Why Can't `shared_from_this()` Be Used Directly in Constructors?

Understanding the Limitations of shared_from_this() in Constructors

The concept of shared_from_this() may seem convenient for establishing shared pointers to an object from within its constructor. However, it poses a technical limitation that prevents its direct use. To understand the issue, let's explore the underlying mechanism:

The Puzzle: Self-Storing shared_ptr and Construction Timing

According to the book reference, the shared_ptr is stored within a private member of enable_shared_from_this within the Person's base class. The confusion arises from two key points:

• Self-Storing shared_ptr: The shared_ptr that is stored is the actual shared_ptr created outside the class.
• Construction Timing: The storage of the shared_ptr occurs at the end of Person's construction process.

Why is shared_from_this() Restricted in Constructors?

The limitation of shared_from_this() in constructors is due to the sequential nature of construction. While the constructor of Person is executing, there is no shared_ptr yet created that could point to the Person object. The shared_ptr is created externally, after Person's construction.

Underlying Mechanism of shared_from_this()

enable_shared_from_this provides a way to establish a hidden weak_ptr within the object. This weak_ptr is then used by the first shared_ptr created outside the object to initialize itself. However, this initialization only occurs when a shared_ptr is created externally.

Conclusion

While shared_from_this() enables the creation of shared pointers to objects from outside their class, its use within the constructor is limited by the construction sequence. The shared_ptr must be created externally, after the object's construction is complete. This is a technical limitation that ensures the correct functioning of shared pointers.

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