Understanding C++ Design Patterns: Policy, Traits, and Strategy

A detailed exploration of essential C++ design patterns and idioms.

1. Policy-Based Design

Policy-based design is a technique where behavior or functionality is provided via policy classes. A policy class is a type that provides a particular piece of functionality or behavior.

Concept:

Policies are used to define or change behavior at compile-time through template parameters. By combining different policies, you can achieve different functionalities.

Example: Policy-Based Design

#include <iostream>
#include <type_traits>

// Base policy class
template<typename T>
class AddPolicy {
public:
    void add(T value) {
        std::cout << "Adding value: " << value << std::endl;
    }
};

// Different policy
template<typename T>
class MultiplyPolicy {
public:
    void multiply(T value) {
        std::cout << "Multiplying value: " << value << std::endl;
    }
};

// Using policies
template<typename T, typename Policy = AddPolicy<T>>
class Calculator : public Policy {
public:
    void doSomething(T value) {
        Policy::add(value);  // Calls the policy's method
    }
};

int main() {
    Calculator<int> calcAdd;
    calcAdd.doSomething(5);  // Calls AddPolicy::add()

    Calculator<int, MultiplyPolicy<int>> calcMultiply;
    calcMultiply.doSomething(5);  // Calls MultiplyPolicy::multiply()

    return 0;
}
                  

Usefulness:

  • Extensibility: Add or change features by introducing new policies.
  • Reusability: Policies can be reused across different classes or functions.
  • Separation of Concerns: Keeps different functionalities separate and modular.

2. Traits Classes

Traits classes provide a way to associate additional properties or behaviors with types. They are often used to define properties like type characteristics or to provide helper functions.

Concept:

Traits classes are a way to introspect or adapt types based on their properties.

Example: Traits Classes

#include <iostream>
#include <type_traits>

// Define traits for different types
template<typename T>
struct IsIntegral {
    static const bool value = false;
};

template<>
struct IsIntegral<int> {
    static const bool value = true;
};

template<>
struct IsIntegral<long> {
    static const bool value = true;
};

// A utility function to check if a type is integral
template<typename T>
void checkIntegral() {
    if (IsIntegral<T>::value) {
        std::cout << "Type is integral\n";
    } else {
        std::cout << "Type is not integral\n";
    }
}

int main() {
    checkIntegral<int>();    // Output: Type is integral
    checkIntegral<double>(); // Output: Type is not integral

    return 0;
}
                

Usefulness:

  • Type Introspection: Check properties of types.
  • Conditional Compilation: Adjust code based on type properties.
  • Generic Programming: Implement type-specific behavior.

3. Strategy Pattern

Strategy pattern is a design pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable. This pattern lets the algorithm vary independently from the clients that use it.

Concept:

Strategies are used to change the behavior of an algorithm at runtime. Each strategy implements a common interface.

Example: Strategy Pattern

#include <iostream>
#include <memory>

// Strategy interface
class SortingStrategy {
public:
    virtual void sort() const = 0;
};

// Concrete strategy A
class QuickSort : public SortingStrategy {
public:
    void sort() const override {
        std::cout << "Sorting using QuickSort\n";
    }
};

// Concrete strategy B
class MergeSort : public SortingStrategy {
public:
    void sort() const override {
        std::cout << "Sorting using MergeSort\n";
    }
};

// Context
class Context {
    std::unique_ptr<SortingStrategy> strategy;
public:
    void setStrategy(SortingStrategy* strat) {
        strategy.reset(strat);
    }

    void performSort() {
        strategy->sort();
    }
};

int main() {
    Context context;

    context.setStrategy(new QuickSort());
    context.performSort();  // Uses QuickSort

    context.setStrategy(new MergeSort());
    context.performSort();  // Uses MergeSort

    return 0;
}
                

Usefulness:

  • Algorithm Flexibility: Switch between algorithms at runtime.
  • Code Organization: Encapsulates algorithms and separates them from the context.
  • Open/Closed Principle: Extending functionality without modifying existing code.

Summary Table

Term Description Usefulness
Policy Class templates providing specific behaviors or features Extensibility, reusability, and separation of concerns
Traits Classes providing type properties or helper functions Type introspection, conditional compilation, and generic programming
Strategy Design pattern for defining interchangeable algorithms Flexibility in choosing algorithms, encapsulation of algorithms, and runtime behavior changes

More Examples and References

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