[Development] RFC: more liberal 'auto' rules?
Marc Mutz
marc.mutz at kdab.com
Mon Dec 7 17:42:50 CET 2015
On Monday 07 December 2015 14:41:00 Knoll Lars wrote:
> On 07/12/15 15:44, "Development on behalf of Marc Mutz" <development-
bounces at qt-project.org on behalf of marc.mutz at kdab.com> wrote:
> >On Monday 07 December 2015 13:48:58 Ziller Eike wrote:
> >> I do not think that more usage of ‘auto’ will make any code (or
> >> refactorings of it) ‘safer’. IMO this is only about convenience and
> >> readability.
> >>
> > std::map<std::string, std::string> stdMap = ...;
> >
> > for (const std::pair<std::string, std::string> &e : stdMap)
> >
> > doSomething(e.first, e.second);
> >
> > for (const auto &e : stdMap)
> >
> > doSomething(e.first, e.second);
> >
> >The second loop is at least two orders of magnitude faster (doSomething()
> >is an out-of-line no-op).
>
> I think the summary here is that auto gives you one guarantee: It won’t do
> an implicit conversion for the initial assignment.
Correct. The first line is missing the const in the pair's first type, thus
forces an implicit conversion to the declared type (the const-& conveniently
extending the lifetime of the temporary so everything appears to work, execept
you're deep-copying two std::strings now).
--
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt Experts
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