slot-missing class object slot-name operation &optional new-value ⇒ {result}*
| class | the class of object. |
| object | an object. |
| slot-name | a symbol (the name of a would-be slot). |
| operation | one of the symbols setf, or slot-value. |
| new-value | an object. |
| result | an object. |
The generic function slot-missing is invoked when an attempt is made to access a slot in an object whose metaclass is standard-class and the slot of the name slot-name is not a name of a slot in that class. The default method signals an error.
The generic function slot-missing is not intended to be called by programmers. Programmers may write methods for it.
The generic function slot-missing may be called during
evaluation of slot-value, (setf slot-value),
slot-boundp, and slot-makunbound. For each
of these operations the corresponding symbol
for the operation
argument is slot-value, setf, slot-boundp,
and slot-makunbound respectively.
The optional new-value argument to slot-missing is used when the operation is attempting to set the value of the slot.
If slot-missing returns, its values will be treated as follows:
If the operation is setf or slot-makunbound, any values will be ignored by the caller.
If the operation is slot-value, only the primary value will be used by the caller, and all other values will be ignored.
If the operation is slot-boundp, any boolean equivalent of the primary value of the method might be is used, and all other values will be ignored.
The default method on slot-missing signals an error of type error.
The set of arguments (including the class of the instance) facilitates defining methods on the metaclass for slot-missing.