Accessing Slots (ANSI and GNU Common Lisp Document)

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#### 7.5.2 Accessing Slots

Slots can be accessed in two ways: by use of the primitive function slot-value and by use of generic functions generated by the defclass form.

The function slot-value can be used with any of the slot names specified in the defclass form to access a specific slot accessible in an instance of the given class.

The macro defclass provides syntax for generating methods to read and write slots. If a reader method is requested, a method is automatically generated for reading the value of the slot, but no method for storing a value into it is generated. If a writer method is requested, a method is automatically generated for storing a value into the slot, but no method for reading its value is generated. If an accessor method is requested, a method for reading the value of the slot and a method for storing a value into the slot are automatically generated. Reader and writer methods are implemented using slot-value.

When a reader or writer method is specified for a slot, the name of the generic function to which the generated method belongs is directly specified. If the name specified for the writer method is the symbol `name`, the name of the generic function for writing the slot is the symbol `name`, and the generic function takes two arguments: the new value and the instance, in that order. If the name specified for the accessor method is the symbol `name`, the name of the generic function for reading the slot is the symbol `name`, and the name of the generic function for writing the slot is the list `(setf name)`.

A generic function created or modified by supplying `:reader`, `:writer`, or `:accessor` slot options can be treated exactly as an ordinary generic function.

Note that slot-value can be used to read or write the value of a slot whether or not reader or writer methods exist for that slot. When slot-value is used, no reader or writer methods are invoked.

The macro with-slots can be used to establish a lexical environment in which specified slots are lexically available as if they were variables. The macro with-slots invokes the function slot-value to access the specified slots.

The macro with-accessors can be used to establish a lexical environment in which specified slots are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the specified slots.

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