C++ - std::enable_shared_from_this

std::enable_shared_from_this Example (C++)

✅ Basic Example

#include <iostream>
#include <memory>

class MyClass : public std::enable_shared_from_this<MyClass> {
public:
    void show() {
        std::cout << "this = " << this << "\n";

        // Create another shared_ptr to this object
        std::shared_ptr<MyClass> self = shared_from_this();
        std::cout << "shared_from_this().get() = " << self.get() << "\n";
    }
};

int main() {
    // Must be managed by shared_ptr
    std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();
    obj->show();
}

Output

this = 0x7f8e94004b40
shared_from_this().get() = 0x7f8e94004b40

---

❌ Incorrect Usage (Throws std::bad_weak_ptr)

int main() {
    MyClass obj;  // ❌ not owned by shared_ptr
    obj.show();   // ❌ shared_from_this() will throw std::bad_weak_ptr
}

---

🧠 Why enable_shared_from_this Is Needed

If you try:

std::shared_ptr<MyClass> self(this);  // ❌ do NOT do this

This creates a second shared_ptr control block → double delete → crash.

shared_from_this() uses the existing shared ownership safely.

---

⭐ Advanced Example: Returning shared_ptr from a Method

#include <memory>
#include <iostream>

class Connection : public std::enable_shared_from_this<Connection> {
public:
    std::shared_ptr<Connection> getPtr() {
        return shared_from_this();
    }
};

int main() {
    auto c = std::make_shared<Connection>();
    auto c2 = c->getPtr();

    std::cout << c.use_count() << "\n";  // prints 2
}

---

🌟 Common Pattern in Networking / Async Code

class Session : public std::enable_shared_from_this<Session> {
public:
    void start() {
        auto self = shared_from_this();  // keep object alive
        doRead();
    }

private:
    void doRead() {
        auto self = shared_from_this();
        // perform async operation using self...
    }
};