Motivation. Synthetic OOD concepts and reuse Lecture 4: Separation of concerns. Problem. Solution. Deleting composites that share parts. Or is it?

Transcription

1 Synthtic OOD concpts and rus Lctur 4: Sparation of concrns Topics: Complx concrn: Mmory managmnt Exampl: Complx oprations on composit structurs Problm: Mmory laks Solution: Rfrnc counting Motivation Suppos w dfin a function: Expr* mkexpr(void) Expr* l1 = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l1,v); Expr* l2 = nw Litral(2); Expr* t2 = nw Multiply(l2,t1); Problm Solution Suppos a clint program xcuts th cod: Expr* = mkexpr(); dlt ; What happns? How could w nsur that whn a composit objct is dltd, its parts ar also dltd? class Expr virtual ~Expr() ; class BinaryExpr : public Expr ~BinaryExpr() dlt(lftoprand); dlt(rightoprand); ; Or is it? Oftn dsirabl for two or mor composit objcts to shar th sam componnt Exampl: Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw Hr, objct l is th child of two composit objcts, t1 and t2. Will our mmory-rclamation schm work now? Dlting composits that shar parts Not saf for dstructor of a composit class to dlt its childrn if ths objcts might b childrn of multipl composit objcts Solution: dsign a protocol that tracks rfrncs to objcts and that dlts an objct only whn its rfrnc count rachs 0. Clints nvr call dlt Objcts call dlt on thmslvs! Clints must xplicitly notify objcts whn rfrncs to ths objcts ar assignd or ovrwrittn 1

2 Abstract rf-counting class class RCObjct public: void addrf() ++rfcount; void rmovrf() if(--rfcount <= 0) dlt this; protctd: RCObjct() : rfcount(0) virtual ~RCObjct() privat: int rfcount; ; Exampl Modify class Expr as follows: class Expr : public RCObjct ; Subsquntly, vry class that drivs from Expr will inhrit rfrnc counting infrastructur Exampl Modify BinaryExpr as follows: class BinaryExpr : public Expr BinaryExpr( Expr* l, Expr* r ) : lftoprand(l), rightoprand(r) l->addrf(); r->addrf(); ~BinaryExpr() l->rmovrf(); r->rmovrf(); ; Exampl (continud) Now w writ a function that crats an xprssion: Expr* cratexprssion(void) Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw rturn t2; Rquirs clint to incrmnt rfcount of root objct (pointd to hr by t2 bfor it gos out of scop) Stp 1 Stp 2 Expr* cratexprssion(void) Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw rturn t2; Expr* cratexprssion(void) Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw rturn t2; 2

3 Stp 3 Stp 4 Expr* cratexprssion(void) Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw rturn t2; Expr* cratexprssion(void) Expr* l = nw Litral(2); Expr* v = nw Variabl( x ); Expr* t1 = nw Add(l,v); Expr* t2 = nw rturn t2; rfcount=2 Exampl (continud) Clint cod thn uss as follows: Expr* = cratexprssion(); // nw lin // rathr than dlt() Adding rfrnc to rturnd objct rfcount=2 Printing xprssion Rmoving rf: Stp 1 rfcount=2 3

4 Rmoving rf: Stp 2 Rmoving rf: Stp 3 Rmoving rf: Stp 4 Rmoving rf: Stp 5 Rmoving rf: Stp 6 Rmoving rf: Stp 7 4

5 Rmoving rf: Stp 8 Concptual (but flawd) altrnativ implmntation of cratexprssion() Expr* cratexprssion(void) Expr* l = nw Litral(2); l->addrf(); Expr* v = nw Variabl( x ); v->addrf(); Expr* t1 = nw Add(l,v); t1->addrf(); Expr* t2 = nw t2->addrf(); l->rmovrf(); v->rmovrf(); t1->rmovrf(); t2->rmovrf(); // What happns hr??? rturn t2; Mixin classs RCObjct is an xampl of a mixin class So-calld bcaus dsignd to ncapsulat orthogonal functionality that may b mixd into classs in an xisting hirarchy Mixin classs ar always abstract May us multipl inhritanc to incorporat functionality in a mixin class if bas class dos not nd it or if bas class nds functionality from multipl mixin classs 5