Next: Association Lists, Previous: Writing Methods, Up: Top

## 7 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.