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

[Piotr Padlewski via llvm-dev]

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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