CLHS: Section 7.1.7

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## 7.1.7 Definitions of Make-Instance and Initialize-Instance

The generic function make-instance behaves as if it were defined as follows, except that certain optimizations are permitted:

     (defmethod make-instance ((class standard-class) &rest initargs)
       ...
       (let ((instance (apply #'allocate-instance class initargs)))
         (apply #'initialize-instance instance initargs)
         instance))

     (defmethod make-instance ((class-name symbol) &rest initargs)
       (apply #'make-instance (find-class class-name) initargs))

The elided code in the definition of make-instance augments the `initargs` with any defaulted initialization arguments and checks the resulting initialization arguments to determine whether an initialization argument was supplied that neither filled a slot nor supplied an argument to an applicable method.

The generic function initialize-instance behaves as if it were defined as follows, except that certain optimizations are permitted:

     (defmethod initialize-instance ((instance standard-object) &rest initargs)
       (apply #'shared-initialize instance t initargs)))

These procedures can be customized.

Customizing at the Programmer Interface level includes using the `:initform`, `:initarg`, and `:default-initargs` options to defclass, as well as defining methods for make-instance, allocate-instance, and initialize-instance. It is also possible to define methods for shared-initialize, which would be invoked by the generic functions reinitialize-instance, update-instance-for-redefined-class, update-instance-for-different-class, and initialize-instance. The meta-object level supports additional customization.

Implementations are permitted to make certain optimizations to initialize-instance and shared-initialize. The description of shared-initialize in Chapter 7 mentions the possible optimizations.

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