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PROGRAMMING LANGUAGE
A t hesis s u b m i t t e d t o t h e U ni v e r s i t y of Du bl i n , T r i n i t y College, in ful fi l l ment o f t h e r e q u i r e m e n t s for t h e d e g r e e of
D o c t o r o f IMiilosophy ( C o m | ) u t e r Science)
' l i l m a n S c h a f e r
D i s t r i b u t e d S y s t e m s ( I r o u p D e i ) a r t i n e i i l o f C o n i p t i t e r S c i e n c e I' rinity Col l e ge , Uiiivei’s i t y of D u b l i n
1. t h e u n cl er s i g i i ed , d e c l a r e t h a t t h i s w o r k h a s n o t j ^ r e vi ous l y b e e n s u b m i t t e d t o l l i i s an>' o t h e r U n i v e r s i t y , a n d t l i a t u n l e s s o t h e r w i s e s t a t e d , it is e n t i r e l y m y o w n w o r k .
I ' i l m a n S c h a f e r
I, t h e u n d e r s i g n e d , a g r e e t h a t T r i n i t y C o l l e g e L i b r a r y m a y l e nd o r c o | ) y t h i s t h e s i s u p o n r ( ' ( | u e s t .
I'iliiian S c h a f e r
'J’h e w o r k d e s c r i b e d in t h i s t h e s i s w o u l d n e v e r h a v e b e e n p o s s i b l e w i t h o u t t h e h e l p a n d s u i ) ] ) o r t o f a n u m b e r o f p e o p l e . F i r s t o f all, I w o u l d like t o t h a n k m y s u p e r v i s o r . D r . \ ’i n n y C a h i l l , f or p r o v i d i n g d e d i c a t e d a n d p r o f o u n d g u i d a n c e , m o r a l s u p p o r t a n d c o n s t r u c t i v e c r i t i c i s m o v e r t h e e n t i r e t i m e .
I w o u l d a l s o like t o t h a n k t h e p e o p l e in t h e I g u a n a p r o j e c t w h o h a v e b e e n a g r e a t c o m p a n y o v e r t h e l ast co u [ ) l e o f y e a r s a n d w h o c o n t r i b u t e d t o m a n y o f t h e i d e a s p r e s e n t e d in t h i s l li esi s, n a m e l y l \ H e r H a r a s z t i , . )im D o w l i n g a n d l i a r r y l l e d m o n d .
S o f t w a r e e n g i n e e r i n g in a d i s t r i b u t e d , h e t e r o g e n e o u s e n v i r o n m e n t is faced wi t h a, n u m b e r o f c h a l l en g e s . W i t h d i s t r i b u t i o n c o m e s t h e need for s y n c h r o n i s a t i o n , t r a n s a c t i o n s , a n d f aul t - t o l e r a n c e whi l e s u p p o r t for di fferent o p e r a t i n g s y s t e m s also h a s t o be t a k e n i n t o a c c o u n t . I'he d i s t r i b u t e d a n d d y n a m i c n a t u r e o f a p p l i c a t i o n s in s u c h a n e n v i r o n m e n t al so i m p o s e s s peci al r ec | u i r e m e n t s in t e r m s o f a d a p t a b i l i t y : c o m p o n e n t s b e i n g a d d e d t o or r e m o v e d f r o m t h e s y s t e m m a y recpiire a p p l i c a t i o n s t o a d a p t t h e m s e l v e s t o t h e c h a n g e d e n v i r o n m e n t . i ) ev e l o| )i ne n t s h o ul d m o r e o v e r not be r e s t r i c t e d t o o n e p a r t i c u l a r p r o g r a m m i n g l a n g u a g e . Il s o o n b e c o m e s a p | ) a r e n t t h a t a l ar ge a m o u n t o f effort in t h e d e v e l o p m e n t o f d i s t r i b u t e d a p p l i c a t i o n s is s]>ent on t h e S])ecihcation a n d i m p l e m e n t a t i o n o f t h e s e n o n - f u n c t i o n a l re q u i r e m e n t s . .Moreover, t h e a c t u a l f u n c t i o n a l p a r t s o f t h e a])pl i cat i on b e c o m e m o r e a n d inor(' i n t e r t w i n e d wi t h a n d d e p e n d e n t on t h e [)arts that, deal wi t h t h e fulfillment of non- i ' unctional reciuirement.s. As a c o n s e q u e n c e , little cha.nges in t h e desi gn of an aj)])lication m a y I'esult in t h e m o d i f i ca t i o n o f l ar ge a m o u n t s of c o d e t h r o u g h o u t , t h e s y s t e m .
In o r d e r t o t a c k l e t h e s e p r o b l e m s m o d e r n s o f t w a r e e n g i n e e r i n g is cl ear l y a d v a n c i n g t o w a r d s m i d d l e w a r e a n d coni]>onent b a se d s y s t e m s s u c h as t h e C o m m o n O b j e c t Hecpiest B r o k e r . Ar c h i t e c t ur e ( C ' O R B A ) . E n t e r p r i s e .Java B e a n s (H!.]H) a n d t h e S y s t e m O b j e c t .\lf)del ( S O M ) . T h e s e s y s t e m s defi ne a | ) l a t f o r m - i n d e p e n d e n t o b j e c t m o de l , u su a l l y w i t h s u p p o r t for d i s t r i b u t i o n , p e r s i s t e n c e a n d t r a n s a c t i o n s , whi ch allow t h e r ap i d devel o])m(' ut o f a p p l i c a t i o n s in a h e t e r o g e n e o u s , d i s t r i b u t e d e n v i r o n m e n t .
( ' o i n ] )u t a ti oi ia l r eflection a n d inet a-l evel a r c h i t e c t u r e s offer a n a l t e r n a t i v e a p p r o a c h t o w a r d s t h e d e v e l o ] ) m e n t o f c o m p l e x s y s t e m s in t h a t t h e y p ro v i d e a cl ean s e p a r a t i o n o f c o n c e r n s a n d a s t r u c t u r e d a p p r o a c h o f m a k i n g i m p l e m e n t a t i o n s m o r e o p e n . In o r d e r t o p r o v i d e g e n e r i c l a n g u a g e s u p p o r t for d i s t r i b u t e d c o m p u t i n g a s well as t h e a d v a n t a g e s o f coni|:>onent b a s e d p r o g i ’a m m i n g , we h a v e d e v e l o p e d a c o m p o s a b l e a n d e x t e n s i b l e meta-U' vel a r c h i t e c t u r e for a c o m p i l e d , o b j e c t - o r i e n t e d p r o g r a m m i n g l a n g u a g e . T h e m o d e l p r o p o s e d offers a high d e g r e e o f flexibility a n d can be u s e d t o e x t e n d o b j e c t - o r i e n t e d p r o g r a m m i n g l a n g u a g e s t o s u | ) p o r t a v a ri e t y o f a p p l i c a t i o n - i n d e p e n d e n t b e h a v i o u r s s u c h a s d i s t r i b u ti on. p e r s i s t e n c e , t r a n s a c t i o n s a n d f a u l t - t o l e r a n c e . I ' h e m a i n c o n t r i b u t i o n s o f t h e w o r k d e s c r i b e d in t h i s t h e s i s c o n s i s t of t h e pr ovi si on o f a reflective o b j e c t - m o d e l t h a t is
• C o n i p o s a b l e : W e p r o v i d e a m e c h a n i s m t h a t all ows t h e a u t o m a t i c c o m p o s i t i o n of i n d i v i d u a l l y d e f i ne d o b j e c t b e h a v i o u r s . If t h e d e f a u l t s e m a n t i c s d o e s n o t yield t h e d e si r ed b e h a v i o u r , user def i ned com])Osition r ules c a n be a pp l i e d ma nu al l y .
• D y n a m i c ; l i u n n i n g a p p l i c a t i o n s c an d y n a m i c a l l y s w i t c h b e t w e e n d i f f er e nt o b j e c t m o d e l s , a l lo w i n g o b j e c t s t o evol ve in c h a n g i n g e n v i r o n m e n t s . T h i s kind o f f u n c t i o n a l i ty c o u l d ])reviously o n l y be a c h i e v e d in i n t e r p r e t e d p l a t f o r m s .
• Efficient: I5y c h o o s i n g a c o m p i l e d l a n g u a g e we a c hi e ve p e r f o r m a n c e a d v a n t a g e s o v e r i n t e r p r e t e d l a n g u a g e s a n d pr e s e n t a n a r c h i t e c t u r e t h a t is s u i t a b l e for a p | ) l i c at i o n d o m a i n s s u c h as legacy s y s t e m s , o ]) e r a t i u g s y s t e m s a n d e m b e d d e d s y s t e m s .
1 I n t r o d u c t i o n
1.1 O p e n l n i | ) l e n i e n t a t i o n ... 1.2 I n t r o f l u c t i o n t o R e f l e c t i o n ... 1.2.1 ( ' o r n | > u t a t i o n a l R e f l e c t i o n ... 1.2.2 R e i f i c a t i o n ... 1.2.3 S t i ' u c t n r a l v e r s u s l i e l i a v i o u r a l R e f l e c t i o n . . . . 1.2.1 B a s e M e t a Leve l S e | ; a r a t i o n ... 1. 2.5 M e t a o b j e c t s a n d M e t a o b j e c t C l a s s e s ... 1.2.(i M e t a o b j e c t P r o t o c o l ... 1.2.7 . M e t a c l a s s e s ... I .:5 R e f l e c t i v e P r o g r a i n n i i n g L a n g u a g e s ... 1.1 . Aims a n d O L i j e c t i v e s ... 1.5 I g u a n a ... !.() ( 'out ril>ution o f t h i s I ' l i e s i s ... 1.7 I J o a d i n a p ...
2 R e f l e c t i v e , O b j e c t - O r i e n t e d P r o g r a m m i n g L a n g u a g e s
2.1 ( i c n e r a l o v e r v i e w ... 2. 2 S i n a l l t a l k ... 2.:5 C + + ... 2. 1 ( ) b j e c t i v e - C ... 2 . 5 . Java...
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2-1'2.6 Ope]iC-|--|- v l ...29
2.6.1 T y p e o f R e f l e c t i o n ...29
2.6.2 R eflectiv e F a c i l i t i e s ...29
2.6.3 P r o g r a m m i n g M o d e l ... 30
2.6.4 K n o w n A p p l i c a t i o n s ... 32
2.6.5 P e r f o r m a n c e ... 33
2.6.6 S u p p o r t for M e t a - T y p e s ... 33
2.7 OpenC'-|--|- v 2 ...33
2.7. J 'Fyi^e o f R e f l e c t i o n ... 33
2.7.2 R efle c tiv e F a c i l i t i e s ...34
2.7.3 P r o g r a m m i n g M o d e l ... 35
2.7.4 i^ e if o r m a n c e ... 37
2.7.5 K n o w n .‘\ p p l i c a 1 i o n s ... 3^
2.7.6 .Support for M e t a - T y p e s ... 39
2.8 C'odA ... 40
2.5.1 ry])e o f l i e f l e c t i o u ...40
2.8.2 R eflective F a c i l i t i e s ...40
2.8.3 P r o g r a m m i n g M o d e l ... 42
2.8.4 K n o w n ,‘\ p | ) l i c a t i o n s ... 42
2.8.5 P e r f o r m a n c e ... 43
2.8.6 S u p ])o rt for M e t a - T y p e s ... 43
2.9 A B C I . / R ... 44
2.9.1 4’y])e o f R e f l e c t i o n ...44
2.9.2 R eflective F a c i l i t i e s ... 44
2.9.3 P ro g i'a iiin iiiig M o d e l ... 45
2.9.4 I V r f o r m a n c e ... 47
2.9.5 K n o w n . ' \ p ] ) l i c a t i o n s ... 18
2.9.6 S u p p o r t for M e t a - T y p e s ... 48
3 T h e I g u a n a R e f l e c t i v e P r o g r a m m i n g M o d e l 5 0
3.1 ( i e n e r a l O v e r v i e w ... 51
3.2 Rei fi cat i on C a t e g o i ' i e s ... 51
3.2.1 S t r u c t u r a l Rei f i cat i on C a t e g o r i e s ... 52
3.2.2 B e h a v i o u r a l Rei fi ca t io n C a t e g o r i e s ... 52
3.2.3 E x t e n t of Refl ect i on ... 53
3.2.4 l)ei)en(lencies b e t we e n I^eification C a t e g o r i e s ... 53
3.2.5 P r o t o c o l s a n d P r o t o c o l S e l e c t i o n ... 54
3.2.6 S h a r e d B e h a v i o u r ... 57
3.3 M e t a o b j e c t C o m p o s i t i o n ... 58
3.3.1 Def aul t C o m p o s i t i o n S e m a n t i c s ... 59
3.3.2 M o d i f y i n g C o m | ) o s i t i o n ... 61
3.3.3 D i s c u s s i o n ... 64
3.3.4 The I g u a n a S y n t a x ... 64
3.4 S u m m a r y ... 65
4 T h e I g u a n a / C + + I m p l e m e n t a t i o n 6 7 1.1 .V|)|)lying tiie iModel ... 67
4.2 ( i e n e r a l O v e r v i e w ... (i8
4.3 4 he I g u a n a Met a - f j evel Cl as s l l i e r a i c h y ... 69
4.4 I ' h e P r e - l ’r o c e s s o r ... 71
4.5 S o u r c e - t o - S o u r c e T r a n s l a t i o n ... 72
4.5.1 P r o t o c o l Def i ni t i ons ... 72
4. 5. 2 A d d i n g I n t r o s p e c t i o n ... 73
4.5. 3 Boot s t , r a p| )i ng ... 74
4.5.4 .Adding i nt er ce ss i on ... 75
4.5. 5 I n s t a n c e p r ot o c ol s e l e c t i o n ... 79
4.5.() R u n - t i me C h e c k s ... 79
4.6 D y n a m i c M e t a - T y p e S e l e c t i o n ... 81
4.6.1 .Meta-level R e c o n f i g u r a t i o n ... 81
4.7 ' I’he D e f a u l t P r o t o c o l 85 4.7.1 M e t h o d I n v o c a t i o n ... 85
4.7. 2 S t a t e Access ... 85
4.7..’] O b j e c t C r e a t i o n ... 87
4.8 R e s t r i c t i o n s ...87
4.8.1 . • \ ut oni a t i c o b j e c t s ... 87
4.8. 2 . A r r a y s ... 88
4.8. 3 . A l i a s i n g ... 88
4.9 S u m m a r y ... 89
5 R e f l e c t i v e P r o g r a m m i n g w i t h I g u a n a 91 5.1 Usi ng l n t r o s | j e c t i o n ... 91
5.2 B o u n d a r y C h e c k s for . A r r a y s ... 92
5.3 H u n - t i i n e . Ad ap t a t i o n of . Syst ems S o f t w a r e ... 9;}
5.3.1 I he i ni ni ma i Buffer M a n a g e r ... 94
5.3.2 .A(la|)tation us i ng Design P a t t e r n s ... 94
5.3.3 . Ad ap t a t i o n us i ng R e f l e c t i o n ... 95
5.3.1 . State Transfer ... 97
5.3.5 D i s c i h s s i o n ... 98
5.4 .A M e t a - T y | ) e fc;r P e r s i s t e n t O b j e c t s ... 98
5.4.1 O v e r v i e w of O b j e c t P e r s i s t e n c e ... 99
5.4.2 I m p l e m e n t i n g I’e rs i s t e n t O b j e c t s us i n g l g u a n a / C ' - | - - | - ... 99
5.4.3 Us i n g P e r s i s t e n t O b j e c t s ... 101
5.4.4 . Ad ap t i ng t h e M e t a - L e v e l ... 104
5.4.5 D i s c u s s i o n ... 105
5.5 S u m m a r y ... 106
(i.l O v e r h e a d , W h e r e a n d W h y ...108
(i.l.J Desi gn Level ... 109
6.1. 2 I m p l e m e n t a t i o n Level ...109
G.1.3 l l o s t - L a n g u a g e L e v e l ... I l l 6.1. 4 A p p l i c a t i o n L e v e l ... 112
6.2 D i s c u s s i o n ...113
6.3 O t h e r 0|:>tiuiisation T e c h n i ( | u e s ... 114
6.3.1 P a r t i a l E v a l u a t i o n ...114
6.-3.2 P a r t i a l E v a l u a t i o n o f l g u a n a / C ' + + ... 116
6.3.3 E l i m i n a t i o n o f R u n - T i m e C h e c k s ... 117
6.1 S u m m a r _ y ... 117
7 C o n c l u s i o n a n d F u t u r e W o r k 1 1 8 7.1 U n d e r s t a n d i n g Ref l ect i ve P r o g r a m m i n g ... 120
7.2 P e r f o r m a n c e a nd O p t i m i s a t i o n ...121
7.-3 L' uture W o r k ...121
7.3.1 Ref l ect i on a n d De si gn P a t t e r n s ... 121
7.3.2 ( 'omi^osition o f M e t a - T y [ ) e s ... 121
7.3.3 C o m | ) i l e r S u | ) p o r t for Ref l ect i ve P r o g r a m m i n g L a n g u a g e s ... 122
7.3.4 I ' or ma J i s at i o n of M e t a - T y p e s ... 122
I n t r o d u c t i o n
R E F L E C T I O N , n . A n a c t i o n o f t h e m i n d w h e r e b y we o b t a i n a c l e ar e r \'iew of o u r r e l a t ion t o t h e t h i n g s o f y e s t e r d a y a n d a r e abl e t o avoi d t h e |)erils t h a t we shall n o t ag a i n e n c o u n t e r . A m b r o s e Bierce. The D e v il's DictioiKirij.
. Modern s o f t w a r e e ng i n e e r i n g h a s t o keep p a c e w i t h ever e vol vi ng r u n t i m e c n v i r o u n i e n t s t h a l pl ace new d e m a n d s on t h e d e v e l o p m e n t o f c o m p u t e r a] ) pl i cat i ons. Si nce t h e ad- \ e n t a n d p o p u l a r i t y o f t h e I n t e r n e t , for e x a m p l e , a c o m p l e t e l y different | ) r o g r a m m i n g e n \ ii'onmen1 h a s e m e r g e d , i n t i ' o d u c i n g a f l e m a n d foi' d i s t r i b u t e d a n d c o n c u r r e n t a])i)lica- l i o n s . I.ong- i ' unni ng s y s t e m s such as o ] ) e r a t i n g s y s t e m s a re m o r e o v e r faced w i t h d y n a m i c c h a n g e s t a k i n g |)lace at r u n - t i m e , w i t h c o n i | ) o n e n t s be i ng a d d e d t o o r r e m o v e d f r om t he ( ' i i v i r on me n t , recpiiring t h e s y s t e m t o ada|)t, itself t o t h e c h a n g e d s u r r o u n d i n g s . .As s ys t e m s g r ow l a r g e r a n d b e c o m e i n c re as in g l y c o m p l e x , t h e q u e s t i o n a r i ses as t o w h a t e x t e n t a p r o g r a m m i n g l a n g u a g e c an assist dev e l oj ie r s in t h e t a s k of d e v e l o p i n g s u c h a p p l i c a t i o n s . O n e ai^proach is t o | jr ovi de d i r e c t s np] ) or t for s o m e e l e m e n t of d i s t r i b u t e d c o m p u t i n g IVom witliin t h e | ) r o g r a m m i n g l a n g u a g e . I']xam])les of t h o s e l a n g u a g e s i n c l u d e ( ' * * [Tay93, \' ( ' (1P93] , I'Jiierald [.)LI1B88] a n d .Java [CJ.IS96]. C'** is an e x t e n s i o n o f C++ a n d |:>rovides I l u ' a| )| )lication p r o g r a m m e r wi t h s u | ) p o r t for p e r si st e n c e , d i s t r i b u t i o n , a n d t r a n s a c t i o n s , h j ii e r ak i p r ov i de s object, m i g r a t i o n a n d an e x t e n d e d e x c e p t i o n h a n d l i n g m e c h a n i s m to rf'cover fr om p a r t i a l failures [Hut9()]. .Java on t h e o t h e r h a n d f e a t u r e s built-in s u p i )o r t for m u l t i - t h r e a d i n g a n d s t a n d a r d l i br ar i es for r e m o t e m e t h o d i n v o c a t i o n , m a k i n g t h e d e v e l o p m e n t of c o n c u r r e n t , d i s t r i b u t e d a p p l i c a t i o n s easier.
s u p p o r t e d f e a t u r e usu a l l y recpiires a specific s y n t a x a n d / o r r u n - t i m e s u p p o r t , m a k i n g t h e l a n g u a g e m o r e c o m p l e x a n d difficult t o i m p l e m e n t .
A different a p p r o a c h t h a t a d d r e s s e s t h e s e p r o b l e m s is e x e mpl i f i ed by c o m p o n e n t - b a s e d s y s t e m s s uch as S u n ’s E n t e r p r i s e J a v a B e a n s ( E J B ) [MicOl], t h e C o m m o n O b j e c t Recpiest B r o k e r A r c h i t e c t u r e ( C O I i B A ) [Gro95] a n d I B M ’s S y s t e m O b j e c t M o d e l ( S O M ) [Lau94], f h e s e s y s t e m s p r o v i d e a p l a t f o r m - i n d e p e n d e n t f r a m e w o r k for b u i l d i n g a p p l i c a t i o n s in a d i s t r i b u t e d e n v i r o n m e n t a n d u s u a l l y c o m e w'ith bui l t -i n s u p p o r t for d i s t r i b u t i o n , p er si s t e n c e a n d t r a n s a c t i o n s , f ^ r o g r a m m e r s s i m p l y sel ect o r d e p l o y a pr e - d e f i n e d c o m p o n e n t i nt o t h e i r aj ^plication. s u b j e c t t o s o m e p r o g r a m m i n g c o n v e n t i o n s t h a t o n e h a s t o obey. Ho we v e r , t h e s e s y s t e m s a r e p r i m a r i l y t a r g e t e d t o w a r d s t h e d e v e l o p m e n t of b us i n e s s a p p l i c a t i o n s a n d can h a r d l y be e mp l o y e d for t h e d e v e l o p m e n t o f low-level, p e r f o r m a n c e critical a p p l i c a t i o n s s u c h as o p e r a t i n g s y s t e m s a n d e m b e d d e d s y s t e m s . M o r e o v e r , t h e s e s y s t e m s a r e ' c l o s e d ’ s y s t e m s , m e a n i n g t h a t u s e r s d o n o t h a v e d i r e c t c o n t r o l ov e r t h e i m p l e m e n t a t i on of t h e v ar io u s facilities a n d c a n in g e n e r a l not p r o v i d e c u s t o m i s e d s er vi ces. S u p p o r t for di l ferent o p e r a t i n g s y s t e m s , a m a j o r prerecpiisite for t h e d e v e l o p m e n t in a h e t e r o g e neous. d i s t r i b u t e d e n v i r o n m e n t , m i gh t also n o t a l w a y s be avai l abl e. I nt er ope r a . bi l it y b e t ween t h e s e s y s t e m s d o e s e x i s t t o s o m e e x t e n t , for exam])le. in t h e f o r m o f C ' O R B A - C O M briflges. but t h e s e c o m e a t t h e price o f i n c r e as e d c o m p l e x i t y a n d i n t r o d u c e c o m | ) a t i b i l i t y p r o b l e m s .
Si nce t h e p r ov i d e r o f a s o f t w a r e c o m p o n e n t c a n in g en er a l not foresee all p o s s ib l e us a g e s c e n a r i o s a n d s y s t e m r ecpi i r ement s , a new p r o g r a m m i n g mo de l is n e ed e d t h a t is in s o m e seiis(' " o p e n " a n d d o e s not restrict, its u s e r s by t h e de c i s i ons m a d e by its d e s i g n e i ’s.
1.1
O pen Im p lem e n ta tio n
I'he o p e n i ni ] ) le n i e n t a t io n a p p r o a c h sli oul d n o t be seen as a c o n t r a d i c t i o n t o t h e cla.ssicai l)lack b ox a p ] ) r oach. I n s t e a d, it s h o ul d be u n d e r s t o o d as an e x t e n s i o n t o t h e f o r m e r . a d \ o c a t i n g a fai r er t r a d e - o f f b e t w e en p e r f o r m a n c e on t h e o n e h a n d a n d a b s t r a c t i o n on t h e o t h e r , l a k e t h e U n i x s y s t e m call m a d v i s e as a n e x a m p l e . It allow's ] ) r o gr a m n ) e rs t o ‘a d v i s e ’ t h e u n d e r l y i n g v i r t u a l m e m o r y m a n a g e r a b o u t t h e u s a g e p a t t e r n o f a block of m e m o r y m a p p e d i n t o t h e a p p l i c a t i o n ’s a d d r e s s s p a ce . It t h e r e f o r e e x p o s e s s o m e h i d de n f u n c t i o n a l i t y o f t h e u n d e r l y i n g o p e r a t i n g s y s t e m t o t h e user, r e s u l t i n g in a m o r e efficient i n i | ) l e m e n t a t i o n .
Ho we v e r , t h i s e x a m p l e c an m e r e l y be r e g a r d e d as an a d hoc a p p r o a c h t o oj)en i m p l e m e n t a t i o n s a n d o n l y al l ows ve r y r e s t r i c t e d ac c e s s t o t h e o p e r a t i n g s y s t e m ’s i n t e r n a l s . W h a t
is t her ef or e ne e de d is a m e t h o d o l o g y t h a t al l ows a m o r e r i g o r o u s a n d s t r u c t u r e d access t o a c o m p o n e n t ’s i n t e r n a l s . C o m p u t a t i o n a l reflection, as d e s c r i b e d in t h e f ol lowing s e ct io n s , c a n s e r ve as an u n d e r l y i n g t e c h n o l o g y for b u i l d i n g o p e n i n i ] ) l e m e n t a t i o n s in t h a t it e q u i p s a c o m p u t a t i o n a l s y s t e m w i t h a s e j i a r a t e r e p r e s e n t a t i o n of itself.
I nevi tabl y, a n u m b e r o f issues arise, n a m e l y t o w h a t e x t e n t i n t e r n a l i m p l e m e n t a t i o n d e t a i l s s h o u l d or c a n be e x p o s e d t o t h e user a n d how t h e e r r o n e o u s o r d e l i b e r a t e m i s u s e o f s uch i n f o r m a t i o n can be | )r ev en t e d . T h e s e issues will follow us t h r o u g h t h i s t h e si s a n d will be d('alt w i t h m o r e t h o r o u g h l y as t h e y arise.
1.2
I n t r o d u c t i o n t o R e f l e c t i o n
' f h i s s e c t i on i n t r o d u c e s s o m e o f t h e m a i n c o n c e p t s a n d t e r m i n o l o g y used t h r o u g h o u t t h e r e m a i n d e r of t his t hesi s. In m o s t c a s e s t h e c o n c e p t s a n d t e r m i n o l o g y p r e s e n t e d her e a r e not. specific t o t h e a u t h o r ’s view a n d a r e wi del y used in t h e reflection c o m m u n i t y a t large. I nf oi ' t una t e l y, t h e r e is n o t c o m p l e t e c o n s e n s u s on t h e use o f all o f t h e s e t e r m s even w i t h i n t h e I ' cf l e c t i o n c o m n u i n i t y .
1. 2.1
C o m p u t a t i o n a l R e f l e c t i o n
it i l l u s t r a t e s s o m e of t h e f e a t u r e s t h a t a r e c h a r a c t e r i s t i c o f a reflective s y s t e m , n a m e l y:
I n t r o s p e c t i o n : Int r o s p e c t i o n is t h e p r oc e s s o f o b s e r v i n g o n e ’s i n t e r n a l s t a t e o r s t r u c t u r e . Kor a s y s t e m t o be a bl e t o i n t r o s p e c t a n d r e as o n a b o u t itself it n e e d s a s e l f repre.se n ta tio n .
S e l f r e p r e s e n t a t i o n : In t h e e x a m p l e a bo v e , ’I ’ r e p r e s e n t s D e s c a r t e s himself, his n o t i o n o f himself.
C a u s a l c o n n e c t i o n : A c o m p u t a t i o n a l s y s t e m c o n t a i n s an i n t e r n a l r e p r e s e n t a t i o n of s o m e ])art of t h e e x t e r n a l wor l d, its d o m a i n (for e x a m p l e in f o r m o f d a t a a n d c o d e ) . A s y s t e m is said t o be c au s a l l y c o n n e c t e d t o its d o m a i n if t h e i n t e r n a l s t r u c t ur es a n d t he d o m a i n t h e y r e p r e s e n t a r e linked in s u c h a w a y t h a t if o n e o f t h e m c h a n g e s , t hi s l e a d s t o a c o r r e s p o n d i n g effect u p o n t h e o t h e r [Mae87]. A reflective s y s t e m is o n e w h o s e s e l f - r e| ) r es en t a t i o n is c au s a l l y c o n n e c t e d t o itself, o r whi ch is s el f - r ef erent i al . A g a i n , as D e s c a r t e s d e d u c e s “t h e r e f o r e I a m ” , he g e n e r a t e s n e w k n o w l e d g e wh i c h in t u r n feeds back i nt o his s e l f - r e p r e s en t a t i o n in a c a u sa l l y c o n n e c t e d way.
Reflection c a n be d i ve rs a n d o c c u r s in m a n y different d o m a i n s . So h o w d o e s t h e a b o v e re l a t e t o o b j e c t - o r i e n t e d |)rogi’amiriiiig l a n g u a g e s ? An o b j e c t - o r i e n t e d p r o g r a m m i n g l a n g u a g e (o o i’l) p r ov i d es c o u ce | ) t s s uch as classes, i n h e r i t a n c e , m e t h o d i n v o c a t i o n a n d s t a t e acc('ss. Ap | ) l i c a t i o n s w r i t t e n in such a l a n g u a g e coiice]>tually consi st o f a set o f o b j e c t s e x c h a n g i n g m e s s a g e s a n d p e r f o r m i n g c om] Mi t a t i ons u])on r e ce pt i o n o f m e s s a g e s . In a re- flec'tive p r o g r a m m i n g l a n g u a g e , i nt r osj^ection e n a b l e s o b j e c t s t o q u e r y t h e i r o w n s t a t e , for e x a m p l e t o h n d o u t a b o u t t h e i r class, t h e m e t h o d s a n d a t t r i b u t e s t h e y ])rovide, e t c . ' .Moi'co\'er, t h e self r e p r e s e n t a t i o n m a y also p ro vi d e a descri])tion of t h e o b j e c t ' s b e h a v i o u r , i.e.. c o d e t h a t d e s c r i b e s t h e o b j e c t sjjccific s e m a n t i c of m e s s a g e s e n d i n g / r e c e i v i n g , m e t h o d d i s | ) a t c hi n g , etc.
1. 2 .2
R e i f i c a t i o n
.Any O O l ' L s u p p o r t s a n u m b e r o f f e a t u r e s such as o b j e c t c r e a t i o n , m e t h o d i n v o c a t ion, s t a t e access a n d i n h e r i t a n c e . A p r o g r a m m i n g l a n g u a g e t h e r ef or e p r ov i d e s a s y n t a x t h a t is used l)y t h e | > r o g r a m m e r t o e x p r e s s a p a r t i c u l a r conce])t as well as a s e m a n t i c t h a t d e s c r i b e s how t h e concej j t is i n t e r p i ’e t e d o r e x e c u t e d . In t r a d i l i o n a J O O P L s t h e s y n t a x / s e m a n t i c s rel at i onshi ]) is fixed, m e a n i n g t h a t t h e r e is no (easy) wa y for a | ) r o g r a m n i e r t o m o d i f y t h e
s y n t a x o f t h e l a n g u a g e o r t o mo di f y t h e wa y a s p e ci h c l a n g u a g e c o n s t r u c t is i n t e r p r e t e d . lk:iJication refers t o t h e p r o ce ss o f m a k i n g t h e i m p l e m e nt a . t i o n o f t h e s e f e a t u r e s ex|)licit. w i t h i n t h e l a n g u a g e so t h a t t h e y c a n be reflected u p o n . In p a r t i c u l a r , r ei f i cat i on allows t h e i m p l e m e n t a t i o n o f t h e s e f e a t u r e s t o be modi f i ed f r o m w i t h i n t h e l a n g u a g e .
1 . 2 . 3
S t r u c t u r a l v e r s u s B e h a v i o u r a l R e f l e c t i o n
S t r u c t u r a l reflection is t h e abi l i t y o f a l a n g u a g e t o p ro v i d e a c o m p l e t e rei fi cat i on o f b o t h t h e p r o g r a m c u r r e n t l y b e i n g e x e c u t e d as well as c o m p l e t e reifi cat i on o f its a b s t r a c t d a t a tyi )es (e.g. its classes) [DM95]. l i e h a v i o u r a l reflection on t h e o t h e r h a n d is t h e ability' o f t h e l a n g u a g e t o |)rovi de a coni]>lete r eification of its s e m a n t i c s . 13ehavioural reflection r e q u i r e s a m e c h a n i s m t o i n t e r v e n e in t h e e x e c u t i o n o f t h e c u r r e n t p r o g r a m in o r d e r t o ( ' x e c u t e s o m e reflective c o d e t h a t will an.alyse o r m o d i f y t h e c o u r s e o f e ve nt s.
1 . 2 . 4
B a s e - M e t a L e v e l S e p a r a t i o n
In g e n e r a l we c an d i s t i n g u i s h b e t we e n t h e base-level o f a s y s t e m a n d its m e t a - l e v e l . 'I'he base-level c o n t a i n s t h e c o d e a n d d a t a c o n c e r n e d wi t h t h e ap])l i cat i on d o m a i n whi l e t h e n u ' t a- l evel c o n t a i n s t h e c o d e a n d f la t a c o n c e r n e d wi th t h e base-level. We c a n t a l k ai ) out a s y s t e m e x e c u t i n g at t h e base-level w h e n p e r f o r m i n g d o m a i n - s p ec i f i c c o m p u t a t i o n a n d e x e c u t i n g at t h e me t a- le v e l w h en c a r r y i n g o u t c o m ] ) u t a t i o n a b o u t t h e base-level. ' Fhus 1 h(' ( s e l f -) re | ) re se nt a t i o n o f t h e base-level e xi s ts as t h e d a t a o f t he me ta -l e v e l a n d reflective c o m| ) U t a t i o n is p e r f o r m e d wh e n e x e c u t i n g at t h e me t a - le v e l . T h e t r a n s i t i o n f r o m p e r f o r m i n g d o m a i n c o m p u t a t i o n t o | ) e r f o r m i n g reflective c o m p u t a t i o n is c h a r a c t e r i s e d as a t i a n s i t i o n f r om b a s e t o me t a - le v e l e x e c u t i o n .
1 . 2 . 5
M e t a o b j e c t s a n d M e t a o b j e c t C l a s s e s
In a I'eflective OOIM,, a ni et aobj er t (also k n o w n as a ni ( ta - l e v el objcrt) is a n object, t h a t liolds i n f o r m a t i o n a b o u t t h e i m p l e m e n t a t i o n a n d i n t e r p r e t a t i o n o f s o m e o b j e c t [Mae87]. r i i e set of n i e t a o b j e c t s t h a t r e pr es e nt a p a r t i c u l a r o b j e c t t o g e t h e r f o r m i t s incia-level. T h e set o f m e t a o b j e c t s r e p r e s e n t i n g all t h e base-level o b j e c t s t h a t m a k e u p a n a]j|)lica1ion t o g e t h e r f o r m t h e a p | ) l i c a t i o n ’s rneta-level.
even t h e c o d e of its m e t h o d s . O n t h e o t h e r h a n d , b e h a v i o u r a l n i e t a o b j e c t s a r e used t o i m p l e m e n t v ar io u s as i )e c t s o f an o b j e c t ’s b e h a v i o u r . F o r e x a m p l e , b e h a v i o u r a l m e t a o b j e c t s m i g h t be used t o i m| > l eme nt o b j e c t c r e a t i o n or m e t h o d i n vo c at i o n .
1 . 2 . 6
M e t a o b j e c t P r o t o c o l
M e t a o b j e c t s |)rovide t h e n e c e s s a r y f u n c t i o n a l i t y t o c o n t r o l a n d re]>resent base-level o b j e c t s . C o m m u n i c a t i o n b e t w e e n b a s e a n d me t a -l e v e l o b j e c t s t a k e s ])lace t h r o u g h a well def i ned s e t o f i n t e r f ac e f u n c t i o n s . ' Fhe set o f t h e s e i n t e r f ac e f u n c t i o n s is r ef e r r ed t o as t h e m e tn o b je c t p rotocol [KP96] , or M O P for s h o r t .
1. 2.7
M e t a c l a s s e s
S o m e o b j e c t - o r i e n t e d p i ' o g r a n i m i n g e n v i r o n m e n t s , i n c l u d i n g S m a l l t a l k a n d S O M . p r ov i d e n u t a r la s s e s as t h e n a t u r a l b a s e o f t h e i r o b j e c t m o d e l: m e t a c l a s s e s a r e c l asses w h o s e i n s t a n c e s a r e t h e m s e l v e s cl asses. I ' h e y defi ne tlie c o m m o n |>ro])erties t h a t all cl as s e s s ho u l d proN'ide. for exairi|)le. how me t h o d s a r e d i s p a t c h e d . In S m a l l t a l k t h e r e is a fixed o n e - t o - o n e r e l a t i o n s h i p b e t w e en a c l ass a n d i t s m e t a c l a s s , e v e ry met acla,ss is def i ned by t h e S m a l l t a l k r u n - t i m e envii-onnuMit a n d c a n n o t be a l t e r e d by t h e a]:>plication ] >r ogr a mme r . S O M on t h e o t h e r h a n d allows t h e (levelojMnent a n d c o m p o s i t i o n of i n d e p e n d e n t met acl a. sses [l''l)M!)4].
1.3
R e fle c tiv e P r o g r a m m in g L a n g u a g e s
R a t h e r t h a n s u p p o r t i n g a fixed s e t o f facilities for a specific p r o g r a m m i n g e n v i r o n m e n t , F('liective i i r o g r a m m i n g l a n g u a g e s a r e d e s i g n e d so t h a t t h e i r i m p l e m e n t a t i o n is m o r e o p e n , al l owi ng a d a p t a t i o n of t he l a n g u a g e t o v a r yi n g , n o n - f u n c t i o n a l r ec i ui r ement s . In a reflective | ) r o gr a ni i n i n g l a n g u a g e , p r o g r a m m e r s c a n a l t e r t h e s y n t a x a n d s e m a n t i c s of t h e l a n g u a g e itself a n d t her eb y a d j u s t t he b e h a v i o u r o f t h e l a n g u a g e c o n s t r u c t s t o build a | ) r o g r a m m i n g e n v i r o n m e n t t h a t best fits t h e i r needs. F o r e x a m p l e , if t h e l a n g u a g e d o e s n o t h a v e bui l t -i n suppor t , foi', l e t ' s sa\'. d i s t r i b u t i o n , it c a n be e x t e n d e d t o allow t h e i n v o c a t i o n o f r e m o t e m e t h o d s by m o d i f yi n g t h e s e m a n t i c s o f a n o r m a l m e t h o d call.
I.isp for exani])le is a f u nc t i o n a l p r o g r a m m i n g l a n g u a g e w i t h reflective facilities: f u n c t i o n s a re first class e n t i t i e s t h a t c a n be mani | Mi l at ed a s c a n a n y o t h e r val ues. T h i s is a c h i e v e d
1.4
A im s and O b jec tiv es
( i e i i e r a l l y s j ^ e a k i n g , w e w a n t t o e x t e n d t l i e e x i s t i n g o b j e c t - m o d e l o f a p r o g r a m m i n g l a n g u a g e ill s u c h a w a y t h a t it is p o s s i b l e t o i n t r o d u c e n e w o b j e c t s e m a n t i c s t o t h a t l a n g u a g e .
I'his e x t e n s i o n s h o u l d ( f r o m t h e a p p l i c a t i o n p r o g r a m m e r ’s ] )oi nt o f v i e w ) be:
• I ' r a n s p a r e n t : t h e r e f l e c t i v e e x t e n s i o n s h o u l d b e a s t r a n s p a r e n t a s p o s s i b l e , w i t h o u t t h e n e e d f o r a p p l i c a t i o n p r o g r a m m e r s t o e x p l i c i t l y i n v o k e m e t a - l e v c l c o m p u t a t i o n .
• C ' o m p o s a b l e : it s h o u l d b e p o s s i b l e t o c o m b i n e a n d a u g m e n t o b j e c t b e h a v i o u r s in a m e a n i n g f u l w a y , s o f o r e x a m p l e t o a l l o w t h e c o n s t r u c t i o n o f b o t h p e r s i s t e n t a n d r e m o t e l y a c c e s s i b l e o b j e c t s .
• D y n a m i c : t h e o b j e c t m o d e l s h o u l d b e a b l e t o e v o l v e o v e r t i m e , e n a b l i n g o b j e c t s t o a d a p t t h e m s e l v e s t o a c h a n g i n g e n v i r o n m e n t .
• K x t e n s i b l e : t h e r e s h o u l d b e n o f i xed s e t o f o b j e c t b e h a v i o u r s . It s h o u l d b e p o s s i b l e t o d e v e l o p n e w oi' m o d i f y e x i s t i n g o b j e c t m o d e l s .
[DM9.5] d e s c r i b e s w h a t a n i d e a l r e f l e c t i v e l a n g u a g e s h o u l d offer:
" I d e a l l y , a r e f l e c t i v e l a n g u a g e s h o u l d s u j ) p o r t a m e t h o d o l o g y o f r e ( l e c t i \ e c o m p u t a t i o n s g i v i n g i t s u s e r s a s m u c h f l e x i b i l i t y a s p o s s i b l e , it s h o u l d b e |)0s s i b l e t o m o d i f y t h e b e h a v i o u r o f s o m e c o n s t r u c t f o r t h e w h o l e e x e c u t i on o f a p r o g r a m , o r f or s h o r t p e r i o d o f ( e x e c u t i o n ) t i m e . It s h o u l d a l s o b e | ) o s s i b l e t o m o d i f y t h e b e h a v i o u r o f s o m e c o n s t r u c t f o r all t h e p r o g r a m o r o n l y for s o m e o f i t s s u b p a r t s . N o n e o f t h e l a n g u a g e s c u r r e n t l y p r o | ) o s e d a. chi eves t h i s level o f ffexi bi l i t y"
M a i n l y b e c a u s e o f e f f i c i e n c y r e a s o n s , p r o v i d i n g all o f t h e f u n c t i o n a l i t i e s a s d e s c r i b e d a b o v e m a y n o t b e feasil)le. W e t h e r e f o r e h a v e t o f i nd a g o o d c o m p r o m i s e b e t w e e n f u n c t i o n a l i t y o n t h e o n e h a n d a n d p e r f o r m a n c e o n t h e o t h e r h a n d .
1.5
Iguana
w i t h e v o l v i n g r u n - t i m e e n v i r o n m e n t s , r e q u i r i n g t h e a p p l i c a t i o n t o a d a p t itself t o t h e mnv e n v i r o n m e n t , s uch as o p e r a t i n g s y s t e m s a n d m i d d l e w a r e .
A desi gn pri nci pl e t h a t h a s b e e n i n f lu e n c i n g t h e I g u a n a m o d e l t o a g r e a t e x t e n t w a s t h e idea o f a coni | ) l et e decom]:)osition o f t h e u n d e r l y i n g o b j e c t m o de l , al lowi ng a ver y fine g r a i n e d c o n t r o l over t h e reification o f l a n g u a g e f e a t u r e s . Ho we v e r , in p r a c t i c a l t e r m s t hi s a p | ) r o a c h h a s been foujid as t o be t o o c o m p l i c a t e d , l e ad i n g t o a n o v e r s pe c i a l i s a t i o n of M O l ' s [Pa,w98]. T h e following is a c r i t i q u e t h a t s u m m a r i s e s t h e m a i n deficiencies o f t h e I g u a n a mo d e l in its pi' evious version:
• C o m p l e x i t y : T h e m o d e l w a s t o o c o m p l e x , c o n f r o n t i n g t h e u s e r w i t h a n u m b e r of f e a t u r e s w h o s e s e m a n t i c s we r e n o t fully u n d e r s t o o d a n d / o r p r a c t i c a l . It also r e qu i r e d a k n ow l e d ge of t h e i nt r i ns i c m e c h a n i s m s o f t h e h o s t p r o g r a m m i n g l a n g u a g e wh i c h d e f e a t e d l a n g u a g e i n d e p e n d e n c e a n d e as e o f use.
• S a f e t y a n d r o b u s t n e s s : M a n y issues c o n c e r n i n g s a f e t y a n d r o b u s t n e s s in reflective p r o g r a m m i n g ha.d n o t been a d d r e s s e d sufficiently. O p e n i n g u p a l a n g u a g e a n d ;il- l owi ng t h e p r o g r a m m e r t o i n t e r c e d e wi t h t h e u n d e r l y i n g o b j e c t mo de l recpiii'es a p r o f o u n d u n d e r s t a n d i n g o f t h e l a n g u a g e ’s i n t r in s i c m e c h a n i s m s . Us e r s o f a r eflective s y s t e m not onl>’ h a \ e t o m a k e s u r e t h a t t h e i r base-level a p p l i c a t i o n is s e m a n t i c a l l y cori' ect. t h e y must also ensui' e t h a t t h e c o d e at. t h e met a- l evel is c o n s i s t e n t . T h e t h o u g h t l e s s use o f reflection c a n easily affect t h e e n t i r e s y s t e m a n d m a k e s d e b u g g i n g ( ' \ en hai'd('r as e r r o r s c a n o c c u r at b o t h t h e b a s e a n d me t a- l evel .
• r ia ns ] ) a r e i i cy: The | )r evi ons ver si on di d n o t s e ] ) a r a t e sufficiently b e t w e e n base a n d m e t a - l e \ ’el cotie a n d e x p o s e d t o o m a n y i m p l e m e n t a t i o n d e t a i l s t o t h e api^licjiticin | ) r o g r a m m e r . .'Vgain, t h i s rai ses t h e issue as t o w h a t e x t e n t a n i m ] ) l e m e n t a t i o n c a n be o| )ened up w i t h o u t l e a di n g t,o an o v er l oa d o f accessi bl e f e a t u r e s . M o r e o v e r , it w a s not possi bl e t o use reflective a n d n o n - re fl ec ti ve o b j e c t s i n t e r c h a n g e a b l y .
• I m p l e m e n t a t i o n : O n l y a p a r t i a l i m p l e m e n t a t i o n e x i s t e d . 'Flie f e a s a b i li t y a n d aj j pl i - c ab i l i ty o f t h e I g u a n a m o d e l in p r a c t i c a l t e r m s still h a d t o be p r ov e n .
1.6
C ontribution o f this T h esis
T h e c h a l l e n ge w a s t o m a i n t a i n t h e flexibility or i gi nal l y e nv i s a g e d a n d t o find a user i n t e r f a c e whi ch is a t t h e s a m e t i m e s i mp l e , r o b u s t a n d ex p r es s i v e . A s a n o u t c o m e of o u r wo r k we i n t r o d u c e t h e c o n c e p t of a m e t a - t y p e as s uch a n i n t e r f ac e t h a t s e rv e s o u r p u r p o s e , ' f i l e m e t a - t y p e o f a n o b j e c t c h a r a c t e r i s e s its o b j e c t m o d e l a n d as s u c h its n o n - f u n c t i o n a l ij e h a v i o u r . F o r e x a m p l e , a m e t a - t y ] ) e Persi st ent m i g h t c o r r e s p o n d t o an o b j e c t - m o d e l si i p] ) or t i ng o b j e c t j^ersistence whi l e a di f f e r e nt m e t a - t y p e R e mo t e m i g h t s u p p o r t r e m o t e m e t h o d i nv o c a t i o n . W e also p r o v i d e a s e m a n t i c for c o m b i n i n g mul t i | )l e m e t a - t y p e s by a m e c h a n i s m s i mi l a r t o class i n h e r i t a n c e . .4s will b e c o m e c l ea r e r l a t e r , met a- t y] >es differ fr om i n e t a c l a s s e s as d e s c r i b e d a b o v e in t h a t we allow' m e t a - t y p e s t o be d y n a m i c a l l y se lected by i n s t a n c e s o f p o t e n t i a l l y a n y class, s u b j e c t t o a few r es t r i c t i o n s . M e t a c l a s s e s on t h e o t h e r h a n d defi ne t h e b e h a v i o u r of a class a n d t h e r e b y t h e b e h a v i o u r o f all i n s t a n c e s of t h a t class.
By a p p l y i n g tlie I g u a n a m o d e l t o a com| )i l ed l a n g u a g e , in t h i s c as e C+ + , we al so d e m o n s t r a t e d t h a t c o m p i l e d p l a t f o r m s c an e xh i b i t a flexibility c o m p a r a b l e t o inter]M'eted l a n g u a g e s , while at t h e s a m e t i m e i n t r o d u c i n g on l y lit t l e o r no o v e r h e a d w h e n t h e reflective f e a t u r e s a re not used.
We h a v e chosen C++ a s a h o s t l a n g u a g e for a c ou p l e o f r ea s o n s . F i r s t , it is w id e ly used. Se co nd , it allows low-level a ccess a n d is highl y p e r f o r m a n t , b o t h cr uci al p r o p e r t i e s for t h e de\-elo|)inent o f o | ) e r a t i n g s y s t e m s a n d emi ^edded s y s t e m s . T h e ,A.pertos o | ) e r a t i n g s y s t e m as (h' scri bed in [’N'KI.94, LYil9-5] for exani])le uses reflection as a n u n d e r l y i n g de s i gn p r i n ciple'. i'lfliciency h a s been re c ogni se d as a m a j o r c o n c e r n . Su[)|>orting r u n - t i m e I'eflection in a coiii|)iled l a n g u a g e c a n a d v a n c e t h e r es e a r ch in t h a t a r e a by p r o v i d i n g a d e v e l o p m e n t p l a t f o r m for efficient a n d low-level | ) r o g r a m m i n g .
It s h o u l d l>e n o t e d h o w e ve r t h a t t h e ] ) r o g r a m m i n g m od e l descrii)ed in t h i s t h es i s is i n t e n d e d t(j be l a n g u a g e i nde i )e n d e n t a n d not. r e s t r i c t e d t o C++ in ] )ar t i cul ar. In f act , c u r r e n t w o r k is invest i g a t i n g t he i m p l e m e n t a t i o n o f t h e I g u a n a m o d e l in .Java [RCOO]. M o r e specifically, we a d d r e s s e d t h e follow'ing issues in o u r i m p l e m e n t a t i o n of m e t a - t y p e s :
M e t a o b j e c t c o m p o s i t i o n : I g u a n a ’s m o d u l a r desi gn al l ows t h e c o m p o s i t i o n o f m e t a level o b j e c t s in o r d e r t o c o m b i n e t w o or m o r e b e h a v i o u r s . ,‘\ s a n e w f e a t u r e , we p r o v i d e a u t o m a t i c m e t a o b j e c t c on i | ) o s it i on . If t h e d ef a u l t s e m a n t i c s d o e s not. yield t h e de s i r e d b e h a \ ' i o u r . user defi ned com| )Osi t i on r ules c a n be a pp l i ed easily.
a s i g n i fi c a n t i m p r o v e m e n t t o e a rl ie r ve r si o n s o f I g u a n a , we n o w p r o v i d e a saf e t r a n s i t i on b e t w e e n d i ff er en t me ta -l e ve l c o n f i g u r a t i o n s , a c c o m p l i s h e d by a c o m b i n a t i o n o f s t a t i c s u b - t y p i n g rules a n d r u n - t i m e m e t a - t y p e ciiecking.
U n d e r s t a n d i n g m e t a - l e v e l p r o g r a m m i n g : in c o n t r a s t t o o b j e c t - o r i e n t a t i o n , reflec t ion r e m a i n s a r a t h e r e so t e r ic p r o g r a m m i n g p a r a d i g m a n d h a s n o t f o u n d its w a y i n t o e x i s t i n g c o m p i l e r t ecluiologies. In a n u m b e r of cas e s t u d i e s we s h o w h o w me t a -l e v el p r o g r a m m i n g c a n p r o v i d e a g e ne r ic f r a m e w o r k for b u i l d i n g c e r t a i n t y p e s of a p p l i c a t i o n s , for exam])l e, s y s t e m s t h a t need t o d y n a m i c a l l y a d a p t t o c h a n g i n g recj ui r ement s .
O p t i m i s a t i o n : T h e flexibility g a i n e d by e x t e n d i n g t h e l a n g u a g e a n d g i vi ng t h e users liie o p p o r t u n i t y t o a d a p t t h e l a n g u a g e t o t h e i r n e ed s u su a l l y i n c u r s s u b s t a n t i a l i n t e r p r e t a t i v e o v e r h e a d . A l t h o u gi i t h i s issue h a s been a d d r e s s e d by a n u m b e r o f r e s e a r c h er s , it h a s not sufficiently been sol ved in a wa y t h a t it wo u l d h a v e i n c re as ed t h e accei:>tance of r ellective | :)rogrammi ng. We ident i f y | )rofi t abl e t a r g e t s a n d technic[ues for o ] ) t i m i s a t i o n . a l l owi ng a m o r e efficient i m] ) le me n t a 1 i on o f t h e reflective p r o g r a m m i n g f e a t u r e s . P r a c t i c a l e x p e r i e n c e s w i t h usi ng reflection a s a t ool for p r o v i d i n g pr o] ) er t i es s u c h as f a u l t - t o l e r a n c e a n d group-l.iased d i s t r i b u t e d s y s t e m s h o we v e r h a v e s h o w n t h a t t h e c o s t s d u e t o t h e use of a iiiet aol )j ect ])rotocol a r e negligible wi t h r e sp e ct t o t h e e xe c u t i o n c o s t s o f t h e ni et a- Ini i ct i oual | )ro| )ert i es [I''P98].
1.7
R o a d m a p
R e fle c tiv e , O b j e c t - O r ie n t e d P r o g r a m m in g
L a n g u a g e s
R E V I E W , v . t . T o s e t y o u r w i s d o m ( h o l d i n g n o t a d o u b t o f it. A l t h o u g h in t r u t h t l i e r e ' s n e i t h e r b o n e n o r skin t o it)
At wo r k u])on a b o o k , a n d so r ead o u t o f it r ii e ( | ualities t h a t you h av e first r ead i n t o it.
At i i bro. ‘<f H i e i r f . I'hf D e v / T s J ) i r t i o n a r i / .
W ith t r a d i t i o n a l , o l) j e c t - o r i e n t e d l a n g u a g e s , t h e d e v e l o | ) me nt o f s y s t e m s t h a t a r e c h a r a c t e r is e d by a st r o n g i n t e r d e p e n d e n c y of f u nc t i o n a l a n d n o n - f u n c t i o n a l r e q u i r e m e n t s can lesult in c o d e w h e r e p a r t s d e a l i n g w i t h t h e f u n c t i o n a l a s p e c t s of t h e a | ) pl i c at i o n a r e t o a large e x t e nt i n t e r t w i n e d w i t h p a r t s t h a t deal wi t h t h e n o n - f u n c t i o n a l a s | )ect s . T a k e r e m o t e m e t h o d i n v o c a t i o n s as an e x a m p l e . I m p l e m e n t i n g r e m o t e o b j e c t s us i n g an a r c h i t e c t u r e such as C ' O R l i A r e qui r e s t h a t a r g u m e n t s t o r e m o t e m e t h o d s a r e w r a p p e d in s peci al w r a p - pei- o b j e c t s ' . A l t h o u g h m u c h o f t h e f u n c t i o n a l i t y u n d e r l y i n g r e m o t e o b j e c t s c a n be h i d d e n by us i ng n o r m a l class i n h e r i t a n c e , t h e a p p l i c a t i o n now c o n t a i n s cocie t o w r a p a n d u n w r aj ) a r g u m e n t s a n d r e t u r n val ues of r e m o t e m e t h o d s .
Reflective p r o g r a m m i n g l a n g u a g e s |)rovi de a n a l t e r n a t i v e a|:)|:)roach in t h e d e v e l o | ) m e n t of such s y s t e m s in t h a t t h e y allow t h e i m j ^ l e m e nt a t i o n of t h e h o s t l a n g u a g e ’s o b j e c t m o d e l t o be m a d e moi ’(' o| )en. a ll o wi n g d i f f er en t o b j e c t m o d e l s t o be s u p p o r t e d s i m u l t a n e o u s l y wi t hi n a single l an g u a g e . In a r eflective a r c h i t e c t u r e , c o d e d e al i n g w i t h t h e f u n c t i o n a l I ' c q u i r eme n t s resides a t t h e base-level w h e r e a s c o d e d e a l i n g w i t h n o n - f u n c t i o n a l recpiire- m e nt s , such as |X’r si s t e n c e a n d d i s t r i b u t i o n , resides a t a s e p a r a t e m e ta l e v e l. C o m r n u
ni cat i oi i b e t w e e n b a s e a n d me t a - l e ve l is a ch i e v e d by i n t e r c e d i n g w i t h t h e e x e c u t i o n or i n t e r | ) r e t a t i o n of l a n g u a g e o p e r a t i o n s .
H o w e ve r , d e s p i t e i t s o b v i o u s a d v a n t a g e s , reflective p r o g r a m m i n g r e m a i n s t o be a r a t h e r ne gl ec te d | > r o g r a n i mi n g p a r a d i g m a n d h a s n ’t f ou n d its w a y i n t o m a i n s t r e a m s o f t w a r e d e sign a n d i m p l e m e n t a t i o n techniciues. T h i s lack o f p o p u l a r i t y s t e m s f r o m t h e f a c t t h a t wi t h t lie a d d i t i o n a l f u n c t i o n a l i t y c o m e s c om p l e x i t y , l e a di n g t o tlie p e r c e p t i o n t h a t niet a-l evel ]:>rogramming is h a r d a n d c o m p l i c a t e d . In f ac t , o u r view is t h a t me ta -l e v e l p r o g r a m m i n g is n o t t r i vi al a n d r eq ui r e s a t h o r o u g h u n d e r s t a n d i n g o f t h e s e m a n t i c s o f b o t h t h e u n d e r lyi ng o b j e c t m o d e l a n d t h e n o n - f u n c t i o n a l r e q u i r e m e n t s t h a t a r e t o be e m b e d d e d i n t o t h e l a n g u a g e ’s o b j e c t m od e l . O n t h e o t h e r h a n d , o n ce t h i s s e p a r a t i o n h a s been a c h i e ve d , t h e le.ss e x p e r i e n c e d base-level p r o g r a m m e r c a n g a i n f r om t h e cl ean s e p a r a t i o n of c o n c e r n s a nd s h o u l d be a b l e t o a p p l y t h e e x t e n d e d o b j e c t b e h a v i o u r w i t h o u t o t h e r w i s e b e i n g c o n c e r n e d a b o u t t h e e x i s t e n c e of t h e me t a -l e v e l a r cl i i t e ct u r e.
W h a t is t h e r e f o r e n e e de d is a n e w level o f a b s t r a c t i o n t h a t e n c a p s u l a t e s m o s t o f t h e ini])le- m e i i t a t i o n d e t a i l s o f reflective pi’o g r a m m i n g for bot h t h e b a s e a n d me ta -l ev e l j i r o g r a m m e r . I'o d r a w an a n al o g y, t h e conce])t o f i n h e r i t a n c e in OOPLs p r ov i d es a power f ul t o o l for e n c a p s u l a t i n g a n d c o m b i n i n g s o f t w a r e c o m p o n e n t s . I n h e r i t a n c e as a conce| )t wo u l d n o t have been s uccessful if p r o g r a m m e r s w e r e n ’t sufficiently shi el ded f r om its i m p l e m e n t a t i o n d e tails. so for e x a m] ) le if t h e y h a d t o ex])licitly acce.ss attd initialise v i r t u a l f u n c t i o n t a b l e s . I nfort uiiately, t h i s is e x a c t l y w h a t a n u m b e r o f reflective p r o g r a m m i n g l a n g u a g e s reciuire. n a m e l y t h e di r e c t anti ex])licit a c c e s s of met a- l evel i n f o r m a t i o n .
In oi'dc'r t o a d d r e s s t h e p r ol j l ems d e s c r i b e d a bo v e , w’e a d v o c a t e a s t r o n g s e ] )a r at i o n of t h e roles of base-level (or aj^plication | ) r o g r a m m e r ) a n d me t a -l e v e l p r o g r a m m e r . T h e l)ase-level p r o g r a m m e r s h o u l d |>rimarily be c o n c e r n e d w i t h t h e i m p l e m e n t a t i o n of t h e f u n c t i o n a l re q u i r e m e n t s o f an a p| ) l ic a t i o n a n d s h o u l d t o a g r e a t e s t e x t e n t be u n a w a r e of t h e e x i s t e n c e of t h e I'ellective l a n g u a g e e x t e n s i o n . 'Idie ( m o r e e x p e r i e n c ed ) me t a - l e ve l p r o g r a m m e r s ho u l d l)e a bl e t o p r o v i d e t h e n o n - f u n c t i o n a l re c i ui re men t s i n d e | ) e n d e n t o f t h e a c t u a l base-level c o d e . . \ s an i n t e r f a c e for b o t h t h e a p p l i c a t i o n a n d me t a - l e v el p r o g r a m m e r , we i n t r o d u c e d t h e c o n c e p t o f a m e t a - t y p e . F r om t h e a])pl i cat i on p r o g r a m m e r p o i n t of view, m e t a - t y p e s e n c a | ) s u l a t e m o s t o f t h e f u n c t i o n a l i t y of reflective p r o g r a m m i n g a n d c o n s t i t u t e s i m p l y c o i i q ) o n e n t s t h a t c a n be sel e c t e d i n t o a n a p p l i c a t i o n . F r o m t h e e x p e r i e n c e d niet a-l evel p r o g r a m m e r p o i n t of view, m e t a - t y p e s build a f r a me w’o r k for d e si g n i n g , c o m | ) o s i n g a nd i m p l e m e n t i n g o b j e c t mo d e l s. C o n c e p t u a l l y , m e t a - t y p e s s h o u l d be t r a n s p a r e n t , e x t e n s i b l e a n d efficient in t h e i r i m p l e m e n t a t i o n .
2.1
G eneral overview
Keflective facilities c a n be f o u n d t o a g r e a t e r o r lesser e x t e n t in m o s t o b j e c t - o r i e n t e d |>ro- g i ' a ni ni i n g l a n g ua ge s . C++ for e x a m p l e o n l y p r o v i d e s r u d i m e n t a r y i n t r o s | j e c t i o n in t h e i'orm o f r u n - t i m e ty])e i n f o r m a t i o n ( i n ' T l ) [Str91], w h e r e a s .Java is e q u i p p e d w i t h a full i nt r os ] ) ect i on AIM [Mic99]. However , m a i n s t r e a m [ )rogra. mming l a n g u a g e s o nl y p r o v i de basic reflective f e a t u r e s , in g en e r a l on l y s t r u c t u r a l r eflection, whi ch is n o t e n o u g h t o s u p - pcM't m e t a - t y p e s . T h e r e f o r e , a n u m b e r of o b j e c t - o r i e n t e d l a n g u a g e s h a v e been e x t e n d e d in or t l er t o j jrovide a m o r e r i g o r o us sup] ) or t for r eflection.
Pi ' ovi di ng full s t r u c t u r a l a n d b e h a v i o u r a l r eflection is a t a s k t h a t largel y d e p e n d s on w h e t h e r t h e u n d e r l y i n g p r o g r a m m i n g l a n g u a g e is i n t e r p r e t e d o r c o m p i l e d . In a n i n t e r p r e t e d l a n g u a g e for e x a m p l e , t h e i n t e r p r e t e r a l r e a d y c o n s t r u c t s a s u b s t a n t i a l a m o u n t of meta.-level i n f o r m a t i o n a b o u t t h e pi’o g r a m t o be i n t e r p r e t e d . E x t e n d i n g t h e l a n g u a g e t o be r('Ilective onl>' i nvolves t h e e x p o s i t i o n of t h i s me t a -l e v e l i n f o r m a t i o n a n d t h e i n t e r p r e t a t i o n m e c h a n i s m t o t h e a p p l i c a t i o n p r o g r a m m e r .
In a t y p i c a l c o m p i l e d l a n g u a g e ( m o s t n o t a b l y C+ + ) on t h e o t h e r h a n d l i ttle or no m e t a - level i n f o r m a t i o n is k e p t in t h e r u n - t i m e i m a g e . A d d i n g reflection is d o m i n a t e d by t h e pi'oljlem of m a i n t a i n i n g t h e s t r u c t u r a l i n f o r m a t i o n b e y o n d t h e c o m p i l a t i o n ])rocess a n d of ( ' x t e n d i n g t h e c o d e wi t h t h e a p i >r o p r ia t e h o o k s t o expl oi t t h e me t a - l e v e l i n f o r m a t i o n . h'igure 2.1 de])icts t h e t a x a n o m y o f r eflective p r o g r a n i i i i i n g l a n g ua ge s . D e | ) e n d i n g on 1 Ik' t>'pe of p r o g r a m m i n g l a n g u a g e , c o m p i l e d o r inter]>reted, we c an d i s t i n g u i s h b e t w e en v a r i ous t y p e s of r efh' ction: com| )il(' -time, r u n - t i m e a n d l o a d - t i m e reflection.
W’itii c o m | ) i l e - t i m e reflection, t h e comj ) il a t i o n ])rocess o f a p r o g r a m is c o n t r ol kx i by a cor- r c' spondi ng met a- l evel p r o g r a m . It c a n be vi ewed a s a kind o f ’s m a r t p r e- | ) r o c es s i n g' wiiere th(' s e m a n t i c s o f tlu' a p p l i c a t i o n c o d e a r e mo d i f i e d in a c o n t e x t s e ns i ti v e way. E x a m | ) l e s o f t h e s e a r c h i t e c t u r e s a r e ( ) p e n C + + v2 [Chi96] a n d O p e n . I a v a [Chi9-5].
H u n - t i m e reflection all ows t h e i n t e r p r e t a t i o n / e x e c u t i o n o f l a n g u a g e c o n s t r u c t s t o i)e m o d ified at r u n - t i m e . T h i s is in g en e r a l ac hi e ve d by i n t e r c e d i n g w i t h t h e e x e c u t i o n of l a n g u a g e o p e i' a t io n s . R u n - t i m e reflection offers t h e h i g h e s t d e g r e e o f flexibility b u t u su a l l y inciirs 1 he m o s t o v e r h e a d .
Object-Oriented Programming Languages
Interpreted (Java, Smalltalk, ABCL/1)
OpenC++ v2 OpenJava
Compiled (C++, Objective C)
Compile- Time
OpenC++ v1 Iguana
Metaxa Guarana ABCL/R CodA
Binary Component Adaptation (BCA) Javassist
l''ig n re 2.1: I'a x a n o in y o f re fle c tiv e , o b je c t-o rie n te d p r o g r a iiin iin g languages.
a fivanced lin k e r /lo a d e r th a t in s p e c ts m od ule s to be d y n a m ic a lly lin k e d in to a ])])lic a tio n s . Taking a p p lic a tio n specific b e h a v io u r in to a c c o u n t^, it is jio s s ib le to load specialised im p le m e n ta tio n s o f syste m ca lls such as f o r k and m a llo c .
H o a d m a p
In th e re m a in d e r o f th is c h a |)te r we w ill firs t b rie fly review som e o f th e m ore co m m o n o b je c t-o rie n te d ])ro g ra m m in g languages and discuss th e in h e re n t re fle c tiv e fe a tu re s th e y pi'ovid e. 'Then we w ill e x a m in e in t \irn a n u m b e r o f re fle c tiv e e x te n s io n s u n d e r th e fo llo w in g aspects:
T y p e o f r e f l e c t i o n : D escribes w h e th e r th e |)la tfo rm |)ro vid e s c o m p ile -tim e , r u n -tim e or lo a d -tim e re fle c tio n .
R e f le c t iv e f a c i li t i e s : T h is section describes w h ich fe a tu re s o f th e la n gu ag e are reified and how th e y can be accessed.
P r o g r a m m i n g m o d e l: 'T his se ction describes th e user in te rfa c e ]>rovided by each la n guage. M o re s p e c ific a lly , it describes how th e p ro g ra m m e r can d efin e a new m o p im p le m e n ta tio n and how it can be a ])p lie d to th e a p ])lic a tio n .
P e r f o r m a n c e : In t h i s s ec t i on we d i s c us s t h e p e r f o r m a n c e t r ad e -o f fs whi ch a r e i n c u r r e d w i t h t h e use o f reflection.
K n o w n a p p l i c a t i o n s : Descri l j es c o n c r e t e a j j p l i c at i o n s t h a t h a v e b e e n w r i t t e n in t h e s p e cific l a n g u a g e .
S u p p o r t f o r m e t a - t y p e s : 13iscusses t o w h a t e x t e n t t h e l a n g u a g e s u p p o r t s t h e c o n c e p t o f n i e t a - t y p e s . M o r e specifically, we d i s c u s s w h e t h e r t h e reflective l a n g u a g e e x t e n s i o n a ll ows t o c o m p o s e a n d d y n a m i c a l l y reselect niet a-l evel c o n f i g u r a t i o n s , h o w t r a n s p a r e n t l y t h e e x t e n s i o n c a n be a pp l i e d a n d w h e t h e r t y p e o r t h o g o n a l i t y is a c h i e v e d .
2.2
S m a llta lk
S m a l l t a l k [GRS.'j] w a s d e ve l o p e d in t h e Xe ro x P a l o A l t o R e s e a r c h C e n t e r a n d w a s first released in 1980. It is an inter|M’e t e d , fully o b j e c t - o r i e n t e d p r o g r a m m i n g l a n g u a g e f e a t u r i n g w e a k t y p i n g , m u l t i p l e i n h e r i t a n c e a n d a u t o m a t i c g a r b a g e col l ect i on).
I n h e r e n t r e f l e c t i v e f e a t u r e s
S m a l l t a l k c o n t a i n s m a n y e l e m e n t s t h a t c an o n l y be d e s c r i b e d as reflective. I ' h i s can lie s('('n f r om its conseciuent- u n d e r l y i n g p r o g r a m m i n g p a r a d i g m : e v e r y t h i n g is an o b j e c t . C l a s s e s in S m a l l t a l k a r e first, c l ass e nt i t i e s , i n s t a n c e s o f a s u b c l a s s o f Met aCl ass. l i v er y met acla.ss has e x a c t l y o n e i n s t a n c e , n a m e l y t h e class o f t h e s a m e n a m e .
riie class i n t e r f a c e de f i n e s a n u m b e r of m e t h o d s t h a t m a k e it v e r y e a s y t o i n t r o s p e c t on cl asses a n d m e t h o d s , a s t h e f ol lowing e x a m p l e i l l us tr at e s .
[ 0 1 ] l i s t M e t h o d s : t a r g e t O b j e c t [ 0 2 ] I c l m I
[ 0 3 ] c l := t a r g e t O b j e c t c l a s s . [ 0 4 ] m := c l m e t h o d D i c t i o n a r y . [ 0 5 ] m i n s p e c t .
f i g u r e 2.2'. I n t r o s p e c t i o n in S m a l l t a l k : t h e f u n c t i o n l i s t M e t h o d s lists all m e t h o d s of t h e o b j e c t t a r g e t O b j e c t .
objeci targetObject. f i r s t , t he class ( ni ct a- ) obj ec t is looked u|) an d stored (line [0.'3]). 'I’lie me t h od methodDictionary c o m p u t e s an orde red collection of all m e t h o d s of t h a t class, which is finally printed o u t in line [05].
[l'.J89] discusses how fur the r reflective features, such as cust omi sed m e t h o d di spat chi ng, could be i nc or por at ed into Smalllalk.
2. 3
C + +
('++ is an object- oriented extension of the C p r o g r a m m i n g l anguage and was designed l)y Bj arne S t r o u s t r u p . it was s t an da r d i se d by t he ISO C++ s t a n d a r d s c o m m i t t e e in 1997. Because it originated from C, C++ is a hybrid language t h a t s u p p o r t s t he full r ange of low- level p ro g r a mmi ng , jjrocedural | )r ogrammi ng, obj ect -ori ent ed p r o g r a m m i n g up t o generic p r o g r a m m i n g using temi)lates.
I n l i e r e n t r e f l e c t i v e f e a t u r e s
('++ allows t he redefinition or o \ er l o a d i ug of specific oi>erators. "Fhe new o | ) e r at or for i nst ance is a special met hod t hat is called whenever a new object is to be cr ea t ed on the lu'a|). Its task is to allocate t he ap p io |) r ia t e a m o u n t of raw m e mo r y to contain t he ol)ject' s att riliutes. 'I’iie delete o|>eratoi' on t he o t he r hand is invoked when an object is deletf'd in
O l d e r to free t he memor y held by t he object.
Ov('rloadiiig t h e n e w / de le t e o| ) e r at or can be viewed as reifying t he m e m o r y m a n a g e m e n t of o b j ec t s and provides a me a ns of interceding in t h e c r e at i o n / d e l e t i o n mecha ni sm of ol)jects. riiis can l>e useful t o im|)lement g a r b a g e collection or to allow o b j ec t s t o be
l o c a t e d at specific addresses in memory. C hoices [MKIC'92] for ex a mp l e is a f r ame wor k loi' c o n s t r u c t i n g object - ori ented o | ) e r a t i n g s y st e m s t h a t uses a reified heap m a n a g e r which
c a n l)e replaced in a I ' u n n i n g s yst em. T h e code in figure 2.3 is an ex amp l e of a customi sed new o |) e ra l or t h a t |)rints out a message every ti me an o bj e c t is cr eat ed, sz c on t a i ns t he size (in bytes) of t h e object.
RTTI
t o |)ei'forin o p t i m i s e d o p e r a t i o n s on o b j e c t s , t o realise o b j e c t - o r i e n t e d d a t a b a s e s o r for d e b u g g i n g p u r p o s e s . In t h e e x a m p l e bel ow, t h e s e ri a li s a ti on o f a d a t a s t r u c t u r e c a n be iini)ieiriented in d i f fe r en t w a y s if t h e a c t u a l t y p e o f t h e s t r u c t u r e is k n ow n :
c l a s s list {
/ / i n i p l e m e n t a t i o n o f a list };
c l a s s linkecLlist : p u b l i c list {
/ / i i n p l e m e n t a t i o n o f a lin k e d list
i n t seri al i ze(l i st *ls, o s t r e a m *os) { / / d u m p a list to a n o u t p u t s t r e a m c o n s t t y p e J n f o & t i = t y p e i d ( * l s ) ; / / *ls can b( o f type list o r l i n k e d J i s t i f (ti = = t y | ) e i d ( l i n k e d _ l is t ) ) {
/ / d o l i n k e d J i s t stufj' } e l s e {
/ / do list s t u f f }
}
l']\| )loiting t y p e i n f o r m a t i o n as in t h e e x a mj j l e a b o v e h o we ve r s h o u l d be a^’oi d e d as it d e f e a t s t r a n s | ) a r e n c y a n d i n h i b i t s s o f t w a r e reuse. I n s t e a d , p o l y m o r p h i c m e m b e r f u n c t i o n s woul d be b e t t e r used t o i m p l e m e n t ty|:>e si)ecific o p e r a t i o n s .
c l a s s V e rb os e {
v o i d * o | ) e r a t o r n e w ( s i z e _ t sz){
c o u t < < ’’C a l l i ng V e r b o s e : : n e w ( ” < < sz < < ” ) ” < < endl; r e t u r n inal l oc(sz);
}
