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

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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.

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