OOP Patterns Skill

Production-Grade Learning Skill | Object-Oriented Design

Master C++ OOP concepts and industry-standard design patterns.

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The Four Pillars

1. Encapsulation

class BankAccount {
private:
    std::string id_;
    double balance_{0.0};  // Default member init

public:
    explicit BankAccount(std::string id) : id_(std::move(id)) {}

    // Read-only access
    [[nodiscard]] double balance() const { return balance_; }
    [[nodiscard]] const std::string& id() const { return id_; }

    // Controlled mutation with validation
    void deposit(double amount) {
        if (amount <= 0) {
            throw std::invalid_argument("Amount must be positive");
        }
        balance_ += amount;
    }

    bool withdraw(double amount) {
        if (amount <= 0 || amount > balance_) return false;
        balance_ -= amount;
        return true;
    }
};

2. Inheritance

// Abstract base class
class Shape {
protected:
    std::string name_;

public:
    explicit Shape(std::string name) : name_(std::move(name)) {}
    virtual ~Shape() = default;  // ALWAYS virtual destructor!

    // Pure virtual functions (interface)
    [[nodiscard]] virtual double area() const = 0;
    [[nodiscard]] virtual double perimeter() const = 0;

    // Non-virtual (shared behavior)
    [[nodiscard]] const std::string& name() const { return name_; }
};

class Circle final : public Shape {  // final prevents further inheritance
    double radius_;
public:
    explicit Circle(double r) : Shape("Circle"), radius_(r) {}

    [[nodiscard]] double area() const override {
        return std::numbers::pi * radius_ * radius_;
    }

    [[nodiscard]] double perimeter() const override {
        return 2 * std::numbers::pi * radius_;
    }
};

3. Polymorphism

// Runtime polymorphism via virtual functions
void printShape(const Shape& shape) {
    std::cout << shape.name() << ": area = " << shape.area() << "\n";
}

// Usage - same function, different behavior
std::vector<std::unique_ptr<Shape>> shapes;
shapes.push_back(std::make_unique<Circle>(5.0));
shapes.push_back(std::make_unique<Rectangle>(3.0, 4.0));

for (const auto& shape : shapes) {
    printShape(*shape);  // Polymorphic dispatch
}

4. Abstraction

// Pure interface (all pure virtual)
class ILogger {
public:
    virtual ~ILogger() = default;
    virtual void log(std::string_view message) = 0;
    virtual void setLevel(int level) = 0;
};

// Multiple implementations
class ConsoleLogger : public ILogger {
public:
    void log(std::string_view message) override {
        std::cout << "[LOG] " << message << "\n";
    }
    void setLevel(int) override { }
};

class FileLogger : public ILogger {
    std::ofstream file_;
public:
    explicit FileLogger(const std::string& path) : file_(path) {}
    void log(std::string_view message) override {
        file_ << message << "\n";
    }
    void setLevel(int) override { }
};

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SOLID Principles

Principle	Summary	C++ Example
Single Responsibility	One class, one reason to change	Separate User from UserSerializer
Open/Closed	Open for extension, closed for modification	Virtual functions, inheritance
Liskov Substitution	Subtypes must be substitutable	Square shouldn't inherit Rectangle
Interface Segregation	Prefer small, specific interfaces	Split IWorker into IWorkable, IFeedable
Dependency Inversion	Depend on abstractions	Inject IDatabase& not MySQLDatabase

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Design Patterns

Factory Pattern

class ShapeFactory {
public:
    static std::unique_ptr<Shape> create(std::string_view type, double param) {
        if (type == "circle") {
            return std::make_unique<Circle>(param);
        }
        if (type == "square") {
            return std::make_unique<Square>(param);
        }
        throw std::invalid_argument("Unknown shape type");
    }
};

// Usage
auto shape = ShapeFactory::create("circle", 5.0);

Singleton Pattern (Thread-Safe)

class Logger {
private:
    Logger() = default;

public:
    Logger(const Logger&) = delete;
    Logger& operator=(const Logger&) = delete;

    static Logger& instance() {
        static Logger instance;  // Thread-safe in C++11
        return instance;
    }

    void log(std::string_view msg) { std::cout << msg << "\n"; }
};

Observer Pattern

class IObserver {
public:
    virtual ~IObserver() = default;
    virtual void onNotify(const std::string& event) = 0;
};

class Subject {
    std::vector<IObserver*> observers_;
public:
    void attach(IObserver* obs) { observers_.push_back(obs); }
    void detach(IObserver* obs) {
        observers_.erase(std::remove(observers_.begin(), observers_.end(), obs),
                        observers_.end());
    }
    void notify(const std::string& event) {
        for (auto* obs : observers_) {
            obs->onNotify(event);
        }
    }
};

Strategy Pattern

class ICompressionStrategy {
public:
    virtual ~ICompressionStrategy() = default;
    virtual std::vector<uint8_t> compress(std::span<const uint8_t> data) = 0;
};

class ZipStrategy : public ICompressionStrategy {
public:
    std::vector<uint8_t> compress(std::span<const uint8_t> data) override {
        // ZIP compression
        return {};
    }
};

class Compressor {
    std::unique_ptr<ICompressionStrategy> strategy_;
public:
    void setStrategy(std::unique_ptr<ICompressionStrategy> s) {
        strategy_ = std::move(s);
    }
    std::vector<uint8_t> compress(std::span<const uint8_t> data) {
        return strategy_->compress(data);
    }
};

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Troubleshooting

Issue	Cause	Solution
Object slicing	Passing by value	Use references/pointers
Memory leak	Missing virtual destructor	Add virtual ~Base() = default;
Diamond problem	Multiple inheritance	Use virtual inheritance
Tight coupling	Direct dependencies	Inject interfaces

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Unit Test Template

#include <gtest/gtest.h>

TEST(ShapeTest, CircleArea) {
    Circle c(5.0);
    EXPECT_NEAR(c.area(), 78.5398, 0.0001);
}

TEST(ShapeTest, Polymorphism) {
    std::unique_ptr<Shape> shape = std::make_unique<Circle>(1.0);
    EXPECT_EQ(shape->name(), "Circle");
}

TEST(FactoryTest, CreateCircle) {
    auto shape = ShapeFactory::create("circle", 5.0);
    EXPECT_NE(shape, nullptr);
    EXPECT_EQ(shape->name(), "Circle");
}

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C++ Plugin v3.0.0 - Production-Grade Learning Skill