJson and setJson do not have this limitation and can handle any 64-bit integer.

To set booleans:

output.instance.setBoolean('my_boolean', True)

To set strings:

output.instance.setString('my_string', 'Hello, World!')

As an alternative to the previous setters, the type-independent method set can be used as follows:

// The set method works on all basic types
output.instance.set('my_double', 2.14)
output.instance.set('my_boolean', true)
output.instance.set('my_string', 'Hello, World!')

In all cases, the type of the assigned value must be consistent with the type of the field, as defined in the configuration file.

Similarly, to get a field in a Input() sample, use the appropriate getter: SampleIterator.getNumber(), SampleIterator.getBoolean(), SampleIterator.getString(), or the type-independent SampleIterator.get(). getString also works with numeric fields, returning the number as a string:

for (const sample of input.samples.validDataIter) {
    // Use the basic type specific getters
    let value = sample.getNumber('my_double')
    value = sample.getBoolean('my_boolean')
    value = sample.getString('my_string')

    // or alternatively, use the type-independent get method
    value = sample.get('my_double')
    value = sample.get('my_boolean')
    value = sample.get('my_string')

    // get a number as string:
    value = sample.getString('my_double')
}

Note

The typed getters and setters perform better than set and get in applications that write or read at high rates. Also prefer getJson and setFromJson over set and get when accessing all or most of the fields of a sample (see previous section).

Note

If a field my_string, defined as a string in the configuration file, contains a value that can be interpreted as a number, sample.get('my_string') returns a number, not a string.

7.1.3. Accessing structs

To access a nested member, use . to identify the fully-qualified fieldName and pass it to the corresponding setter or getter.

output.instance.setNumber('my_point.x', 10)
output.instance.setNumber('my_point.y', 20)

// alternatively:
output.instance.set('my_point.x', 10)
output.instance.set('my_point.y', 20)

It is possible to reset the value of a complex member back to its default:

output.instance.clearMember('my_point') // x and y are now 0

It is also possible to reset members using the set method:

output.instance.set('my_point', null)

Structs are set via JSON objects as follows:

output.instance.setFromJson({ 'my_point': { 'x':10, 'y':20 } })

When an member of a struct is not set, it retains its previous value. If we run the following code after the previous call to setFromJson:

output.instance.setFromJson({ 'my_point': { 'y': 200 } })

The value of my_point is now { 'x': 10, 'y':200 }. If you do not want the values to be retained you must clear the value first (as described above).

It is possible to obtain the JSON object of a nested struct:

for (const sample of input.samples.validDataIter) {
   let point = sample.getJson('my_point')
}

memberName must be one of the following types: array, sequence, struct, value or union. If not, the call to getJson will fail:

 for (let sample of input.samples.validDataIter) {
    try {
       let long = sample.getJson('my_long')
    } catch (err) {
       // Error was thrown since my_long is a basic type
    }
}

It is also possible to obtain the JSON of a struct using the SampleIterator.get() method:

 for (const sample of input.samples.validDataIter) {
     let point = sample.get('my_point')
     // point is a JSON object
}

The same limitations described in Accessing basic members (numbers, strings and booleans) of using SampleIterator.get() apply here.

7.1.4. Accessing arrays and sequences

Use fieldName[index] to access an element of a sequence or array, where 0 <= index < length:

let value = input.samples.get(0).getNumber('my_int_sequence[1]')
value = input.samples.get(0).getNumber('my_point_sequence[2].y')

To obtain the length of a sequence in an Input() sample, append # to the fieldName:

let length = input.samples[0].getNumber('my_int_sequence#')

Another option is to use SampleIterator.getJson('fieldName') to obtain a JSON object containing all of the elements of the array or sequence with name fieldName:

for (let sample of input.samples.validDataIter) {
    let thePointSequence = sample.getJson('my_point_sequence')
}

You can also get a specific element as a dictionary (if the element type is complex):

for (let sample of input.samples.validDataIter) {
   let pointElement = sample.getJson('my_point_sequence[1]')
}

In an Output(), sequences are automatically resized:

output.instance.setNumber('my_int_sequence[5]', 10) // length is now 6
output.instance.setNumber('my_int_sequence[4]', 9) // length still 6

You can clear a sequence:

output.instance.clearMember('my_int_sequence') // my_int_sequence is now empty

In JSON objects, sequences and arrays are represented as lists. For example:

output.instance.setFromJson({
    my_int_sequence: [1, 2],
    my_point_sequence: [{ x: 1, y: 1 }, { x: 2, y: 2 }]
})

Arrays have a constant length that can’t be changed. When you don’t set all the elements of an array, the remaining elements retain their previous values. However, sequences are always overwritten. See the following example:

output.instance.setFromJson({
    my_point_sequence: [{ x: 1, y: 1 }, { x: 2, y: 2 }],
    my_point_array: [{ x: 1, y: 1 }, { x: 2, y: 2 }, { x: 3, y: 3 }] })

output.instance.setFromJson({
    my_point_sequence: [{ x: 100 }],
    my_point_array: [{ x: 100}, { y: 200}] })

After the second call to setFromJson, the contents of my_point_sequence are [{ x: 100, y: 0 }], but the contents of my_point_array are: [{ x: 100, y: 1 }, { x: 2, y: 200 }, {x: 3, y: 3 }].

7.1.5. Accessing optional members

A optional member is a member that applications can decide to send or not as part of every published sample. Therefore, optional members may have a value or not. They are accessed the same way as non-optional members, except that null is a possible value.

On an Input, any of the getters may return null if the field is optional:

if (input.samples.get(0).getNumber('my_optional_long') == null) {
    console.log('my_optional_long not set')
}

if (input.samples.get(0).getNumber('my_optional_point.x') == null) {
    console.log('my_optional_point not set')
}

SampleIterator.getJson() returns a JSON object that doesn’t include unset optional members.

To set an optional member on an Output:

output.instance.setNumber('my_optional_long', 10)

If the type of the optional member is not primitive, when any of its members is first set, the rest are initialized to their default values:

output.instance.setNumber('my_optional_point.x', 10)

If my_optional_point was not previously set, the previous code also sets y to 0.

There are several ways to reset an optional member. If the type is primitive:

output.instance.setNumber('my_optional_long', null) // Option 1
output.instance.clearMember('my_optional_long') // Option 2
output.instance.set('my_optional_long', null) // Option 3

If the member type is complex, all the above options except option 1 are available:

output.instance.clearMember('my_optional_point')
output.instance.set('my_optional_point', null)

Note that Instance.setFromJson() doesn’t clear those members that are not specified; their values remain. For example:

output.instance.setNumber('my_optional_long', 5)
output.instance.setFromJson({ my_double: 3.3, my_long: 4 })
// my_optional_long is still 5

To clear a member, set it to null explicitly:

output.instance.setFromJson({ my_double: 3.3, my_long: 4, my_optional_long: null })

For more information about optional members in DDS, see the Getting Started Guide Addendum for Extensible Types, Section 3.2 Optional Members.

7.1.6. Accessing unions

In an Output(), the union member is automatically selected when you set it:

output.instance.setNumber('my_union.point.x', 10)

You can change it later:

output.instance.setNumber('my_union.my_long', 10)

In an Input(), you can obtain the selected member as a string:

if (input.samples.get(0).getString('my_union#') == 'point') {
    value = input.samples.get(0).getNumber('my_union.point.x')
}