The generic function make-instance creates and returns a new
instance of a class. The first argument is a class or
the name of a class, and the remaining arguments form an
initialization argument list.
The initialization of a new instance consists of several distinct
steps, including the following: combining the explicitly supplied initialization
arguments with default values for the unsupplied initialization arguments,
checking the validity of the initialization arguments, allocating storage
for the instance, filling slots with
values, and executing user-supplied methods that perform additional
initialization. Each step of make-instance is implemented by a
generic function to provide a mechanism for customizing that step.
In addition, make-instance is itself a generic function
and thus also can be customized.
The object system specifies system-supplied primary methods for each step and thus specifies a well-defined standard behavior for the entire initialization process. The standard behavior provides four simple mechanisms for controlling initialization:
defclass. This provides a mechanism
for supplying a value for a slot in a call to make-instance.
defclass. If an
initialization argument is not explicitly provided
as an argument to make-instance, the default value form is
evaluated in the lexical environment of the defclass form that
defined it, and the resulting value is used as the value of the
initialization argument.
defclass. If no initialization
argument associated with that slot is given as an argument to
make-instance or is defaulted by :default-initargs, this
default initial value form is evaluated in the lexical environment of
the defclass form that defined it, and the resulting value is
stored in the slot. The :initform form for a
local slot may be used when creating an instance, when
updating an instance to conform to a redefined class,
or when updating an instance to conform to the definition of a
different class. The :initform form for a
shared slot may be used when defining or re-defining the class.
initialize-instance and
shared-initialize. The slot-filling behavior described above is
implemented by a system-supplied primary method for
initialize-instance which invokes shared-initialize. The
generic function shared-initialize implements the parts of
initialization shared by these 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. The system-supplied
primary method for shared-initialize directly implements the
slot-filling behavior described above, and initialize-instance
simply invokes shared-initialize.