C++ – the usefulness of `enable_shared_from_this`

boostboost-asioc++tr1

I ran across enable_shared_from_this while reading the Boost.Asio examples and after reading the documentation I am still lost for how this should correctly be used. Can someone please give me an example and explanation of when using this class makes sense.

Best Solution

It enables you to get a valid shared_ptr instance to this, when all you have is this. Without it, you would have no way of getting a shared_ptr to this, unless you already had one as a member. This example from the boost documentation for enable_shared_from_this:

class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_from_this();
    }
}

int main()
{
    shared_ptr<Y> p(new Y);
    shared_ptr<Y> q = p->f();
    assert(p == q);
    assert(!(p < q || q < p)); // p and q must share ownership
}

The method f() returns a valid shared_ptr, even though it had no member instance. Note that you cannot simply do this:

class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_ptr<Y>(this);
    }
}

The shared pointer that this returned will have a different reference count from the "proper" one, and one of them will end up losing and holding a dangling reference when the object is deleted.

enable_shared_from_this has become part of C++ 11 standard. You can also get it from there as well as from boost.