std::unique_ptr: विशिष्ट ओनरशिप

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

struct Connection {
    std::string host;
    int port;
    Connection(std::string h, int p) : host(std::move(h)), port(p) {
        std::cout << "Connected to " << host << ":" << port << '\n';
    }
    ~Connection() {
        std::cout << "Disconnected from " << host << '\n';
    }
    void query(const std::string& sql) {
        std::cout << "Executing: " << sql << '\n';
    }
};

int main() {
    // CORRECT: use make_unique
    auto conn = std::make_unique<Connection>("localhost", 5432);
    conn->query("SELECT * FROM users");

    // Access the raw pointer (non-owning)
    Connection* raw = conn.get();  // does NOT transfer ownership

    // Transfer ownership
    auto conn2 = std::move(conn);
    // conn is now nullptr
    if (!conn) std::cout << "conn is null after move\n";

    // conn2 destroyed here — destructor called automatically
}  // Output: "Disconnected from localhost"

#include <cstdio>
#include <memory>
#include <iostream>

// Custom deleter for FILE*
struct FileDeleter {
    void operator()(FILE* f) const {
        if (f) {
            std::fclose(f);
            std::cout << "File closed\n";
        }
    }
};

// Type alias for clarity
using UniqueFile = std::unique_ptr<FILE, FileDeleter>;

// Factory function
UniqueFile open_file(const char* path, const char* mode) {
    FILE* f = std::fopen(path, mode);
    if (!f) throw std::runtime_error("Cannot open file");
    return UniqueFile(f);
}

int main() {
    try {
        auto file = open_file("/tmp/test.txt", "w");
        std::fputs("Hello, RAII!\n", file.get());
        // file automatically closed when scope exits
    } catch (const std::exception& e) {
        std::cerr << e.what() << '\n';
    }

    // Lambda deleter (less common, but useful for one-offs)
    auto deleter = [](int* p) { std::cout << "custom free\n"; delete p; };
    std::unique_ptr<int, decltype(deleter)> p(new int(42), deleter);
}

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

class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;
    virtual std::string name() const = 0;
};

class Circle : public Shape {
    double radius_;
public:
    explicit Circle(double r) : radius_(r) {}
    double area() const override { return 3.14159 * radius_ * radius_; }
    std::string name() const override { return "Circle"; }
};

class Rectangle : public Shape {
    double w_, h_;
public:
    Rectangle(double w, double h) : w_(w), h_(h) {}
    double area() const override { return w_ * h_; }
    std::string name() const override { return "Rectangle"; }
};

// Factory — caller owns the result
std::unique_ptr<Shape> make_shape(const std::string& type, double a, double b = 0) {
    if (type == "circle")    return std::make_unique<Circle>(a);
    if (type == "rectangle") return std::make_unique<Rectangle>(a, b);
    return nullptr;  // unknown shape
}

int main() {
    auto s = make_shape("circle", 5.0);
    if (s) {
        std::cout << s->name() << " area: " << s->area() << '\n';
    }
    // s automatically deleted here
}