Object models for computer aided design

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59 ciE􁪽􋴺:􁪽􋵥ed as bein9 o􁪽􋵴 type attribute list, the field becorres a 􁪽􋵩is􁪽􋵨 hea6er. Ey forcing eaeh 􁪽􋵴ttritute to a􁪽􋵰pear in exactly one attribu􁪽􋵥e list, we are a􁪽􋵬le to e􁪽􋵳sure that the res􁪽􋵬lti􁪽􋵧g 􁪽􋴚􁪽􋴺:􁪽􋵩i-level list struc􁪽􋴚􁪽􋵲re is ec􁪽􋵩ivalent to a􁪽􋵮 n-􁪽􋵲rv tree whict is rooted in the object 􁪽􋵥ea􁪽􋵥er. In order to allo􁪽􋵡 attributes to be shared in a 􁪽􋵯ore an "attribute pointer" field type is defined. Attribute pointer fields ffay ref&rence any attrib􁪽􋵴te within the same object. 􁪽􋵨hereas an at􁪽􋵲r:bute list field references all of the attributes which are in the list, an attribute pointer field references only a single attribute. Since each attribute must appear in exactly one at􁪽􋵲ribute list, any other reference to an a:tribute 􁪽􋵵ust be by an attribute pointer. One normally tr:es to desisn objects so that each attribute will appear _" en e t t r i c u t e list of an e t t r i bu t e v.·:-::'c􁪽􋵣 car: r e e s or a cl y be considered its parent. If there are several potential parents o􁪽􋵴 the same type (e.g. several in polyhedron which share a common point), it 􁪽􋵡ay be necessary to assign the shared attribute to an attribute li􁪽􋴚􁪽􋵷 which originates in some higher level ancestor (e.g. the fo:y􁪽􋵥edron 􁪽􋵡ay have an attribute list fer points and each e=se references its end peir.ts 􁪽􋵩ith ctt:ibute pcir.ters--see t i c ur e 5.4).