A default value form can be supplied for an initialization
argument by using the :default-initargs class option. If an
initialization argument is declared valid by some particular class,
its default value form might be specified by a different class.
In this case :default-initargs is used to supply a default value
for an inherited initialization argument.
The :default-initargs option is used only to provide default
values for initialization arguments; it does not declare a symbol
as a valid initialization argument name. Furthermore,
the :default-initargs option is used only to provide default values for
initialization arguments when making an instance.
The argument to the :default-initargs class
option is a list of
alternating initialization argument names and forms.
Each form is the
default value form for the corresponding initialization
argument. The default value form of an initialization
argument is used and evaluated only if that initialization argument
does not appear in the arguments to make-instance and is not
defaulted by a more specific class. The default value form is
evaluated in the lexical environment of the defclass form that
supplied it; the resulting value is used as the initialization
argument’s value.
The initialization arguments supplied to make-instance are combined with defaulted initialization arguments to produce a defaulted initialization argument list. A defaulted initialization argument list is a list of alternating initialization argument names and values in which unsupplied initialization arguments are defaulted and in which the explicitly supplied initialization arguments appear earlier in the list than the defaulted initialization arguments. Defaulted initialization arguments are ordered according to the order in the class precedence list of the classes that supplied the default values.
There is a distinction between the purposes of the
:default-initargs and the :initform options with respect to the
initialization of slots. The :default-initargs
class option
provides a mechanism for the user to give a default value form
for an initialization argument without knowing whether the
initialization argument initializes a slot
or is passed to a method.
If that initialization argument is not explicitly supplied in a call
to make-instance, the default value form is used, just
as if it had been supplied in the call. In contrast, the
:initform slot option provides a mechanism for the user to give a
default initial value form for a slot. An :initform form is
used to initialize a slot only if no initialization argument
associated with that slot is given as an argument to
make-instance or is defaulted by :default-initargs.
The order of evaluation of default value forms for initialization
arguments and the order of evaluation of :initform forms are
undefined. If the order of evaluation is important,
initialize-instance or shared-initialize methods
should be used
instead.