> On May 15, 2018, at 7:23 PM, Richard Smith <rich...@metafoo.co.uk> wrote: > On 15 May 2018 at 16:01, John McCall via cfe-dev <cfe-...@lists.llvm.org > <mailto:cfe-...@lists.llvm.org>> wrote: >> On May 15, 2018, at 6:05 PM, George Karpenkov via cfe-dev >> <cfe-...@lists.llvm.org <mailto:cfe-...@lists.llvm.org>> wrote: >> >> +cfe-dev >> >> Hi Andrea, >> >> I think you might get more luck asking on the cfe-dev mailing list. >> >> George >> >>> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users >>> <cfe-users@lists.llvm.org <mailto:cfe-users@lists.llvm.org>> wrote: >>> >>> Dear all, >>> Recently, my team suffered from a bug due to a double bad usage of C++. >>> >>> We have a function returning a reference to an object: >>> >>> Object& GetObject(); >>> >>> Sometimes we use this function like this: >>> >>> auto obj = GetObject(); >>> >>> This triggers a copy of the object, which we really don't mean. The two >>> problems are: >>> 1. Object does not delete the copy constructor, nor does it declare one. We >>> have a policy of never using implicitly-declared constructors, we either >>> use `=delete` or `=default`. Nevertheless we missed this class. > > Implicitly-defined copy constructors are ubiquitous in idiomatic C++. Maybe > that's not true in your project, but still, this seems too special-case for > the compiler. Maybe a linter that has a more sophisticated model of what > code is yours vs. the standard library. > >>> 2. A reference is demoted to a rvalue due to the usage of `auto` instead of >>> `auto&`. > > This is a more reasonable thing to try to warn about. I have two concerns: > - I don't know a reasonable way to suppress the warning if you really do > want to load from the l-value. > - I have a non-specific worry that it'll disrupt some important idiom that > I'm just not thinking of. > > How about: > > vector<some_complex_type> v = get_vector(); > auto x = v.front(); > > or, worse: > > auto y = get_vector().front();
Yep, good example. John.
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