[boost] Safe bool idiom

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-09-11 08:36:15

It seems to me that this went unnoticed because it was in a sub-thread, 
so I'd like to give it another push with its own thread now :)

Ed Brey wrote (abbridged):

<quote>

For instance, when I examine the optimized code generated by VC 7.1 from 
the technique used by shared_ptr et al, I find code that does a run-time 
fetch of the "unspecified" member address from a non-immediate variable. 
  Another way to put it is that for shared_ptr, these two fragments 
generate different code:

if (ptr.get()) ...
if (ptr) ...       // Less efficient.

</quote>

This inefficiency is caused by shared_ptr's current safe-bool-idiom. A 
solution is the use of the safe-bool-idiom I first showed here: 
http://lists.boost.org/MailArchives/boost/msg38164.php and that found 
it's way into the operators library, see the CVS version and have a look 
at bool_testable.

This new idiom should be as safe as the current one, but it uses a real 
operator bool() for bool conversion, so no overhead can slip in. To be 
fair, it also has one drawback: The error message a user receives when 
converting a shared_ptr to int, float, ... is not as nice as today.

As I think that efficiency is more important than a nice error message, 
I'd like to propose changing shared_ptr's code appropriately to avoid 
the overhead imposed by the current safe-bool-idiom. Comments?

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial training, research and technology
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