id 1763797213

CMakeLists.txt

@@ -103,6 +103,7 @@ set(APP_SOURCES
     "src/patterns/behavioral/Iterator.cpp"
     "src/patterns/behavioral/Mediator.cpp"
     "src/patterns/behavioral/Memento.cpp"
+    "src/patterns/behavioral/Visitor.cpp"
 )
 
 # Test files

src/patterns/behavioral/Visitor.cpp

@@ -0,0 +1,284 @@
+// Visitor is a behavioral design pattern that lets you separate algorithms from the objects on which they operate.
+// Appicability:
+// (*)   when you need to perform an operation on all elements of a complex object structure (for example, an object tree, document).
+// (**)  to clean up the business logic of auxiliary behaviors.
+// (**)  when a behavior makes sense only in some classes of a class hierarchy, but not in others.
+// UML: docs/uml/patterns_behavioral_visitor.drawio.svg
+
+#include <iostream>
+#include <string>
+#include <vector>
+
+namespace
+{
+    namespace Visitor
+    {
+        class TextConcreteElement;
+        class ImageConcreteElement;
+        class TableConcreteElement;
+
+        /**
+         * The Visitor interface declares a set of visiting methods that can take concrete elements of an object structure as arguments.
+         * These methods may have the same names if the program is written in a language that supports overloading,
+         * but the type of their parameters must be different.
+         */
+        class IVisitor
+        {
+        public:
+            virtual ~IVisitor() = default;
+
+            virtual void visitText(const TextConcreteElement *t) = 0;
+            virtual void visitImage(const ImageConcreteElement *i) = 0;
+            virtual void visitTable(const TableConcreteElement *t) = 0;
+        };
+
+        /**
+         * The Element interface declares a method for “accepting” visitors.
+         * This method should have one parameter declared with the type of the visitor interface.
+         */
+        class IElement
+        {
+        public:
+            virtual ~IElement() = default;
+            // [P1]
+            // if we implement exportHtml(), exportMarkdown(), exportJson(), etc.. here,
+            // every time we add a new export format we must modify ALL element classes.
+            // This violates the Open/Closed Principle
+            // and makes each element hold multiple unrelated behaviors.
+            // => Use `Visitor Pattern` to separate export logic from the element classes.
+
+            virtual void accept(IVisitor *visitor) = 0;
+        };
+
+        /**
+         * The complex object structure
+         */
+        class DocumentConcreteStructure
+        {
+        private:
+            std::vector<IElement *> m_elements;
+
+        public:
+            ~DocumentConcreteStructure()
+            {
+                for (IElement *e : this->m_elements)
+                {
+                    delete e;
+                }
+                m_elements.clear();
+            }
+
+            void add(IElement *e)
+            {
+                m_elements.push_back(e);
+            }
+
+            std::vector<IElement *> &get()
+            {
+                return this->m_elements;
+            };
+        };
+
+        /**
+         * Each Concrete Element must implement the acceptance method.
+         * The purpose of this method is to redirect the call to the proper visitor’s method corresponding to the current element class.
+         * Be aware that even if a base element class implements this method, all subclasses must still override this method
+         * in their own classes and call the appropriate method on the visitor object.
+         */
+        class TextConcreteElement : public IElement
+        {
+        private:
+            std::string m_content;
+
+        public:
+            explicit TextConcreteElement(const std::string &c) : m_content{c} {};
+            std::string getContent() const
+            {
+                return this->m_content;
+            }
+
+            void accept(IVisitor *visitor) override
+            {
+                visitor->visitText(this);
+            }
+        };
+
+        class ImageConcreteElement : public IElement
+        {
+        private:
+            std::string m_path;
+
+        public:
+            explicit ImageConcreteElement(const std::string &p) : m_path{p} {};
+
+            std::string getPath() const
+            {
+                return this->m_path;
+            }
+
+            void accept(IVisitor *visitor) override
+            {
+                visitor->visitImage(this);
+            }
+        };
+
+        class TableConcreteElement : public IElement
+        {
+        private:
+            int m_rows, m_cols;
+
+        public:
+            TableConcreteElement(int r, int c) : m_rows{r}, m_cols{c} {};
+            int getRows() const { return this->m_rows; }
+            int getCols() const { return this->m_cols; }
+
+            void accept(IVisitor *visitor) override
+            {
+                visitor->visitTable(this);
+            }
+        };
+
+        /**
+         * Each Concrete Visitor implements several versions of the same behaviors,
+         * tailored for different concrete element classes.
+         */
+        class HtmlExportConcreteVisitor : public IVisitor
+        {
+            void visitText(const TextConcreteElement *t) override
+            {
+                std::cout << "<p>" + t->getContent() + "</p>\n";
+            }
+
+            void visitImage(const ImageConcreteElement *i) override
+            {
+                std::cout << "<img src=\"" + i->getPath() + "\" />\n";
+            }
+
+            void visitTable(const TableConcreteElement *t) override
+            {
+                std::cout << "<table>\n";
+                for (int r = 0; r < t->getRows(); ++r)
+                {
+                    std::cout << "  <tr>";
+                    for (int c = 0; c < t->getCols(); ++c)
+                    {
+                        std::cout << "<td>cell</td>";
+                    }
+                    std::cout << "</tr>\n";
+                }
+                std::cout << "</table>\n";
+            }
+        };
+
+        class JsonExportConcreteVisitor : public IVisitor
+        {
+            void visitText(const TextConcreteElement *t) override
+            {
+                std::cout << "{ \"type\": \"text\", \"content\": \"" << t->getContent() << "\" }\n";
+            }
+            void visitImage(const ImageConcreteElement *i) override
+            {
+                std::cout << "{ \"type\": \"image\", \"path\": \"" << i->getPath() << "\" }\n";
+            }
+
+            void visitTable(const TableConcreteElement *t) override
+            {
+                std::cout << "{ \"type\": \"table\", \"rows\": " << t->getRows()
+                          << ", \"cols\": " << t->getCols() << " }\n";
+            }
+        };
+
+        class MarkdownExportConcreteVisitor : public IVisitor
+        {
+            void visitText(const TextConcreteElement *t) override
+            {
+                std::cout << t->getContent() << "\n\n";
+            }
+
+            void visitImage(const ImageConcreteElement *i) override
+            {
+                std::cout << "![](" << i->getPath() << ")\n\n";
+            }
+
+            void visitTable(const TableConcreteElement *t) override
+            {
+                for (int r = 0; r < t->getRows(); ++r)
+                {
+                    for (int c = 0; c < t->getCols(); ++c)
+                    {
+                        std::cout << "| cell ";
+                    }
+                    std::cout << "|\n";
+                }
+                std::cout << "\n";
+            }
+        };
+
+        /**
+         * clients aren’t aware of all the concrete element classes because they work with objects from that collection via some abstract interface.
+         */
+        namespace Client
+        {
+            void clientCode(DocumentConcreteStructure *const doc, IVisitor *const visitor)
+            {
+                // [P2] Instead of adding exportHtml(), exportMarkdown(), exportJson() in each element,
+                // we use the Visitor pattern to separate data from behavior.
+                // This avoids bloating element classes with multiple unrelated functions
+                // and makes it easier to add new export formats without modifying the elements.
+                for (IElement *ele : doc->get())
+                {
+                    ele->accept(visitor);
+                }
+            }
+        }
+        void run()
+        {
+            DocumentConcreteStructure *document = new DocumentConcreteStructure();
+            // Add images
+            document->add(new ImageConcreteElement("header.png"));
+            document->add(new ImageConcreteElement("diagram1.png"));
+            document->add(new ImageConcreteElement("chart.png"));
+
+            // Add text paragraphs
+            document->add(new TextConcreteElement("Introduction: This document demonstrates the Visitor pattern."));
+            document->add(new TextConcreteElement("Section 1: Visitor allows adding new operations without modifying elements."));
+            document->add(new TextConcreteElement("Section 2: Elements are data holders, visitors implement behaviors."));
+
+            // Add tables
+            document->add(new TableConcreteElement(3, 4)); // e.g., 3 rows x 4 columns table
+            document->add(new TableConcreteElement(2, 2)); // smaller table
+            document->add(new TableConcreteElement(4, 3)); // another table
+
+            // Add more text
+            document->add(new TextConcreteElement("Conclusion: Using Visitor pattern makes code open for extension but closed for modification."));
+
+            IVisitor *visitor = new HtmlExportConcreteVisitor();
+            std::cout << " ===HTML Export ===\n";
+            Client::clientCode(document, visitor);
+            std::cout << " ==================\n";
+
+            visitor = new JsonExportConcreteVisitor();
+            std::cout << " ===JSON Export ===\n";
+            Client::clientCode(document, visitor);
+            std::cout << " ==================\n";
+
+            visitor = new MarkdownExportConcreteVisitor();
+            std::cout << " ===MD Export ===\n";
+            Client::clientCode(document, visitor);
+            std::cout << " ==================\n";
+            delete visitor;
+            delete document;
+        }
+    }
+}
+
+struct VisitorAutoRunner
+{
+    VisitorAutoRunner()
+    {
+        std::cout << "\n--- Visitor Pattern Example ---\n";
+        Visitor::run();
+    }
+};
+
+static VisitorAutoRunner instance;