Composite Pattern

Composite Pattern

The intent of Composite pattern is to composite objects into tree structures to represent a “part-whole” hierarchy .The Composite Pattern allow clients to treat individual objects and composite objects uniformly.

UML

Used in Qt

Examples include the QObject hierarchy, QGraphicsItem and QGraphicsScene, and so on.

Some Codes:

1.Defining the Component Interface

#include <iostream>
#include <vector>
#include <memory>// Component (Base class)
class Graphic {
public:virtual ~Graphic() = default;// Pure virtual function for drawingvirtual void draw() const = 0;// Common operation like resizingvirtual void resize(double factor) = 0;
};

2. Defining the Leaf Classes

// Leaf (Concrete) classes
class Circle : public Graphic {
private:double x, y, radius;public:Circle(double x, double y, double radius) : x(x), y(y), radius(radius) {}void draw() const override {std::cout << "Drawing Circle at (" << x << ", " << y << ") with radius " << radius << std::endl;}void resize(double factor) override {radius *= factor;}
};class Square : public Graphic {
private:double sideLength;public:Square(double sideLength) : sideLength(sideLength) {}void draw() const override {std::cout << "Drawing Square with side length " << sideLength << std::endl;}void resize(double factor) override {sideLength *= factor;}
};

3、Defining the Composite Class

// Composite class
class CompositeGraphic : public Graphic {
private:std::vector<std::shared_ptr<Graphic>> children;public:// Add a child graphicvoid add(std::shared_ptr<Graphic> graphic) {children.push_back(graphic);}// Remove a child graphicvoid remove(std::shared_ptr<Graphic> graphic) {children.erase(std::remove(children.begin(), children.end(), graphic), children.end());}// Draw all childrenvoid draw() const override {for (const auto& child : children) {child->draw();  // Recursively draw all children}}// Resize all childrenvoid resize(double factor) override {for (const auto& child : children) {child->resize(factor);  // Recursively resize all children}}
};

4.Client Code

int main() {// Create some leaf objectsstd::shared_ptr<Graphic> circle1 = std::make_shared<Circle>(0.0, 0.0, 5.0);std::shared_ptr<Graphic> circle2 = std::make_shared<Circle>(2.0, 3.0, 3.0);std::shared_ptr<Graphic> square = std::make_shared<Square>(4.0);// Create a composite objectstd::shared_ptr<CompositeGraphic> compositeGraphic = std::make_shared<CompositeGraphic>();compositeGraphic->add(circle1);compositeGraphic->add(circle2);compositeGraphic->add(square);// Draw all graphics (including leaves and composites)std::cout << "Drawing the entire graphic collection:" << std::endl;compositeGraphic->draw();// Resize all graphics (including leaves and composites)compositeGraphic->resize(1.5);std::cout << "\nAfter resizing:" << std::endl;compositeGraphic->draw();return 0;
}