What is `enable_shared_from_this` and how does it safely create `shared_ptr` instances?

What is enable_shared_from_this and Why is it Useful?

Introduction

While exploring Boost.Asio examples, you may encounter enable_shared_from_this, leaving you perplexed about its proper application. This article aims to provide a comprehensive explanation and example of when using this class makes sense.

Understanding enable_shared_from_this

enable_shared_from_this is a utility class that facilitates the conversion of a regular pointer (such as this) into a valid shared_ptr instance. Without it, accessing a shared_ptr to the current object would be impossible unless it was explicitly stored as a member variable.

Example and Explanation

Consider the following example:

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
}

In this example, the f() method returns a valid shared_ptr, even though it does not have a member instance of shared_ptr. The enable_shared_from_this class allows this conversion. It is important to note that the following approach is incorrect:

class Y : public enable_shared_from_this<Y>
{
public:

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

This would result in shared pointers with different reference counts, leading to potential dangling references when the object is deleted.

Conclusion

enable_shared_from_this is a valuable tool for creating a shared_ptr instance of the current object, when you only have access to a regular pointer. It prevents dangling references and ensures proper resource management. This functionality is now also available as part of the C 11 standard.

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