[Development] RFC: more liberal 'auto' rules?

[Julien Blanc]

Le vendredi 04 décembre 2015 à 18:10 -0500, Matthew Woehlke a écrit : 
> On 2015-12-04 17:43, André Pönitz wrote:
> > On Fri, Dec 04, 2015 at 04:33:26PM -0500, Matthew Woehlke wrote:
> >> Which of these is easier to read? (More importantly, which is easier to
> >> *verify* that it is correct?)
> >>
> >>   for (int i = 0; i < list.size(); ++i)
> >>     foo(list[i]);
[snip] 
> 
> >>   for (auto i : qtIndexRange(list.size())
> >>     foo(list[i]);
> > 
> > In any case, this is an uncommon pattern, using some unknown qtIndexRange()
> > function.
> 
> Really?
> 
>   (Python)
>   for i in range(len(list))

Am I the only one to think that this example is inherently broken ? I
mean, why wouldn’t any sane person write :

    for(auto const& element : list)
        foo(element)

or
    std::for_each(list.begin(), list.end(), foo);

Because :
- it’s shorter
- it’s more readable
- it works for non random-access-container types
- bonus : range checking is not needed, so it should be more performant
- bonus : it doesn’t resort to a temporary value, same remark


Now, getting back to what Marc proposed initially, because the debate
has gone far away from his initial proposal :

> - template code (esp., but not necessarily only, when the type name
> would require the use of 'typename')

This is the one i’m not at ease with. Template code tends to be
difficult to read, partly *because* there’s no type information. For
readability concerns, i would prefer a local typedef named by the
concept. Yes, that makes it two lines instead of one, but i think that’s
one case where hinting the reader with a well-chosen type name makes a
lot of sense.

> - all loop variables (both index and iterators)
>   the reason being that spelling out the name is usually wrong:
>  size_t i = stdVector.size() // wrong, should be
>  std::vector::size_type...
>  also helps when porting from Qt containers to STL ones

That looks perfectly fine for me. Index based loops should be avoided
whenever it is possible, and for iterator based loops auto makes no harm
(seriously, who wants to write
std::vector<my_very_long_type>::const_iterator instead of auto ?).

> - all references to elements in a collection (same problem - way too
> easy to misspell std::pair<int, int> for std::pair<const int, int>...)

Same thing.

Regards,

Julien