Cannot predict if shared_from_this will throw

According to
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1450.html,
std::enable_shared_from_this<T>::shared_from_this will throw
std::bad_weak_ptr if the object isn't being referenced by a
shared_ptr. This leads to a problem when one might want to avoid an
exception and doesn't know if it'll occur or not.

Is there any rationale behind this behavior? In other words, why
shared_ptr's ctor that accepts an weak_ptr should throw an exception
if weak_ptr is expired? Why not simply make the shared_ptr empty?

Another solution that doesn't involve changing shared_ptr's ctor
semantics would be adding a member function to enable_shared_from_this
that returned the internal weak_ptr expire status.

Regards,
Rodolfo Lima.

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