Re: [RFA] Algorithms vs operator* and operator==

[chris <caj at cs dot york dot ac dot uk>]

Paolo Carlini wrote:

> Hi,
>
> seems to me the natural development of the improvements to locale_facets
> and stl_numeric but want to make sure I'm not missing something before
> actually starting on it.
>
> The fundamental observation is rather trivial: for a generic 
> input_iterator
> both operator* and operator== can be expensive and we should do our best
> to minimize the number of calls throughout in the library.
>
> This morning I changed tentatively std::replace_copy (see attached) and
> benchmarked this type of test on my home machine:
....

> +	  const _ValueType __value = *__first;
>  
I apologise for the (I suspect) slightly confusing mails I've been 
sending on this subject, but I believe I have finally sorted out what I 
want to say...

This seems to be the line we have trouble with, as it could lead to an 
expensive copy. I haven't fully tested it yet, but I wonder if this line 
would be better expressed as:
typeof(*__first) __value = *__first; (where I've only just found typeof).

I've done some benchmarks, and I'm reasonably convinced that this is 
both safe and the most efficent way of doing things. If *__first is a 
reference, we just store the reference and access it twice. If *__first 
is returned by value, then we only get it by value once and use it 
twice. Does this seem safe / sensible?

Chris