The Composite State Visitor Pattern

Transcription

1 Design Patterns for Data Structures Chapter 8 he Composite State Visitor Pattern

2 Design Patterns for Data Structures Chapter 8 he Composite State Binary ree with the Visitor Pattern BireeCSV

3 Design Patterns for Data Structures Chapter 8 he host system provides he solution a minimal complete set of operations that any plugin developer can use to build the desired plugin an abstract visitor class with methods for the plugin developer to implement a public method named accept() for the end user to execute

4 Design Patterns for Data Structures Chapter 8 he plugin developer writes a plugin by declaring a visitor to be a subclass of the abstract visitor class implements the visitor methods that the abstract visitor class provides

5 Design Patterns for Data Structures Chapter 8 he plugin user instantiates a specific visitor object calls accept(), passing the visitor object as a parameter he host executes accept(), which, in turn, executes the methods implemented by the plugin developer.

6 BireeCSV _root: AcsvNode<> * AcsvNode Host system BireeCSV(rhs: BireeCSV<> const &) BireeCSV(node: AcsvNode<> *) + BireeCSV( ) + ~BireeCSV( ) + accept(visitor: ABireeCSVVis<> &) + accept(visitor: ABireeCSVVis<> &) const copyroot(rhs: BireeCSV<> const &): AcsvNode<> * + insertroot(data: const &) + left( ): BireeCSV<> & + left( ) const: BireeCSV<> const & + operator=(rhs: BireeCSV const &): BireeCSV & + remroot( ): + right( ): BireeCSV<> & + right( ) const: BireeCSV<> const & + root( ): & + root( ) const: const & + setree(tree: BireeCSV<> &) + ~AcsvNode() # accept(owner: BireeCSV<> &, visitor: ABireeCSVVis<> &) # accept(owner: BireeCSV<> const &, visitor: ABireeCSVVis<> &) copyroot(): AcsvNode * # insertroot(owner: BireeCSV<> &, data: const &) # left( ): BireeCSV<> & # left( ) const: BireeCSV<> const & # remroot(owner: BireeCSV<> &): # right( ): BireeCSV<> & # right( ) const: BireeCSV<> const & # root( ): & # root( ) const: const & # setree(owner: BireeCSV<> &, tree: BireeCSV<> &) NEcsNode McsNode _left: BireeCS<> _data: _right: BireeCS<> McsNode( ) NEcsNode(data: const &) NEcsNode(leftNode: AcsNode<> *, data:, rightnode: AcsNode<> *) NEcsNode(rhs: const NEcsNode<> *) <<use>> ABireeCSVVis + ~ABireeCSVVis() Plugin + emptycase(host: BireeCSV<> &) + nonemptycase(host: BireeCSV<> &) + emptycase(host: BireeCSV<> const &) + nonemptycase(host: BireeCSV<> const &) BiCSVisEmptyVis BiCSVclearVis BiCSVmaxVis _result: bool + BiCSVclearVis( ) _result: const * + BiCSVisEmptyVis( ) + result( ) const: bool + BiCSVmaxVis( ) + result( ) const: const &

7 Design Patterns for Data Structures Figure 8.41 BireeCSV _root: AcsvNode<> * AcsvNode BireeCSV(rhs: BireeCSV<> const &) BireeCSV(node: AcsvNode<> *) + BireeCSV( ) + ~BireeCSV( ) + accept(visitor: ABireeCSVVis<> &) + accept(visitor: ABireeCSVVis<> &) const copyroot(rhs: BireeCSV<> const &): AcsvNode<> * + insertroot(data: const &) + left( ): BireeCSV<> & + left( ) const: BireeCSV<> const & + operator=(rhs: BireeCSV const &): BireeCSV & + remroot( ): + right( ): BireeCSV<> & + right( ) const: BireeCSV<> const & + root( ): & + root( ) const: const & + setree(tree: BireeCSV<> &) + ~AcsvNode() # accept(owner: BireeCSV<> &, visitor: ABireeCSVVis<> &) # accept(owner: BireeCSV<> const &, visitor: ABireeCSVVis<> &) copyroot(): AcsvNode * # insertroot(owner: BireeCSV<> &, data: const &) # left( ): BireeCSV<> & # left( ) const: BireeCSV<> const & # remroot(owner: BireeCSV<> &): # right( ): BireeCSV<> & # right( ) const: BireeCSV<> const & # root( ): & # root( ) const: const & # setree(owner: BireeCSV<> &, tree: BireeCSV<> &) NEcsNode McsNode _left: BireeCS<> _data: _right: BireeCS<> McsNode( ) NEcsNode(data: const &) NEcsNode(leftNode: AcsNode<> *, data:, rightnode: AcsNode<> *) NEcsNode(rhs: const NEcsNode<> *)

8 Design Patterns for Data Structures Figure 8.41 <<use>> + ~ABireeCSVVis() ABireeCSVVis + emptycase(host: BireeCSV<> &) + nonemptycase(host: BireeCSV<> &) + emptycase(host: BireeCSV<> const &) + nonemptycase(host: BireeCSV<> const &) BiCSVisEmptyVis BiCSVclearVis BiCSVmaxVis _result: bool + BiCSVclearVis( ) _result: const * + BiCSVisEmptyVis( ) + result( ) const: bool + BiCSVmaxVis( ) + result( ) const: const &

9 Design Patterns for Data Structures BireeCSV Primitive methods he primitive methods for BireeCSV are implemented exactly as they are for BireeCS.

10 Design Patterns for Data Structures Figure 8.42 // ========= accept ========= template<class > void BireeCSV<>::accept(ABireeCSVVis<> &visitor) { _root->accept(*this, visitor); template<class > void McsvNode<>::accept(BireeCSV<> &owner, ABireeCSVVis<> &visitor) { visitor.emptycase(owner); template<class > void NEcsvNode<>::accept(BireeCSV<> &owner, ABireeCSVVis<> &visitor) { visitor.nonemptycase(owner);

11 Design Patterns for Data Structures Figure 8.42 // ========= accept ========= template<class > void BireeCSV<>::accept(ABireeCSVVis<> &visitor) { _root->accept(*this, visitor); template<class > void McsvNode<>::accept(BireeCSV<> &owner, ABireeCSVVis<> &visitor) { visitor.emptycase(owner); template<class > void NEcsvNode<>::accept(BireeCSV<> &owner, ABireeCSVVis<> &visitor) { visitor.nonemptycase(owner);

12 Design Patterns for Data Structures BireeCSV he minimal complete set of methods o access the parts of a binary tree root() left() right() o build a binary tree insertroot() setree() o trim a binary tree remroot()

13 Design Patterns for Data Structures Chapter 8 he signature mechanism A plugin is a subclass of an abstract visitor. It is free to add any number of attributes of any type that are not present in its superclass. he attributes in a visitor correspond to the parameters in a non-plugin method.

14 Design Patterns for Data Structures BireeCSV Visitor translation techniques Input parameters in the native method are attributes in the visitor initialized in the constructor for the visitor. Output parameters in the native method are reference variable attributes in the visitor. Nonvoid methods in the native class require an additional method named result() that returns the same type.

15 Design Patterns for Data Structures BireeCSV BiCSVisEmptyVis he BireeCS version of isempty() is non-void. herefore, the visitor version provides a result() method.

16 Design Patterns for Data Structures BireeCSV o use a visitor for a nonvoid method: Declare a local visitor. Pass it as a parameter in the data structure s accept() method. Call the visitor s result() method to get the result of the computation.

17 Design Patterns for Data Structures Figure 8.43 // ========= BiCSVisEmptyVis ========= template<class > class BiCSVisEmptyVis : public ABireeCSVVis<> { private: bool _result; // Output result. public: // ========= Constructor ========= BiCSVisEmptyVis() { // ========= visit ========= void emptycase(bireecsv<> &host) override { _result = true; void nonemptycase(bireecsv<> &host) override { _result = false;

18 Design Patterns for Data Structures Figure 8.43 ; // ========= result ========= // Pre: his visitor has been accepted by a host tree. // Post: true is returned if the host tree is empty; // otherwise, false is returned. bool result() const { return _result; // Global function for convenience template<class > bool isempty(bireecsv<> const &tree) { BiCSVisEmptyVis<> isemptyvis; tree.accept(isemptyvis); return isemptyvis.result();

19 Design Patterns for Data Structures BireeCSV General translation rules Use host.left() in a BireeCSV visitor to correspond to _left in a BireeCS nonempty node. Use host.root() in a BireeCSV visitor to correspond to _data in a BireeCS nonempty node. Use host.right() in a BireeCSV visitor to correspond to _right in a BireeCS nonempty node.

20 Design Patterns for Data Structures BireeCSV BiCSVpreOrderVis he BireeCS version of preorder() is void. herefore, the visitor version does not provide a result() method. he BireeCS version has output parameter os, so the visitor version has reference variable attribute _os.

21 Design Patterns for Data Structures Figure 8.44 // ========= BiCSVpreOrderVis ========= template<class > class BiCSVpreOrderVis : public ABireeCSVVis<> { private: ostream &_os; // Input paramter. public: // ========= Constructor ========= BiCSVpreOrderVis(ostream &os): _os(os) { // ========= visit ========= // Pre: his visitor has been accepted by a host tree. // Post: A preorder representation of this tree is sent to os. void emptycase(bireecsv<> &host) override { ; void nonemptycase(bireecsv<> &host) override { _os << host.root() << ; host.left().accept(*this); host.right().accept(*this);

22 Design Patterns for Data Structures Figure 8.44 // Global function for convenience template<class > void preorder(ostream &os, BireeCSV<> const &tree) { BiCSVpreOrderVis<> preordervis(os); tree.accept(preordervis);

23 Design Patterns for Data Structures Figure 8.45 // ========= BiCSVnumNodesVis ========= template<class > class BiCSVnumNodesVis : public ABireeCSVVis<> { private: int _result; // Output result. public: // ========= Constructor ========= BiCSVnumNodesVis(): _result(0) { // ========= visit ========= void emptycase(bireecsv<> &host) override { void nonemptycase(bireecsv<> &host) override { host.left().accept(*this); host.right().accept(*this); _result++;

24 Design Patterns for Data Structures Figure 8.45 ; // ========= result ========= // Pre: his visitor has been accepted by a host tree. // Post: he number of nodes of the host tree is returned. int result() const { return _result; // Global function for convenience template<class > int numnodes(bireecsv<> const &tree) { BiCSVnumNodesVis<> numnodesvis; tree.accept(numnodesvis); return numnodesvis.result();

25 Design Patterns for Data Structures Figure 8.46 // ========= BiCSVequalsVis ========= template<class > class BiCSVequalsVis : public ABireeCSVVis<> { private: BireeCSV<> const &_rhs; // Input parameter. bool _result; // Output result. public: // ========= Constructor ========= BiCSVequalsVis(BireeCSV<> const &rhs): _rhs(rhs) { // ========= visit ========= void emptycase(bireecsv<> &host) override { cerr << BiCSVequalsVis: Exercise for the student. << endl; throw -1; void nonemptycase(bireecsv<> &host) override { cerr << BiCSVequalsVis: Exercise for the student. << endl; throw -1;

26 Design Patterns for Data Structures Figure 8.46 ; // ========= result ========= // Pre: his visitor has been accepted by a host tree. // Post: true is returned if the host tree is equal to rhs; // otherwise, false is returned. // wo trees are equal if they contain the same number of // equal elements with the same shape. bool result() const { cerr << BiCSVequalsVis: Exercise for the student. << endl; throw -1; // Global function for convenience. // ========= operator== ========= template<class > bool operator==(bireecsv<> const &lhs, BireeCSV<> const &rhs) { cerr << operator==: Exercise for the student. << endl; throw -1;

27 Design Patterns for Data Structures Figure 8.47 rhs _left _data _right rhs (a) he environment of a BireeCS empty node. (b) he environment of a BireeCS nonempty node. host _rhs _result (c) he environment of a BireeCSV helper visitor.

28 Design Patterns for Data Structures Figure 8.48 // ========= BiCSVequalsHelperVis ========= template<class > class BiCSVequalsHelperVis : public ABireeCSVVis<> { private: const &_data; // Input parameter. BireeCSV<> const &_left; // Input parameter. BireeCSV<> const &_right; // Input parameter. bool _result; // Output result. public: // ========= Constructor ========= BiCSVequalsHelperVis ( const &data, BireeCSV<> const &left, BireeCSV<> const &right): _data(data), _left(left), _right(right) { // ========= visit ========= void emptycase(bireecsv<> &host) override { cerr << BiCSVequalsHelperVis: Exercise for the student. << endl; throw -1; void nonemptycase(bireecsv<> &host) override { cerr << BiCSVequalsHelperVis: Exercise for the student. << endl; throw -1;

29 Design Patterns for Data Structures Figure 8.48 ; // ========= result ========= // Pre: his visitor has been accepted by a host tree. // Post: true is returned if root equals host.root(), // left equals host.left(), and right equals host.right(); // otherwise, false is returned. bool result() const { cerr << BiCSVequalsHelperVis: Exercise for the student. << endl; throw -1;

30 Design Patterns for Data Structures Figure 8.49 _left _data _right data left right (a) he environment of a BireeCS helper empty node. (b) he environment of a BireeCS helper nonempty node. host _left _data _right _result (c) he environment of a BireeCSV helper visitor.

31 Design Patterns for Data Structures Figure 8.50 he clear() operation for BireeCSV host host host host Empty node Empty node Empty node Empty node (a) Original. (b) Clear left. (c) Clear right. (d) Remove root.