CLHS: Section 7.1.5

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## 7.1.5 Shared-Initialize

The generic function shared-initialize is used to fill the slots of an instance using initialization arguments and `:initform` forms when an instance is created, when an instance is re-initialized, when an instance is updated to conform to a redefined class, and when an instance is updated to conform to a different class. It uses standard method combination. It takes the following arguments: the instance to be initialized, a specification of a set of names of slots accessible in that instance, and any number of initialization arguments. The arguments after the first two must form an initialization argument list.

The second argument to shared-initialize may be one of the following:

* It can be a (possibly empty) list of slot names, which specifies the set of those slot names.

* It can be the symbol t, which specifies the set of all of the slots.

There is a system-supplied primary method for shared-initialize whose first parameter specializer is the class standard-object. This method behaves as follows on each slot, whether shared or local:

* If an initialization argument in the initialization argument list specifies a value for that slot, that value is stored into the slot, even if a value has already been stored in the slot before the method is run. The affected slots are independent of which slots are indicated by the second argument to shared-initialize.

* Any slots indicated by the second argument that are still unbound at this point are initialized according to their `:initform` forms. For any such slot that has an `:initform` form, that form is evaluated in the lexical environment of its defining defclass form and the result is stored into the slot. For example, if a before method stores a value in the slot, the `:initform` form will not be used to supply a value for the slot. If the second argument specifies a name that does not correspond to any slots accessible in the instance, the results are unspecified.

* The rules mentioned in Section 7.1.4 (Rules for Initialization Arguments) are obeyed.

The generic function shared-initialize is called by the system-supplied primary methods for reinitialize-instance, update-instance-for-different-class, update-instance-for-redefined-class, and initialize-instance. Thus, methods can be written for shared-initialize to specify actions that should be taken in all of these contexts.

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