describe-object (Standard Generic Function)

Syntax:

Method Signatures:

describe-object (object standard-object) stream

Arguments and Values:

object—an object.

stream—a stream.

Description:

The generic function describe-object prints a description of object to a stream. describe-object is called by describe; it must not be called by the user.

Each implementation is required to provide a method on the class standard-object and methods on enough other classes so as to ensure that there is always an applicable method. Implementations are free to add methods for other classes. Users can write methods for describe-object for their own classes if they do not wish to inherit an implementation-supplied method.

Methods on describe-object can recursively call describe. Indentation, depth limits, and circularity detection are all taken care of automatically, provided that each method handles exactly one level of structure and calls describe recursively if there are more structural levels. The consequences are undefined if this rule is not obeyed.

In some implementations the stream argument passed to a describe-object method is not the original stream, but is an intermediate stream that implements parts of describe. Methods should therefore not depend on the identity of this stream.

Examples:

 (defclass spaceship ()
   ((captain :initarg :captain :accessor spaceship-captain)
    (serial# :initarg :serial-number :accessor spaceship-serial-number)))

 (defclass federation-starship (spaceship) ())

 (defmethod describe-object ((s spaceship) stream)
   (with-slots (captain serial#) s
     (format stream "~&~S is a spaceship of type ~S,~
                     ~%with ~A at the helm ~
                       and with serial number ~D.~%"
             s (type-of s) captain serial#)))

 (make-instance 'federation-starship
                :captain "Rachel Garrett"
                :serial-number "NCC-1701-C")
→ #<FEDERATION-STARSHIP 26312465>

 (describe *)
▷ #<FEDERATION-STARSHIP 26312465> is a spaceship of type FEDERATION-STARSHIP,
▷ with Rachel Garrett at the helm and with serial number NCC-1701-C.
→ <no values>

See Also:

describe

Notes:

The same implementation techniques that are applicable to print-object are applicable to describe-object.

The reason for making the return values for describe-object unspecified is to avoid forcing users to include explicit (values) in all of their methods. describe takes care of that.