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 |
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