Use std::tie for multi-member comparisons

Use std::tie for multi-member comparisons

Per discussion on cxx@chromium.org, update C++ style
guide to allow limited use of std::tie() for implementing
lexicographical comparisons of structures, which are
conceptually simple but verbose and easy to get wrong.

This first CL updates a handful of cases as a proof-of-
concept; a tracking bug is filed to audit the code base.

BUG=555171
R=thakis@chromium.org

Committed: https://crrev.com/a882f62eafc5bc81dbe9804ab5f171f3cb90c7cf
Cr-Commit-Position: refs/heads/master@{#359467}

[jsbell, 2015-11-12 21:00:30]

jsbell@chromium.org changed reviewers:
+ thakis@chromium.org

[jsbell, 2015-11-12 21:00:31]

thakis@ - please take a look?

(Pending cxx@ discussion, of course.)

Other than these cases (which I touched recently), a quick grep shows about 60
other cases in chromium that would benefit from this. I could tackle them all in
one CL, or do one CL per top-level directory, or ...?

[Nico, 2015-11-12 21:02:44]

i think it's good to have a small proof-of-concept CL like this one that also
updates style/c++/whateverthefileiscalled.html in the same change to mark this
as allowed, and then file a tracking bug for convert other operator<s and then
do that in small chunks.

Code looks fine, but please update the style doc to allow using std::tie for
implementing operator<, with a link to the discussion thread etc (make it look
like all the allowed language features on chromium-cpp.appspot.com look)

[jsbell, 2015-11-12 21:35:36]

thanks - how does this look?

[Nico, 2015-11-12 21:57:12]

lgtm, thanks!

[Nico, 2015-11-12 21:57:40]

(if you wanted you could file a bug for converting existing operator<s and put
that in the BUG= line)

[jsbell, 2015-11-13 00:35:42]

Description was changed from

==========
Use std::tie for multi-member comparisons
==========

to

==========
Use std::tie for multi-member comparisons

BUG=555171
==========

[jsbell, 2015-11-13 00:40:45]

Description was changed from

==========
Use std::tie for multi-member comparisons

BUG=555171
==========

to

==========
Use std::tie for multi-member comparisons

Per discussion on cxx@chromium.org, update C++ style
guide to allow limited use of std::tie() for implementing
lexicographical comparisons of structures, which are
conceptually simple but verbose and easy to get wrong.

This first CL updates a handful of cases as a proof-of-
concept; a tracking bug is filed to audit the code base.

BUG=555171
R=thakis@chromium.org
==========

[jsbell, 2015-11-13 00:41:13]

On 2015/11/12 21:57:40, Nico wrote:
> (if you wanted you could file a bug for converting existing operator<s and put
> that in the BUG= line)

Done!

[jsbell, 2015-11-13 00:45:52]

The CQ bit was checked by jsbell@chromium.org

[commit-bot: I haz the power, 2015-11-13 00:46:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1438073002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1438073002/20001

[commit-bot: I haz the power, 2015-11-13 02:02:23]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-13 02:03:07]

Patchset 2 (id:??) landed as
https://crrev.com/a882f62eafc5bc81dbe9804ab5f171f3cb90c7cf
Cr-Commit-Position: refs/heads/master@{#359467}