How Does `enable_shared_from_this` Solve the Problem of Creating Safe `shared_ptr`s to `this`?

Understanding the Role of enable_shared_from_this

The concept of enable_shared_from_this often leaves programmers perplexed, and the documentation can be equally confusing. This article aims to shed light on the purpose and proper usage of this class with the help of concrete examples.

What Does enable_shared_from_this Do?

enable_shared_from_this allows you to create a shared_ptr instance pointing to the current object ("this") when you only have a raw pointer. This functionality is essential for certain scenarios where you need to maintain a shared ownership of the object.

An Illustrative Example:

Consider the following class Y which derives from enable_shared_from_this:

class Y : public enable_shared_from_this<Y> {
public:
    shared_ptr<Y> f() {
        return shared_from_this();
    }
};

In the f method, we can return a valid shared_ptr even though the class does not have any member instance. Let us explore this further:

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
}

In this example, p is the original shared_ptr pointing to the object. When we call p->f(), we obtain another shared_ptr (q) that shares ownership with p. The assertion p == q verifies that they refer to the same object, and the second assertion guarantees that they don't have different values (p and q must always maintain equal reference counts).

Importance of Correct Usage:

It's crucial to emphasize that simply creating a shared_ptr using this (without enable_shared_from_this) will result in an incorrect reference count. Hence, using enable_shared_from_this ensures the proper management of shared ownership.

Availability:

enable_shared_from_this is available in both Boost and the C 11 standard. You can use either of these implementations based on your needs.

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