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

```cpp
// 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 { }
};

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`

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);
    }
};

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

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");
}

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

oop-patterns

OOP Patterns Skill

The Four Pillars

1. Encapsulation