The data values in a deck item can be in three different states, given
by the DeckValue enum in DeckItem.hpp. The three values are:
SET_IN_DECK : The value has been set explictly in the deck.
DEFAULT : The value was not present in the input deck, but a default
value has been supplied in the configuration and that value
has been set.
NOT_SET : No value has been set for this item; it was not explicitly
set in the deck and also not included in the configuration.
If you ask for DeckItem->value which is in state NOT_SET you will get an
exception. The method setInDeck() can be used to check if a value has
been set explicitly in the deck; the method defaultApplied() will check
if a default value has been applied.
Observe that the system for handling defaults is not really well suited
for multi valued data items, as it is only a scalar state variable. In
the case of multi valued data items both defaultApplied() and
setInDeck() might return true.
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