#### 4.3.1.1 Standard Metaclasses ¶

The object system provides a number of predefined metaclasses. These include the classes standard-class, built-in-class, and structure-class:

`*`

  The class standard-class is the default class of classes defined by defclass.

`*`

  The class built-in-class is the class whose instances are classes that have special implementations with restricted capabilities. Any class that corresponds to a standard type might be an instance of built-in-class. The predefined type specifiers that are required to have corresponding classes are listed in Figure~4–8. It is implementation-dependent whether each of these classes is implemented as a built-in class.

`*`

  All classes defined by means of defstruct are instances of the class structure-class.