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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 9 08:24:09 PST 2017

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.

> 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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