[boost] Re: parameters library and rvalue pitfall

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2005-05-09 11:59:11

David Abrahams wrote:
> Daniel Wallin <dalwan01_at_[hidden]> writes:
> 
> 
>>David Abrahams wrote:
>>[...]
>>
>>>The question is, what should we do about this?  It turns out to be
>>>surprisingly hard to write the declarations of these temporary
>>>variables so they work in all cases.  For example, v had to be a
>>>const& because in one case index_result<...>::type turned out to be
>>>
>>>  char const[4]
>>>
>>>and when the default is used, it's 
>>>
>>>  double 
>>>
>>>with p[value || ...] returning a double rvalue (by value).
>>>
>>>Thoughts?
>>
>>Make the user add reference to the Default type instead of
>>index_result<>::type?
> 
> 
> I don't fully understand what you're proposing or what it's supposed
> to address, but I think you might be missing the point.  To make it
> safe to use, ideally
> 
>   index_result<...>::type whatever = p[x | some-expression];
> 
> would always cause a by-value return from p[...] when some-expression
> is an rvalue.

It's perfectly safe for p[...] to always return by reference, even if
some-expression is an rvalue. The only problem is that it's hard for the
user to determine if the result should be held by reference or not. I
propose that the user tells index_result<> if the result of the default
expression should be by reference, so that:

   index_result<.., int>::type whatever = p[x | 0];

whatever stores a copy of 0 in the default-case, and stores a reference
to p[x] otherwise.

   int default_ = ...;
   index_result<.., int const&>::type whatever = p[x | default_];

See what I'm saying now? Am I missing the point?

-- 
Daniel Wallin