In C++, when you create a class that derives from std::enable_shared_from_this, it becomes eligible for special treatment by shared_ptr, allowing the shared_from_this() method to return a shared_ptr to the object. However, certain conditions and constraints must be met, and this functionality is enabled through a mechanism involving a secret weak pointer called weak_this.

Inside STL: The shared_ptr constructor and enable_shared_from_this

By Raymond Chen

From the article:

If you create a class of the form

struct S : std::enable_shared_from_this<S>
{
    /* ... */
};

which derives from std::enable_shared_from_this of itself (using the curiously recurring template pattern), then this class becomes a candidate for special treatment by shared_ptr: The shared_from_this() method will produce a shared_ptr<S>. Some restrictions apply.