[llvm-dev] [cfe-dev] RFC: Modernizing our use of auto (original) (raw)

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Tue Dec 4 02:01:37 PST 2018

On Wed, Nov 28, 2018 at 6:25 PM Chris Lattner via cfe-dev < cfe-dev at lists.llvm.org> wrote:

Generally no IMO, because the cases that produce optional are not obvious.

Just to say, +1 from me too.

> * Can we use auto in c++14 lambda arguments with llvm::findif(C, [](const auto& i) { ... }) for example? > * We need to use auto for structured bindings, if those are used. These both get into “yes, if the type is contextually obvious”. I’m not sure how to specify this though. Perhaps the rule came be rewritten more generally, as in “auto should generally only be used when the inferred type is obvious from context, e.g. as a result of a cast like dyncast or as a result of constructing a value with a constructor containing a type name.”?

Strongly agree.

I understand it may not be as precise and unambiguous as people would like, I still think this is the correct core rule: there needs to be some reason why the type is contextually obvious.

And honestly, I'm very happy erring on the side of writing out the type name. I think people are too eager to use auto because it is easy to write but it makes the types substantially harder for the reader to understand.

In the case that came up in review for me, the code I submitted is > > template <typename BaseT, typename DerivedT> > void registerIfNodeMatcher(...) { > auto NodeKind = ASTNodeKind::getFromNodeKind(); > if (!NodeKind.isNone()) > NodeCtors[NodeKind] = std::makepair(MatcherName, Descriptor); > } > > but it was rejected as unreadable and I was given a link to the coding guidelines.

I agree, this seems overly strict.

I mean maybe. But looking at the example, I don't have a clue what type NodeKind is. I think this is borderline, and I think its fine to be somewhat subjective based on the reviewer that is more familiar with the code and APIs in question.

> I'd also like to update them so that > > llvm::Optional<std::pair<std::string, MatcherCtor>> > getNodeConstructorType(ASTNodeKind targetType) { > auto const &ctors = RegistryData->nodeConstructors(); > auto it = llvm::findif( > ctors, [targetType](const NodeConstructorMap::valuetype &ctor) { > return ctor.first.isSame(targetType); > }); > if (it == ctors.end()) > return llvm::None; > return it->second; > } > > is acceptable. The auto it is already acceptable, but the auto const&_ _ctors depends on an interpretation of the guideline and was rejected in the review I submitted.

This seems like a step too far from me. I don’t know what nodeConstructors is here at all, and you’ve erased all type information about what you’re dealing with.

Strongly agree. The auto it continues to seem fine, but the ctors here seems super confusing to leave off all type information. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181204/19f8e616/attachment.html>

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