Common Lisp the Language, 2nd Edition

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### 28.1.8.2. Standard Metaclasses

The Common Lisp 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 Common Lisp type might be an instance of `built-in-class`. The predefined Common Lisp type specifiers that are required to have corresponding classes are listed in table 28-1. 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 `structure-class`.