Constructors play an important role in object-oriented programming as they allow us to initialize objects with initial values. In the context of a strategy pattern, constructors become even more crucial as they define the behavior of different strategies. In this blog post, we will explore how to implement constructors for strategy patterns in C++.

The Strategy Pattern

Before diving into constructors, let’s briefly recap the strategy pattern. The strategy pattern is a behavioral design pattern that allows the algorithm of a class to be changed at runtime. It consists of three main components:

1. The context class: This is the class that holds a reference to the current strategy and delegates the work to it.
2. The strategy interface: This is an abstract class or interface that defines the common methods that all strategies must implement.
3. The concrete strategy classes: These are the various implementations of the strategy interface, each encapsulating a different algorithm or behavior.

Adding Constructors to Strategy Classes

To add a constructor to a strategy class, simply define a constructor method within the class. This constructor method will be called when an instance of the strategy class is created. Inside the constructor, you can initialize any instance variables or perform any other necessary setup.

Here’s an example of a strategy class ConcreteStrategyA with a constructor:

class ConcreteStrategyA : public Strategy {
public:
    ConcreteStrategyA() {
        // Perform any initialization here
    }
    
    void execute() override {
        // Implement execute method specific to ConcreteStrategyA
    }
};

In this example, the constructor for ConcreteStrategyA is called when an instance of ConcreteStrategyA is created. You can add any necessary initialization code inside the constructor, such as setting default values or allocating resources.

Using Constructors in Context Class

Next, we need to understand how to use these constructors in the context class. In the context class, you would typically have a member variable to hold an instance of the strategy interface. When creating a new strategy, you can use the constructor of the specific concrete strategy class to instantiate the strategy object.

Here’s an example of a context class Context using the strategy ConcreteStrategyA:

class Context {
private:
    Strategy* strategy;

public:
    Context() {
        strategy = new ConcreteStrategyA();   // Using the constructor of ConcreteStrategyA
    }
    
    void setStrategy(Strategy* newStrategy) {
        strategy = newStrategy;
    }
    
    void executeStrategy() {
        strategy->execute();
    }
};

In this example, the Context class has a constructor that instantiates a ConcreteStrategyA object using its constructor. This sets the default strategy to be ConcreteStrategyA. The setStrategy method allows changing the strategy at runtime by accepting a new strategy object.

Conclusion

Constructors play a crucial role in initializing objects in the strategy pattern. By adding constructors to strategy classes, we can ensure that each strategy is properly initialized and ready to fulfill its specific role. By utilizing constructors in the context class, we can easily assign and switch between different strategies, providing flexibility and runtime adaptability.

#cpp #strategydesignpattern