[llvm-dev] [cfe-dev] Modernizing LLVM Coding Style Guide and enforcing Clang-tidy

[Piotr Padlewski via llvm-dev]

2017-01-09 19:16 GMT+01:00 David Blaikie <dblaikie at gmail.com>:

>
>
> On Mon, Jan 9, 2017 at 10:10 AM Piotr Padlewski <piotr.padlewski at gmail.com>
> wrote:
>
> 2017-01-09 17:24 GMT+01:00 David Blaikie <dblaikie at gmail.com>:
>
>
>
> On Mon, Jan 9, 2017 at 6:17 AM Piotr Padlewski via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Are there any other comments about changing style guide?
> I would like to add points like
>
> - prefer "using' instead of "typedef"
> - use default member initialization
> struct A {
>   void *ptr = nullptr;
> };
>
>
> (instead of doing it in constructor)
>
> - use default, override, delete
> - skip "virtual" with override
>
> The last point is to get to consensus with
>
> push_back({first, second})
> or
> emplace_back(first ,second);
>
>
>
> It might be a bit noisy, but I'd be inclined to start a separate thread
> for each of these on llvm-dev with a clear subject line relevant to each
> one. So they don't get lost and some of them don't get drowned out by the
> discussion of others, etc.
>
>
> Sure, I can do it, but at least now I don't see much of attention of
> people to any of the subject, so I would not like to start 5 empty threads.
> I guess as long as the thread is not getting noisy I will keep only one.
> Does it sound ok?
>
>
> Perhaps - if no one else pipes up *shrug*
>
> If that's the case, I'd at least suggest submitting the changes to the
> style guide separately with clear subject/titles so people reading commits
> might see/notice/discuss there.
>
> Good idea.


> FWIW: I'm also in favor of push_back where valid. Though I'm not sure it's
> so much a matter of votes, but justification, etc. As for the others -
> sure, they all sound good to me.
>
> I totally agree with all the points Chandler pointed out except the 2.b
(multiple or no arugments). I just prefer emplace_back, but I find the
push_back({}) also clean. So I guess small voting would be a good way to
figure out what people prefer here.


> Also - once these are in the style guide there's still a separate
> discussion to be had about whether automated cleanup is worthwhile & how
> best to go about that sort of thing.
>
>
That's right, I am not proposing any automatic cleanups (yet).


>
>
>
>
> 2016-12-30 12:26 GMT+01:00 Piotr Padlewski <piotr.padlewski at gmail.com>:
>
>
>
> 2016-12-30 11:34 GMT+01:00 Chandler Carruth <chandlerc at gmail.com>:
>
> On Fri, Dec 30, 2016 at 2:08 AM Piotr Padlewski via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Thanks for very accurate responses.
> - I totally agree what Dave and Chandler said about explicit and implicit
> operations, this is what I meant my first email.
>   I believe there are places like
>     v.emplace_back(A, B);
>   istead of
>     v.push_back(make_pair(A, B));b
>   That can make code simpler.
>
>
> Do you have examples? The only ones i can come up with are the ones where
> the push_back variant literally can't compile because the type isn't
> movable.
>
> Perhaps it would be useful to break down categories of can happen here...
>
> Case 1: there is one object already -- this is a *conversion* of a type.
> - If the author of the conversion made it *implicit*, then
> 'v.push_back(x)' just works.
> - If the author of the conversion made it *explicit* I would like to see
> the name of the type explicitly: 'v.push_back(T(x))'.
>
> Case 2a: There is a collection of objects that are being composed into an
> aggregate. We don't have any interesting logic in the constructor, it takes
> an initializer list.
> - This should work with 'v.push_back({a, b, c})'
> - If it doesn't today, we can fix the type's constructors so that it does.
> - Using 'emplace_back' doesn't help much -- you still need {}s to form the
> std::initializer_list in many cases. Pair and tuple are somewhat unusual in
> not requiring them.
>
>  This sounds extremely reasonable.
>
> Case 2b: A specific constructor needs to be called with an argument list.
> These arguments are not merely being aggregated but are inputs to a
> constructor that contains logic.
> - This is analogous to a case called out w.r.t. '{...}' syntax in the
> coding standards[1]
> - Similar to that rule, I would like to see a *call to the constructor*
> rather than hiding it behind 'emplace_back' as this is a function with
> interesting logic.
> - That means i would write T(a, b, c) anyways, and 'v.push_back(T(a, b,
> c))' works.
>
> Calling emplace_back with 0 or multiple arguments is a clear way of saying
> "this constructor takes multiple arguments".
> We can do it with initializer list with easy way like:
> v.emplace_back()        == v.push_back({})
> v.emplace_back(a, b ,c) == v.push_back({a, b, c})
>
> I personally never liked the initializer syntax because of tricky casees
> like:
>
> vector<string> v{{"abc", "def"}};
> Which is equivalent of
> vector<string> v = {std::string("abc", "def")};
> That will call std::string ctor with 2 iterators likely crashing, and
> putting same string might gives us empty string.
>
> In this case programmer probably meant
> std::vector<std:string> v({"abc", "def"});
> or
> std::vector<std::string> v = {"abc", "def"};
>
> But this case is not possible to mess up with push_back (in the case of
> vector<vector<string>> or something). At least I hope it is not.
> So avoiding braces is my personal preference. It is fine for me if we
> would choose to prefer 'v.push_back({a, b, c})' instead of 'v.emplace_back(a,
> b, c)', ofc as long as most of the community would prefer first form to
> the second :)
>
>
> [1]: http://llvm.org/docs/CodingStandards.html#do-not-
> use-braced-initializer-lists-to-call-a-constructor
>
> Case 3: Passing objects of type 'T' through 'push_back' fails to compile
> because they cannot be copied or moved.
> - You *must* use 'emplace_back' here. No argument (obviously).
>
> My experience with LLVM code and other codebases is that case 3 should be
> extremely rare. The intersection of "types that cannot be moved or copied"
> and "types that you put into containers" is typically small.
>
>
> Anyways, I don't disagree with this point with a tiny fix:
>
> I think in cases like this we can leave it for judgement of contributor.
>
> *or reviewer*. ;]
>
> I continue to think exceptions can be made in rare cases when folks have
> good reasons. But I expect this to be quite rare. =]
>
>
> Piotr
>
>
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