DDSXTY_ — Introduce the notional value "null" that may be used in a content-filter expression

The specification introduces the concept of optional members into DDS data types. However, it does not address how comparisons of these members should work in content-filtering expressions. For example, if a content-filtered topic indicates that all samples are of interest where "x != 5", but x is not present in the sample at all, does the sample pass the filter or not?

Proposed resolution: Introduce the notional value "null" that may be used in a content-filter expression. The null value is not equal to any value in the domain of any data type. (That is, x != null for any non-null value of x.) A member's value in a given sample shall be considered to be equal to null if that member is not present in the given sample. Its value shall not be considered equal to null if it is present.

Optional members:
Introduce the notional value “null” that may be used in a content-filter expression. The null value is not equal to any value in the domain of any data type. (That is, x != null for any non-null value of x.)
• An optional member’s value in a given sample shall be considered to be equal to null if that member is not present in the given sample.
• Its value shall not be considered equal to null if it is present.
• A non-optional member’s value shall never be considered equal to null.
Maps:
Use the same syntax as arrays and sequences, but instead of a numeric index, use the stringified representation of the key to be queried as a string literal. Such an expression evaluates to the “value” element of the map that is associated with the given key. For example:
my_map['foo'] = 'bar'
my_map['5'] = 'baz'
Bit sets:
Use the same syntax as arrays and sequences, but instead of a numeric index, use the enumerated constant identifying the desired bit. Such an expression evaluates to a Boolean value: true if the named bit is set or false if it is not. For example:
my_bitset['MY_BIT_CONSTANT']
Specify the above rules in a new section 7.6.5, “DCPS Queries and Filters”. Include the relevant updates to the DCPS Queries and Filters grammar (from Annex A in the DDS 1.2 specification).
In addition, include a description of the following things that should be described in DDS already but seem to be missing:
• The use square bracket syntax for arrays, strings, and sequences
• Boolean literals
An issue should be filed against DDS for this, but the resolution of this issue should not wait for it to be acted upon.