shared_from_this called from constructor

Question

I have to register an object in a container upon its creation. Without smart pointers I\'d use something like this:

a_class::a_class()
{
    register_somewhere(t         


        

Answer 1

I came up with a helper class for that problem:

template 
class ImmediatelySharedFromThis : public std::enable_shared_from_this {
    typedef std::unique_ptr> MallocGuard;
    typedef std::shared_ptr SharedPtr;

    // disallow `new MyClass(...)`
    static void *operator new(size_t) = delete;
    static void *operator new[](size_t) = delete;
    static void operator delete[](void*) = delete;
protected:
    typedef std::pair SharingCookie;

    ImmediatelySharedFromThis(SharingCookie cookie) {
        MallocGuard &ptr = cookie.first;
        SharedPtr &shared = cookie.second;
        // This single line contains the actual logic:
        shared.reset(reinterpret_cast(ptr.release()));
    }
public:
    // Create new instance and return a shared pointer to it.
    template 
    static SharedPtr create(Args &&...args) {
        // Make sure that the memory is free'd if ImmediatelySharedFromThis
        // is not the first base class, and the initialization
        // of another base class throws an exception.
        MallocGuard ptr(aligned_alloc(alignof(Impl), sizeof(Impl)), free);
        if (!ptr) {
            throw std::runtime_error("OOM");
        }

        SharedPtr result;
        ::new (ptr.get()) Impl(SharingCookie(ptr, result),
                               std::forward(args)...);
        return result;
    }

    static void operator delete(void *ptr) {
        free(ptr);
    }
};

class MyClass : public ImmediatelySharedFromThis {
    friend class ImmediatelySharedFromThis;

    MyClass(SharingCookie cookie, int some, int arguments) :
        ImmediatelySharedFromThis(cookie)
        // You can pass shared_from_this() to other base classes
    {
        // and you can use shared_from_this() in here, too.
    }
public:
    ....
};

...

std::shared_ptr obj = MyClass::create(47, 11); 

A bit ugly, but it works.