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8 Predicates and Utilities

Now that we know how to create classes, access slots, and define methods, it might be useful to verify that everything is doing ok. To help with this a plethora of predicates have been created.

— Function: find-class symbol &optional errorp

Return the class that symbol represents. If there is no class, nil is returned if errorp is nil. If errorp is non-nil, wrong-argument-type is signaled.

— Function: class-p class

Return t if class is a valid class vector. class is a symbol.

— Function: slot-exists-p object-or-class slot

Non-nil if object-or-class has slot.

— Function: slot-boundp object slot

Non-nil if OBJECT's slot is bound. Setting a slot's value makes it bound. Calling slot-makeunbound will make a slot unbound. object can be an instance or a class.

— Function: class-name class

Return a string of the form ‘#<class myclassname>’ which should look similar to other Lisp objects like buffers and processes. Printing a class results only in a symbol.

— Function: class-option class option

Return the value in CLASS of a given OPTION. For example:

          (class-option eieio-default-superclass :documentation)

Will fetch the documentation string for eieio-default-superclass.

— Function: class-constructor class

Return a symbol used as a constructor for class. The constructor is a function used to create new instances of CLASS. This function provides a way to make an object of a class without knowing what it is. This is not a part of CLOS.

— Function: object-name obj

Return a string of the form ‘#<object-class myobjname>’ for obj. This should look like Lisp symbols from other parts of Emacs such as buffers and processes, and is shorter and cleaner than printing the object's vector. It is more useful to use object-print to get and object's print form, as this allows the object to add extra display information into the symbol.

— Function: object-class obj

Returns the class symbol from obj.

— Function: class-of obj

CLOS symbol which does the same thing as object-class

— Function: object-class-fast obj

Same as object-class except this is a macro, and no type-checking is performed.

— Function: object-class-name obj

Returns the symbol of obj's class.

— Function: class-parents class

Returns the direct parents class of class. Returns nil if it is a superclass.

— Function: class-parents-fast class

Just like class-parent except it is a macro and no type checking is performed.

— Function: class-parent class

Deprecated function which returns the first parent of class.

— Function: class-children class

Return the list of classes inheriting from class.

— Function: class-children-fast class

Just like class-children, but with no checks.

— Function: same-class-p obj class

Returns t if obj's class is the same as class.

— Function: same-class-fast-p obj class

Same as same-class-p except this is a macro and no type checking is performed.

— Function: object-of-class-p obj class

Returns t if obj inherits anything from class. This is different from same-class-p because it checks for inheritance.

— Function: child-of-class-p child class

Returns t if child is a subclass of class.

— Function: generic-p method-symbol

Returns t if method-symbol is a generic function, as opposed to a regular Emacs Lisp function.