#### 7.7.5 shared-initialize [Standard Generic Function] ¶

#### Syntax:: ¶

`shared-initialize` instance slot-names &rest initargs &key &allow-other-keys ⇒ instance

#### Method Signatures:: ¶

`shared-initialize` (instance standard-object) slot-names &rest initargs

#### Arguments and Values:: ¶

instance—an object.

slot-names—a list or t.

initargs—a list of keyword/value pairs (of initialization argument names and values).

#### Description:: ¶

The generic function shared-initialize is used to fill the slots of an instance using initargs and `:initform` forms. It is called 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. The generic function shared-initialize is called by the system-supplied primary method for initialize-instance, reinitialize-instance, update-instance-for-redefined-class, and update-instance-for-different-class.

The generic function shared-initialize takes the following arguments: the instance to be initialized, a specification of a set of slot-names accessible in that instance, and any number of initargs. The arguments after the first two must form an initialization argument list. The system-supplied primary method on shared-initialize initializes the slots with values according to the initargs and supplied `:initform` forms. Slot-names indicates which slots should be initialized according to their `:initform` forms if no initargs are provided for those slots.

The system-supplied primary method behaves as follows, regardless of whether the slots are local or shared:

`*`

  If an initarg 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.

`*`

  Any slots indicated by slot-names 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.

`*`

  The rules mentioned in Rules for Initialization Arguments are obeyed.

The slots-names argument specifies the slots that are to be initialized according to their `:initform` forms if no initialization arguments apply. It can be a list of slot names, which specifies the set of those slot names; or it can be the symbol t, which specifies the set of all of the slots.

#### See Also:: ¶

Initialize-Instance , reinitialize-instance [Standard Generic Function] , update-instance-for-redefined-class [Standard Generic Function] , update-instance-for-different-class [Standard Generic Function] , slot-boundp [Function] , slot-makunbound [Function] , Object Creation and Initialization, Rules for Initialization Arguments, Declaring the Validity of Initialization Arguments

#### Notes:: ¶

Initargs are declared as valid by using the `:initarg` option to defclass, or by defining methods for shared-initialize. The keyword name of each keyword parameter specifier in the lambda list of any method defined on shared-initialize is declared as a valid initarg name for all classes for which that method is applicable.

Implementations are permitted to optimize `:initform` forms that neither produce nor depend on side effects, by evaluating these forms and storing them into slots before running any initialize-instance methods, rather than by handling them in the primary initialize-instance method. (This optimization might be implemented by having the allocate-instance method copy a prototype instance.)

Implementations are permitted to optimize default initial value forms for initargs associated with slots by not actually creating the complete initialization argument list when the only method that would receive the complete list is the method on standard-object. In this case default initial value forms can be treated like `:initform` forms. This optimization has no visible effects other than a performance improvement.