Master Software Design Patterns and Architecture in C++/

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Decorator Design Pattern Example

Decorator Design Pattern Example

Understand the implementation of the decorator design pattern.

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In the previous lesson, we saw a coding example of a decorator design pattern. Here, we’ll look at another example of a decorator design pattern. But this time, we’ll make a small change in our code: we won’t create a decorator. Instead, the concrete decorator class will inherit the component class directly.

Example

C++
#include <iostream>
#include <string>
struct Shape{
    virtual std::string str() = 0;
    virtual float getWidth() = 0;
    virtual float getHeight() = 0;
};
class Rectangle : public Shape{
    float width = 10.0f;
    float height = 5.0f;
public:
    std::string str() override{
        return std::string("A rectangle of width ") + std::to_string(width) + " and height " + std::to_string(height);
    }
    float getWidth() override{
        return width;
    }
    float getHeight() override{
        return height;
    }
};
class MarginShape : public Shape{
    float margin;
    Shape& shape;
    float width;
    float height;
public:
    MarginShape(float m, Shape& s) : shape(s){
        margin = m;
        width = shape.getWidth() + 2*margin;
        height = shape.getHeight() + 2*margin;
    }
    std::string str() override{
        return shape.str() + std::string(" with a margin of ") + std::to_string(margin);
    }
    float getWidth() override{
        return width;
    }
    float getHeight() override{
        return height;
    }
};
int main(){
    Rectangle rectangle;
    MarginShape marginShape(2.0f, rectangle);
    std::cout << rectangle.str() << " Width: " << rectangle.getWidth() << " and Height: " << rectangle.getHeight() << std::endl;
    std::cout << marginShape.str() << " Width: " << marginShape.getWidth() << " and Height: " << marginShape.getHeight() << std::endl;
}
Decorator design pattern example

Code explanation

Let’s look at the code line by line:

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