My class derives from std::enable_shared_from_this, but shared_from_this() doesn’t work

If you make a class T that derives from std::enable_shared_from_this<T>, then the creation of a std::shared_ptr to that class will activate the shared_from_this() method to return a shared_ptr that shares ownership with the originally-created std::shared_ptr.

The catch is that the shared_ptr constructor and enable_shared_from_this are in cahoots, and the shared_ptr must be able to access the enable_shared_from_this in order to finish the job. This requires that you publicly derive from std::enable_shared_from_this:

class MyClass : public std::enable_shared_from_this<MyClass>
{
    ...
};

If you forget the public keyword, then the base class defaults to private, and the secret signal between shared_ptr and enable_shared_from_this does not get through.

Here’s how enable_shared_from_this and shared_ptr work together. Note that I’ve ignored edge cases; the idea here is to give the basic idea so you can diagnose enable_shared_from_this issues yourself.

template<typename T>
struct enable_shared_from_this
{
    shared_ptr<T> shared_from_this()
    { return shared_ptr<T>(weak_this); }

    weak_ptr<T> weak_this;
};

template<typename T>
struct shared_ptr
{
    shared_ptr(T* p) : ptr(p)
    {
        if (T derives from enable_shared_from_this) {
            ptr->weak_ptr = *this;
        }
    }

    T* ptr;
    /* other stuff */
};

When a shared_ptr is created, it snoops at the managed object to see if it derives from enable_shared_from_this. If so, then it sets the weak_ptr to hold a weak pointer to the shared object. When you later ask for a shared_from_this(), it promotes this weak pointer to a shared pointer and returns it.

Okay, so we already see some consequences and pitfalls:

First of all, if you fail to derive publicly from enable_shared_from_this, the feature simply fails silently. There is no diagnostic that says, “Hey, like, you’re deriving from enable_shared_from_this, but you did it privately, so it’s not going to work.”¹

Second, notice that the weak pointer is set only when the object is placed inside a shared_ptr, which happens after the shared object has been constructed. This means that you cannot use shared_from_this() in your constructor.

Third, if the object is not wrapped inside a shared_ptr at all, then shared_from_this() will always fail. For example, if somebody constructs the object on the stack, or via new or make_unique, it will not be controlled by a shared_ptr.

There are so many ways enable_shared_from_this can go wrong. Next time, we’ll see what we can do to guard against them.

¹ Maybe it’s possible to add a diagnostic to shared_from_this(). I wonder if the shared type is required to be complete by that point.