Deferred call to weak bound member function

main.cpp

#include <functional>
#include <iostream>
#include <memory>

using namespace std;

class A : public enable_shared_from_this<A>
{
public:
    A() : data_(make_unique<int>(0))
    {
        cout << "A::A() ctor: " << hex << this << endl;
    }
    ~A()
    {
        cout << "A::~A() dtor: " << hex << this << endl;
    }
    void handle(int data) {
        *data_ = data;
        cout << "A::handle() : " << hex << this << ", data: " << dec << *data_ << endl;
    }
    std::function<void()> deferredHandle(int data)
    {
        cout << "A::deferredHandle() : " << hex << this << ", arg=" << dec << data << endl;

        // raw pointer => segfault
        // return [instance=this, data]{
        //     instance->handle(data);
        // };
        
        // shared pointer => object's lifetime is extended by deferred call
        // return [instance=shared_from_this(), data] {
        //     instance->handle(data);
        // };

        // weak pointer => deferred call is non-owning (weak ref)
        return [instance=weak_ptr<A>(shared_from_this()), instanceId=this, data] {
            if (auto lockedInstance = instance.lock())
                lockedInstance->handle(data);
            else
                cout << "Referred instance was already destroyed: " << hex << instanceId << endl;
        };
    }
private:
    unique_ptr<int> data_;
};

int main()
{
    auto a = make_shared<A>();
    a->handle(1);
    a->handle(2);
    a->handle(3);
    auto deferred4 = a->deferredHandle(4);
    auto deferred5 = a->deferredHandle(5);
    auto deferred6 = a->deferredHandle(6);

    deferred4();
    deferred5();

    a.reset();

    deferred6();

    return 0;
}