I have found this decorator design pattern on the internet here :
// CPP program to demonstrate
// Decorator pattern
#include <iostream>
#include <string>
using namespace std;
// Component
class MilkShake
{
public:
virtual string Serve() = 0;
virtual float price() = 0;
};
// Concrete Component
class BaseMilkShake : public MilkShake
{
public:
string Serve()
{
return "MilkShake";
}
float price()
{
return 30;
}
};
// Decorator
class MilkShakeDecorator: public MilkShake
{
protected:
MilkShake *m_MilkShake;
public:
MilkShakeDecorator(MilkShake *baseMilkShake): m_MilkShake(baseMilkShake){}
string Serve()
{
return m_MilkShake->Serve();
}
float price()
{
return m_MilkShake->price();
}
};
// Concrete Decorator
class MangoMilkShake: public MilkShakeDecorator
{
public:
MangoMilkShake(MilkShake *baseMilkShake): MilkShakeDecorator(baseMilkShake){}
string Serve()
{
return m_MilkShake->Serve() + " decorated with Mango ";
}
float price()
{
return m_MilkShake->price() + 40;
}
};
class VanillaMilkShake: public MilkShakeDecorator
{
public:
VanillaMilkShake(MilkShake *baseMilkShake): MilkShakeDecorator(baseMilkShake){}
string Serve()
{
return m_MilkShake->Serve() + " decorated with Vanilla ";
}
float price()
{
return m_MilkShake->price() + 80;
}
};
int main()
{
MilkShake *baseMilkShake = new BaseMilkShake();
cout << "Basic Milk shake \n";
cout << baseMilkShake -> Serve() << endl;
cout << baseMilkShake -> price() << endl;
MilkShake *decoratedMilkShake = new MangoMilkShake(baseMilkShake);
cout << "Mango decorated Milk shake \n";
cout << decoratedMilkShake -> Serve() << endl;
cout << decoratedMilkShake -> price() << endl;
delete decoratedMilkShake;
decoratedMilkShake = new VanillaMilkShake(baseMilkShake);
cout << "Vanilla decorated Milk shake \n";
cout << decoratedMilkShake -> Serve() << endl;
cout << decoratedMilkShake -> price() << endl;
delete decoratedMilkShake;
delete baseMilkShake;
return 0;
}
But I am wondering: why should the decorator class exist? If I delete it and use only the concrete decorators than everything still works as before. The only downside I see is the fact that I have to use a private pointer to MilkShake in the concrete decorators.
// CPP program to demonstrate
// Decorator pattern
#include <iostream>
#include <string>
using namespace std;
// Component
class MilkShake
{
public:
virtual string Serve() = 0;
virtual float price() = 0;
};
// Concrete Component
class BaseMilkShake : public MilkShake
{
public:
string Serve()
{
return "MilkShake";
}
float price()
{
return 30;
}
};
class MangoMilkShake: public MilkShake
{
private:
MilkShake *m_MilkShake;
public:
MangoMilkShake(MilkShake *baseMilkShake): m_MilkShake(baseMilkShake){}
string Serve()
{
return m_MilkShake->Serve() + " decorated with Mango ";
}
float price()
{
return m_MilkShake->price() + 40;
}
};
class VanillaMilkShake: public MilkShake
{
private:
MilkShake *m_MilkShake;
public:
VanillaMilkShake(MilkShake *baseMilkShake): m_MilkShake(baseMilkShake){}
string Serve()
{
return m_MilkShake->Serve() + " decorated with Vanilla ";
}
float price()
{
return m_MilkShake->price() + 80;
}
};
int main()
{
MilkShake *baseMilkShake = new BaseMilkShake();
cout << "Basic Milk shake \n";
cout << baseMilkShake -> Serve() << endl;
cout << baseMilkShake -> price() << endl;
MilkShake *decoratedMilkShake = new MangoMilkShake(baseMilkShake);
cout << "Mango decorated Milk shake \n";
cout << decoratedMilkShake -> Serve() << endl;
cout << decoratedMilkShake -> price() << endl;
delete decoratedMilkShake;
decoratedMilkShake = new VanillaMilkShake(baseMilkShake);
cout << "Vanilla decorated Milk shake \n";
cout << decoratedMilkShake -> Serve() << endl;
cout << decoratedMilkShake -> price() << endl;
delete decoratedMilkShake;
delete baseMilkShake;
return 0;
}
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