Decorator in C++
Decorator is a structural pattern that allows adding new behaviors to objects dynamically by placing them inside special wrapper objects, called decorators.
Using decorators you can wrap objects countless number of times since both target objects and decorators follow the same interface. The resulting object will get a stacking behavior of all wrappers.
Complexity:
Popularity:
Usage examples: The Decorator is pretty standard in C++ code, especially in code related to streams.
Identification: Decorator can be recognized by creation methods or constructors that accept objects of the same class or interface as a current class.
Conceptual Example
This example illustrates the structure of the Decorator design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.cc: Conceptual example
/**
* The base Component interface defines operations that can be altered by
* decorators.
*/
class Component {
public:
virtual ~Component() {}
virtual std::string Operation() const = 0;
};
/**
* Concrete Components provide default implementations of the operations. There
* might be several variations of these classes.
*/
class ConcreteComponent : public Component {
public:
std::string Operation() const override {
return "ConcreteComponent";
}
};
/**
* The base Decorator class follows the same interface as the other components.
* The primary purpose of this class is to define the wrapping interface for all
* concrete decorators. The default implementation of the wrapping code might
* include a field for storing a wrapped component and the means to initialize
* it.
*/
class Decorator : public Component {
/**
* @var Component
*/
protected:
Component* component_;
public:
Decorator(Component* component) : component_(component) {
}
/**
* The Decorator delegates all work to the wrapped component.
*/
std::string Operation() const override {
return this->component_->Operation();
}
};
/**
* Concrete Decorators call the wrapped object and alter its result in some way.
*/
class ConcreteDecoratorA : public Decorator {
/**
* Decorators may call parent implementation of the operation, instead of
* calling the wrapped object directly. This approach simplifies extension of
* decorator classes.
*/
public:
ConcreteDecoratorA(Component* component) : Decorator(component) {
}
std::string Operation() const override {
return "ConcreteDecoratorA(" + Decorator::Operation() + ")";
}
};
/**
* Decorators can execute their behavior either before or after the call to a
* wrapped object.
*/
class ConcreteDecoratorB : public Decorator {
public:
ConcreteDecoratorB(Component* component) : Decorator(component) {
}
std::string Operation() const override {
return "ConcreteDecoratorB(" + Decorator::Operation() + ")";
}
};
/**
* The client code works with all objects using the Component interface. This
* way it can stay independent of the concrete classes of components it works
* with.
*/
void ClientCode(Component* component) {
// ...
std::cout << "RESULT: " << component->Operation();
// ...
}
int main() {
/**
* This way the client code can support both simple components...
*/
Component* simple = new ConcreteComponent;
std::cout << "Client: I've got a simple component:\n";
ClientCode(simple);
std::cout << "\n\n";
/**
* ...as well as decorated ones.
*
* Note how decorators can wrap not only simple components but the other
* decorators as well.
*/
Component* decorator1 = new ConcreteDecoratorA(simple);
Component* decorator2 = new ConcreteDecoratorB(decorator1);
std::cout << "Client: Now I've got a decorated component:\n";
ClientCode(decorator2);
std::cout << "\n";
delete simple;
delete decorator1;
delete decorator2;
return 0;
}
Output.txt: Execution result
Client: I've got a simple component:
RESULT: ConcreteComponent
Client: Now I've got a decorated component:
RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))
