Re: [boost] boost::shared_ptr/weak_ptr

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-04-12 12:01:38

Daniel Frey:

> On Sat, 2008-04-12 at 17:31 +0300, Peter Dimov wrote:
>> http://svn.boost.org/trac/boost/changeset/44344
>
> Wouldn't it be easier to make shared_count(weak_count&) to never throw
> and add
>
> if(!pn) boost::throw_exception( boost::bad_weak_ptr() );
>
> to the non-tagged ctor of the shared_ptr?

No. Consider your proposed

+    template<class Y>
+    explicit shared_ptr(detail::weak_count const & wc, Y * p): pn(wc) // 
may throw
+    {
+        // it is now safe to copy p, as pn(wc) did not throw
+        px = p;
+    }

which would change meaning.

Also consider that

+    template<class Y>
+    shared_ptr(detail::shared_count c, Y * p): px(p) // never throws
+    {
+      pn.swap(c);
+    }

is now equivalent to the weak_count constructor because 
shared_count(weak_count) throws. If it yielded an empty shared_count, you 
wouldn't be able to tell from inside the body whether you need to throw or 
not (because passing a genuine empty shared_count is legitimate).