An initialization argument controls object creation and
initialization. It is often convenient to use keyword symbols
to name initialization arguments, but the name of an
initialization argument can be any symbol, including nil. An
initialization argument can be used in two ways: to fill a slot
with a value or to provide an argument for an initialization
method. A single initialization argument can be used for both
purposes.
An initialization argument list is a
property list of
initialization argument names and values.
Its structure is identical
to a property list and also
to the portion of an argument list
processed for &key parameters.
As in those lists,
if an initialization
argument name appears more than once in an initialization argument list,
the leftmost occurrence supplies the value and the remaining occurrences
are ignored. The arguments to make-instance (after the first
argument) form an initialization argument list.
An initialization argument can be associated with a slot. If the initialization argument has a value in the initialization argument list, the value is stored into the slot of the newly created object, overriding any :initform form associated with the slot. A single initialization argument can initialize more than one slot. An initialization argument that initializes a shared slot stores its value into the shared slot, replacing any previous value.
An initialization argument can be associated with a method. When
an object is created and a particular initialization argument is
supplied, the generic functions initialize-instance,
shared-initialize, and allocate-instance are called
with that initialization argument's name and value as a keyword argument
pair. If a value for the initialization argument is not supplied in the
initialization argument list, the method's
lambda list supplies a default value.
Initialization arguments are used in four situations: when making an instance, when re-initializing an instance, when updating an instance to conform to a redefined class, and when updating an instance to conform to the definition of a different class.
Because initialization arguments are used to control the creation and initialization of an instance of some particular class, we say that an initialization argument is “an initialization argument for” that class.