Common Lisp the Language, 2nd Edition

28.1.9. Object Creation and Initialization

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:

• Declaring a symbol to be an initialization argument for a slot. An initialization argument is declared by using the :initarg slot option to defclass. This provides a mechanism for supplying a value for a slot in a call to make-instance.

• Supplying a default value form for an initialization argument. Default value forms for initialization arguments are defined by using the :default-initargs class option to 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.

• Supplying a default initial value form for a slot. A default initial value form for a slot is defined by using the :initform slot option to 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.

• Defining methods for 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.

• Initialization Arguments
• Declaring the Validity of Initialization Arguments
• Defaulting of Initialization Arguments
• Rules for Initialization Arguments
• Shared-Initialize
• Initialize-Instance
• Definitions of Make-Instance and Initialize-Instance