Introduction to the Factory Pattern
A design pattern is a method of solving recurring problems in software design. The Factory Pattern is one of the design patterns that help to create objects in a flexible and decoupled manner.
The Factory Pattern is a creational design pattern in C++ that allows for object creation without explicitly knowing the object’s class. It provides a way to encapsulate complex object creation logic within a separate factory class. This is done by defining a common interface for creating objects, where the specific class of the object is determined at runtime.
Instead of creating an object directly, we use the factory method to give us an instance of the object we want. This way, we can hide the complicated steps involved in creating the object and make it simpler to change or modify the object in the future.
Why use the Factory Pattern?
There are several reasons why we might want to use the factory pattern. One reason is that it allows decoupling code from specific classes. By using a factory method, we can easily switch out one class for another without having to change any other code and make it more efficient.
Another reason to use the factory pattern is that it can make code more testable. Factory patterns allow us to test code by creating mock objects.
Here are some of the more reasons why the factory pattern is used in C++:
- The Factory Method pattern is used when a class doesn’t know in advance what objects it needs to create.
- The pattern is used when a class relies on its subclasses to specify the objects it creates.
- It is useful when you want to encapsulate the logic for creating a complex object in one place.
Example of Factory Pattern in C++
In C++, we can implement the factory pattern by defining an abstract base class with a virtual factory method. This method should return a pointer to the desired class instance. After that, we can derive concrete classes from this base class and override the factory method to return an instance of the appropriate class.
Here’s an example of how we might use this implementation of the factory pattern:
using namespace std;
class Shape {
public:
virtual void draw() = 0;
static Shape* createShape(int type);
};
class Circle : public Shape {
public:
void draw() {
cout << "Drawing a circle." << endl;
}
};
class Rectangle : public Shape {
public:
void draw() {
cout << "Drawing a rectangle." << endl;
}
};
Shape* Shape::createShape(int type) {
if (type == 1) {
return new Circle();
} else if (type == 2) {
return new Rectangle();
} else {
return nullptr;
}
}
int main() {
Shape* circle = Shape::createShape(1);
circle->draw();
Shape* rectangle = Shape::createShape(2);
rectangle->draw();
return 0;
}
In this example, we have a Shape class with two derived classes: Circle and Rectangle. The Shape class has a pure virtual function draw(), which is implemented by the derived classes. The Shape class also has a static function createShape(int type), which acts as a factory method to create instances of the derived classes based on the specified type. The draw() function is called on the created objects to demonstrate their specific behavior.
Conclusion
The Factory Pattern is a creational design pattern in C++ that allows for object creation without explicitly knowing the object’s class. It provides a simplified interface for creating objects by hiding the complex creation logic behind a factory class. This is done by defining a common interface for creating objects, where the specific class of the object is determined at runtime. Read more about factory patterns in this article.
