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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Section 4.3.1</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="04_c.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="04_c.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="04_caa.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
4.3.1 Introduction to Classes</H2> <P>
A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> that determines the structure and behavior of a set of other <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A>, which are called its <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A>. <P>
A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> can inherit structure and behavior from other <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> whose definition refers to other <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> for the purpose of inheriting from them is said to be a <A REL=DEFINITION HREF="26_glo_s.htm#subclass"><I>subclass</I></A> of each of those <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. The <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> that are designated for purposes of inheritance are said to be <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclasses</I></A> of the inheriting <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. <P>
A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> can have a <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A>. The <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_class_.htm#class-name"><B>class-name</B></A> takes a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> and returns its <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A>. The <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> of an anonymous <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. A <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> can <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. The <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_find_c.htm#find-class"><B>find-class</B></A> takes a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> and returns the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> that the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> names. A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> has a <A REL=DEFINITION HREF="26_glo_p.htm#proper_name"><I>proper name</I></A> if the <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> is a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> and if the <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> names that <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. That is, a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> C has the <A REL=DEFINITION HREF="26_glo_p.htm#proper_name"><I>proper name</I></A> S if S= <TT>(class-name C)</TT> and C= <TT>(find-class S)</TT>. Notice that it is possible for <TT>(find-class S</TT>1<TT>)</TT> = <TT>(find-class S</TT>2<TT>)</TT> and S1/=S2. If C= <TT>(find-class S)</TT>, we say that C is the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <I>named</I> S. <P>
A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> C1 is a <A REL=DEFINITION HREF="26_glo_d.htm#direct_superclass"><I>direct superclass</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> C2 if C2 explicitly designates C1 as a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> in its definition. In this case C2 is a <A REL=DEFINITION HREF="26_glo_d.htm#direct_subclass"><I>direct subclass</I></A> of C1. A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> Cn is a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> C1 if there exists a series of <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> C2,...,Cn-1 such that Ci+1 is a <A REL=DEFINITION HREF="26_glo_d.htm#direct_superclass"><I>direct superclass</I></A> of Ci for 1 &lt;=i&lt;n. In this case, C1 is a <A REL=DEFINITION HREF="26_glo_s.htm#subclass"><I>subclass</I></A> of Cn. A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is considered neither a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> nor a <A REL=DEFINITION HREF="26_glo_s.htm#subclass"><I>subclass</I></A> of itself. That is, if C1 is a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> of C2, then C1 /=C2. The set of <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> consisting of some given <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> C along with all of its <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclasses</I></A> is called ``C and its superclasses.'' <P>
Each <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> has a <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A>, which is a total ordering on the set of the given <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> and its <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclasses</I></A>. The total ordering is expressed as a list ordered from most specific to least specific. The <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> is used in several ways. In general, more specific <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> can <A REL=DEFINITION HREF="26_glo_s.htm#shadow"><I>shadow</I></A>[1] features that would otherwise be inherited from less specific <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. The <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> selection and combination process uses the <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> to order <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> from most specific to least specific. <P>
When a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is defined, the order in which its direct <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclasses</I></A> are mentioned in the defining form is important. Each <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> has a <A REL=DEFINITION HREF="26_glo_l.htm#local_precedence_order"><I>local precedence order</I></A>, which is a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> consisting of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> followed by its <A REL=DEFINITION HREF="26_glo_d.htm#direct_superclass"><I>direct superclasses</I></A> in the order mentioned in the defining <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
A <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> is always consistent with the <A REL=DEFINITION HREF="26_glo_l.htm#local_precedence_order"><I>local precedence order</I></A> of each <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> in the list. The <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> in each <A REL=DEFINITION HREF="26_glo_l.htm#local_precedence_order"><I>local precedence order</I></A> appear within the <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> in the same order. If the <A REL=DEFINITION HREF="26_glo_l.htm#local_precedence_order"><I>local precedence orders</I></A> are inconsistent with each other, no <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> can be constructed, and an error is signaled. The <A REL=DEFINITION HREF="26_glo_c.htm#class_precedence_list"><I>class precedence list</I></A> and its computation is discussed in <A REL=CHILD HREF="04_ce.htm">Section 4.3.5 (Determining the Class Precedence List)</A>. <P>
<A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> are organized into a directed acyclic graph. There are two distinguished <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>, named <A REL=DEFINITION HREF="t_t.htm#t"><B>t</B></A> and <A REL=DEFINITION HREF="t_std_ob.htm#standard-object"><B>standard-object</B></A>. The <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> named <A REL=DEFINITION HREF="t_t.htm#t"><B>t</B></A> has no <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclasses</I></A>. It is a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> of every <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> except itself. The <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> named <A REL=DEFINITION HREF="t_std_ob.htm#standard-object"><B>standard-object</B></A> is an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A> and is a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> of every <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> that is an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A> except itself. <P>
There is a mapping from the object system <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> space into the <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> space. Many of the standard <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>types</I></A> specified in this document have a corresponding <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> that has the same <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> as the <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A>. Some <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>types</I></A> do not have a corresponding <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. The integration of the <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> and <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> systems is discussed in <A REL=CHILD HREF="04_cg.htm">Section 4.3.7 (Integrating Types and Classes)</A>. <P>
<A REL=DEFINITION HREF="26_glo_c.htm#class"><I>Classes</I></A> are represented by <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> that are themselves <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. The <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> is termed the <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> of that <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. When no misinterpretation is possible, the term <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> is used to refer to a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> that has <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> that are themselves <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. The <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> determines the form of inheritance used by the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> that are its <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> and the representation of the <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of those <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A>. The object system provides a default <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A>, <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A>, that is appropriate for most programs. <P>
Except where otherwise specified, all <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> mentioned in this standard are <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A>, all <A REL=DEFINITION HREF="26_glo_g.htm#generic_function"><I>generic functions</I></A> are <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_std_ge.htm#standard-generic-function"><B>standard-generic-function</B></A>, and all <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> are <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_std_me.htm#standard-method"><B>standard-method</B></A>. <P>
<A REL=CHILD HREF="04_caa.htm"><H2>
4.3.1.1 Standard Metaclasses</H2></A><P><HR>
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