टेम्पलेट मेटाप्रोग्रामिंग पैटर्न (Template Metaprogramming Patterns)

#include <iostream>

// CRTP base: provides a counter for each derived class
template <typename Derived>
class Counter {
    static inline int count_ = 0;

protected:
    Counter()  { ++count_; }
    ~Counter() { --count_; }
    Counter(const Counter&) { ++count_; }

public:
    static int alive() { return count_; }
};

// Each derived class gets its OWN counter
class Dog : public Counter<Dog> {
public:
    std::string name;
    Dog(std::string n) : name(std::move(n)) {}
};

class Cat : public Counter<Cat> {
public:
    std::string name;
    Cat(std::string n) : name(std::move(n)) {}
};

// CRTP for static polymorphism: no virtual dispatch
template <typename Derived>
class Shape {
public:
    double area() const {
        // Call derived class method — resolved at compile time
        return static_cast<const Derived*>(this)->area_impl();
    }
};

class Circle : public Shape<Circle> {
    double radius_;
public:
    Circle(double r) : radius_(r) {}
    double area_impl() const { return 3.14159265 * radius_ * radius_; }
};

class Square : public Shape<Square> {
    double side_;
public:
    Square(double s) : side_(s) {}
    double area_impl() const { return side_ * side_; }
};

int main() {
    Dog d1("Rex"), d2("Buddy");
    Cat c1("Whiskers");
    std::cout << "Dogs alive: " << Dog::alive() << '\n'; // 2
    std::cout << "Cats alive: " << Cat::alive() << '\n'; // 1

    Circle circle(5.0);
    Square square(4.0);
    std::cout << "Circle area: " << circle.area() << '\n'; // 78.54
    std::cout << "Square area: " << square.area() << '\n'; // 16.0
}

#include <iostream>
#include <mutex>
#include <stdexcept>

// Threading policies
struct SingleThreaded {
    struct Lock { Lock(SingleThreaded&) {} }; // no-op
};

struct MultiThreaded {
    std::mutex mtx;
    struct Lock {
        std::lock_guard<std::mutex> guard;
        Lock(MultiThreaded& host) : guard(host.mtx) {}
    };
};

// Bounds checking policies
struct NoBoundsCheck {
    static void check(int /*index*/, int /*size*/) {} // no-op
};

struct StrictBoundsCheck {
    static void check(int index, int size) {
        if (index < 0 || index >= size)
            throw std::out_of_range("Index out of bounds");
    }
};

// Policy-based container: users choose their combination
template <typename T,
         typename ThreadingPolicy = SingleThreaded,
         typename BoundsPolicy = NoBoundsCheck>
class SmartArray : private ThreadingPolicy {
    T* data_;
    int size_;

public:
    SmartArray(int size) : data_(new T[size]{}), size_(size) {}
    ~SmartArray() { delete[] data_; }

    T& operator[](int index) {
        typename ThreadingPolicy::Lock lock(*this);
        BoundsPolicy::check(index, size_);
        return data_[index];
    }

    int size() const { return size_; }
};

int main() {
    // Fast, no overhead: no locking, no bounds checks
    SmartArray<int> fast(10);
    fast[0] = 42;

    // Safe: bounds-checked, single-threaded
    SmartArray<int, SingleThreaded, StrictBoundsCheck> safe(10);
    safe[0] = 42;
    try {
        safe[99] = 0; // throws!
    } catch (const std::out_of_range& e) {
        std::cout << e.what() << '\n';
    }

    // Thread-safe with bounds checking
    SmartArray<int, MultiThreaded, StrictBoundsCheck> robust(10);
    robust[5] = 100;
}

#include <iostream>
#include <memory>
#include <string>

// Type-erased "Printable" — can hold any type with a print() method
class Printable {
    // Concept: abstract interface
    struct Concept {
        virtual ~Concept() = default;
        virtual void print(std::ostream& os) const = 0;
        virtual std::unique_ptr<Concept> clone() const = 0;
    };

    // Model: wraps any T that supports operator<<
    template <typename T>
    struct Model : Concept {
        T value;
        Model(T v) : value(std::move(v)) {}
        void print(std::ostream& os) const override { os << value; }
        std::unique_ptr<Concept> clone() const override {
            return std::make_unique<Model>(value);
        }
    };

    std::unique_ptr<Concept> ptr_;

public:
    // Constructor: accepts any printable type
    template <typename T>
    Printable(T value) : ptr_(std::make_unique<Model<T>>(std::move(value))) {}

    // Copy support
    Printable(const Printable& other) : ptr_(other.ptr_->clone()) {}
    Printable& operator=(const Printable& other) {
        ptr_ = other.ptr_->clone();
        return *this;
    }
    Printable(Printable&&) = default;
    Printable& operator=(Printable&&) = default;

    friend std::ostream& operator<<(std::ostream& os, const Printable& p) {
        p.ptr_->print(os);
        return os;
    }
};

int main() {
    // Heterogeneous collection — no common base class needed!
    std::vector<Printable> items;
    items.emplace_back(42);
    items.emplace_back(3.14);
    items.emplace_back(std::string("hello"));
    items.emplace_back('X');

    for (const auto& item : items) {
        std::cout << item << '\n';
    }
    // Output: 42, 3.14, hello, X
}