Why shared_from_this can't be used in constructor from technical standpoint?

Question

In the book The C++ Standard Library at page 91 I have read this about shared_from_this():

The problem is that shared_ptr stores itself in a private member of Person’s base class, enable_shared_from_this<>, at the end of the construction of the Person.

The relevant code snippet from the book is:

class Person : public std::enable_shared_from_this<Person> {    ... }; 

I don't understand two things here:

1. who is this shared_ptr which stores itself?
2. how he can store itself anywhere at the end of the construction of Person? I think construction of Person ends up with the last statement of its constructor which written by me.

I understand that there is weak_ptr which hasn't been initialized yet.

EDIT: Thanks to Angew! shared_from_this will work only after first shared_ptr to Person was created. This shared_ptr will check if Person class inherited from enable_shared_from_this, and if yes then initialize its internal weak_ptr.

Answer 1

The reason is simple: in object X, enable_shared_from_this works by initialising a hidden weak_ptr with a copy of the first shared_ptr which points to object X. However, for a shared_ptr to be able to point to X, X must already exist (it must be already constructed). Therefore, while the constructor of X is running, there is yet no shared_ptr which enable_shared_from_this could use.

Take this piece of code:

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

Before the constructor of p (of the shared_ptr) is even called, its argument must be evaluated. That argument is the expression new Person(). Therefore, the constructor of Person runs before the constructor of p has even begun—before there is any shared_ptr object to which enable_shared_from_this could bind.