bìa giảng chương trình dịch design pattern visitor

Design Pattern: Visitor Ph.D. Nguyen Hua Phung Faculty of CSE, HCMUT 2007 Faculty of CSE, HCMUT Visitor design pattern 2 Motivation • Separate an algorithm from an object structure. A tree structure Task A Task B Task C Faculty of CSE, HCMUT Visitor design pattern 3 Example abstract class Expr { } class BinExpr extends Expr { Expr e1, e2; char op; } class VarExpr extends Expr { Token id; } class IntLiteral extends Expr { Token lit; } (5 + b) * a BinExpr BinExpr VarExpr * IntLiteral VarExpr + 5 b a e1 e2 e1 e2 lit id id Faculty of CSE, HCMUT Visitor design pattern 4 Task 1: Translate to postfix • Imperative programming trans2post(Expr e) { if (e instanceof BinExpr) { BinExpr be = (BinExpr)e; trans2post(be.e1); trans2post(be.e2); System.out.print(be.op); } else if (e instanceof VarExpr) { VarExpr ve = (VarExpr) e; System.out.print(ve.id.Lexeme); } else { IntLiteral ie = (IntLiteral) e; System.out.print(ie.lit.Lexeme); } } BinExpr BinExpr VarExpr * IntLiteral VarExpr + 5 b a e1 e2 e1 e2 lit id id 5 b + a *(5 + b) * a Faculty of CSE, HCMUT Visitor design pattern 5 Task 1: Translate to postfix (cont’d) • Object-oriented programming abstract class Expr { abstract void trans2post(); } class BinExpr extends Expr { Expr e1,e2; char op; void trans2post() { e1.trans2post(); e2.trans2post(); System.out.print(op); } } class VarExpr extends Expr { Token id; void trans2post() { System.out.print( id.Lexeme); } } class IntLiteral extends Expr { Token lit; void trans2post() { System.out.print( lit.Lexeme); } } Faculty of CSE, HCMUT Visitor design pattern 6 Add New Task: Type checking • Object-oriented programming abstract class Expr { abstract void trans2post(); abstract void typechecking(); } class BinExpr extends Expr { Expr e1,e2; char op; void trans2post() { } void typechecking() { } } need to modify code when adding more tasks Faculty of CSE, HCMUT Visitor design pattern 7 Design Pattern: Visitor A Task 1 Task 2 B Task 1 Task 2 C Task 1 Task 2 A B C Task 1 Task 2 Visitor for A for B for C for A for B for C Faculty of CSE, HCMUT Visitor design pattern 8 Task 1: Translate to postfix (cont’d) • Visitor design pattern abstract class Expr { abstract void accept(Visitor v); } class BinExpr extends Expr { Expr e1,e2; char op; void accept(Visitor v) { v.visitBE(this); }} class VarExpr extends Expr { Token id; void accept(Visitor v) { v.visitVE(this); }} abstract class Visitor { abstract void visitBE(BinExpr e); abstract void visitVE(VarExpr e); abstract void visitIL(IntLiteral e); } class In2Pos extends Visitor { void visitBE(BinExpr e) { e.e1.accept(this); e.e2.accept(this); System.out.print(e.op); } void visitVE(VarExpr e) { System.out.print( e.id.Lexeme); } } Faculty of CSE, HCMUT Visitor design pattern 9 Task 1: Translate to postfix (cont’d) class Application { public static main() { Expr e = ; Visitor v = new In2Pos(); e.accept(v); BinExpr BinExpr VarExpr * IntLiteral VarExpr + 5 b a e1 e2 e1 e2 lit id id BinExpr.accept(v) In2Pos.visitBE(BinExpr e) e.e1.accept(v); e.e2.accept(v); System.out.print(op); BinExpr.accept(v) In2Pos.visitBE(BinExpr e) e.e1.accept(v); e.e2.accept(v); System.out.print(op); IntLiteral.accept(v) In2Pos.visitIL(IntLiteral e) VarExpr.accept(v) In2Pos.visitVE(VarExpr e) 5 b + a * Faculty of CSE, HCMUT Visitor design pattern 10 Add New Task: Type checking abstract class Expr { abstract void accept(Visitor v); } class BinExpr extends Expr { Expr e1,e2; char op; void accept(Visitor v) { v.visitBE(this); }} class VarExpr extends Expr { Token id; void accept(Visitor v) { v.visitVE(this); }} abstract class Visitor { abstract void visitBE(BinExpr e); abstract void visitVE(VarExpr e); abstract void visitIL(IntLiteral e); class In2Pos extends Visitor { } class TypeCheck extends Visitor { void visit(BinExpr e) { } void visit(VarExpr e) { }} class Application { public static main() { Expr e = ; Visitor v = new TypeCheck(); e.accept(v); // Type check v = new In2Pos(); e.accept(v); // Translate to postfix [...]...Summary • Visitor design pattern is used to separate an algorithm from a data structure • It is used to implement many algorithms on the same data • Useful to implement type checker, code generation • Less useful at development stage when some new classes might be added Faculty of CSE, HCMUT Visitor design pattern 11 . HCMUT Visitor design pattern 7 Design Pattern: Visitor A Task 1 Task 2 B Task 1 Task 2 C Task 1 Task 2 A B C Task 1 Task 2 Visitor for A for B for C for A for B for C Faculty of CSE, HCMUT Visitor. Design Pattern: Visitor Ph.D. Nguyen Hua Phung Faculty of CSE, HCMUT 2007 Faculty of CSE, HCMUT Visitor design pattern 2 Motivation • Separate an algorithm. B for C Faculty of CSE, HCMUT Visitor design pattern 8 Task 1: Translate to postfix (cont’d) • Visitor design pattern abstract class Expr { abstract void accept (Visitor v); } class BinExpr extends