Why Can't You Use `shared_from_this` in a Constructor?

Understanding the Inability of Shared_from_this in Constructors

The query focuses on the limitations of using shared_from_this within a class constructor. To address this, let's explore the technical reasons behind this restriction.

The Shared_ptr's Hidden Member

Shared_from_this is self-referential, relying on an internal weak_ptr member added to the enable_shared_from_this base class. However, this weak_ptr is only set after the first shared_ptr to Person is created.

Construction Timeline

A class constructor is responsible for initializing its members, including constructing the enable_shared_from_this base class instance. During the initial object construction, there is no shared_ptr pointing to the Person yet.

The Connection between Shared_ptr and Weak_ptr

Consider the following snippet:

 std::shared_ptr<Person> p(new Person());

Before the constructor of p (the shared_ptr) is invoked, the expression new Person() must be evaluated, leading to the Person object's construction. Since the shared_ptr's constructor runs after the Person object is created, the shared_ptr object does not exist during Person's constructor phase, rendering the use of shared_from_this impossible.

Conclusion

Technically, shared_from_this is unusable within a constructor because the required shared_ptr, which initializes the internal weak_ptr, does not exist until the object is fully constructed. This prevents shared_from_this from establishing the necessary self-referential connection.

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