The “Null” Sentinel
Sometimes it is convenient to have an
RWDateTime that is valid yet holds no specific value. The Essential Tools Module includes the “null” state sentinel for this purpose. You can construct a null
RWDateTime or use the constant
RWDateTime::nullSentinel in construction by passing the constant
RWDateTime::nullSentinel to the
RWDateTime(rwint64) constructor. You can also use the constant in direct comparisons with the millisecond count returned by
RWDateTime::milliSeconds().
Comparison operations for equality and inequality do work, with an
RWDateTime in a “null” state always being less than a valid
RWDateTime. Arithmetic manipulation of a null
RWDateTime results in an invalid
RWDateTime. Attempts to use any part extraction functions (
day(),
month(),
extract(), etc.) result in an
RWInternalErr being thrown. The
asString() family of functions always return the string
NULL when called for a null
RWDateTime.
The following examples show how to construct an
RWDateTime with a null state, and demonstrate what happens when it is used in an arithmetic operation:
// construct dt1 with null state:
RWDateTime dt1(RWDateTime::null);
// construct dt2 with null state
RWDateTime dt2(RWDateTime::nullSentinel);
// attempted arithmetic manipulation:
assert(dt2.isValid());
dt2.incrementMillisecond(23); // dt2 set to invalid state
assert(dt2.isValid()); // invalid
