[boost] Re: Constructing shared_ptr from weak_ptr

From: AlisdairM (alisdair.meredith_at_[hidden])
Date: 2003-12-18 11:18:02

"Rani Sharoni" <rani_sharoni_at_[hidden]> wrote in
news:brshl7$r8m$1_at_[hidden]: 

> Why is it important that the shared_ptr(weak_ptr<Y> const & r)
> constructor throws an exception?
> I can't avoid thinking that it can easily affect the exception safety
> of code that uses it and this exception indicates about a bug in many
> cases. 

IIUC, the issue occurs when resource the weak_ptr refers to no longer 
exists, or at least can no longer be tracked through the weak_ptr.

Given the weak reference has become invalid, what behaviour would you 
prefer when trying to make a shared_ptr out of it?

The exception forces you to consider the possibility, and if no-throw 
guarantees or exception-specifications are a concern you must explicitly 
dead with it.  Any alternative would seem to involve some sort of post-
construction test to see if I made a valid shared_ptr.  In other words, my 
constructor may fail to give me a valid object.  This is one of the 
specific problems exceptions exist for, signalling a failure to construct.  
Once you allow objects to construct with invalid states, the rest of your 
code pollutes with state-checks.  It is much simpler to code with that 
exception up front, and not worry about such issues for the remaining 
lifetime of your object.

AlisdairM