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Master Software Design Patterns and Architecture in C++

Composite Design Pattern Example

Learn the composite design pattern with some coding examples.

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Example

In the following example, we have defined a Component named Grafik, which is implemented by GrafikKompositum, a composite object, and Ellipse, a leaf object. First, we add some leaf objects to the composite object and then print it. How will we do it? Let’s have a look at the code and its explanation.

C++
#include <iostream>
#include <set>
class Grafik {
public:
    virtual void print() const = 0;
    virtual ~Grafik() {} 
};
class GrafikKompositum : public Grafik {
    std::set<Grafik const*> children;
    typedef std::set<Grafik const*>::const_iterator grIter;
public:
    void print() const {
        for (grIter it = children.begin(); it != children.end(); it++) (*it)->print();
    }
    void add(Grafik const* component) {
        children.insert(component);
    }
    void remove(Grafik const* component) {
        children.erase(component);
    }
};
class Ellipse: public Grafik {
public:
    void print() const {
        std::cout << "Ellipse\n";
    }
};
int main(){
    Ellipse ellipse1, ellipse2, ellipse3, ellipse4;
    GrafikKompositum grafik1, grafik2, grafikGesamt;
    grafik1.add(&ellipse1);
    grafik1.add(&ellipse2);
    grafik1.add(&ellipse3);
    grafik2.add(&ellipse4);
    grafikGesamt.add(&grafik1);
    grafikGesamt.add(&grafik2);
    grafikGesamt.print();
}

Code explanation

Let’s explain the code above line ...