[cfe-dev] Clang seems to prefer copy-ctor

[Hubert Tong via cfe-dev]

On Thu, Nov 15, 2018 at 11:51 PM Matthew Urquhart via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hiya,
>
> When initialising class members with braces, it seems Clang doesn't have
> the same behaviour as GCC/MSVC. GCC and MSVC try to use the
> initializer-list-constructor if defined, where Clang appears to be using
> the copy-constructor.
>
> My guess is that copy elision is what might be causing this behaviour. Is
> it correct, or is this a bug?
>
https://clang.llvm.org/cxx_dr_status.html#2137


>
> For the way I'm using it, the initializer-list-constructor behaves quite
> differently to the copy-constructor and triggered completely different
> behaviour between Clang and the other compilers.
>
> Super keen to hear any thoughts.
>
> Cheers,
> Matt
>
> Repro case:
>
> struct ContainedObj {
>     ContainedObj() = default;
>     ContainedObj(ContainedObj const& other) { std::cout << this << ": copy
> ctor" << std::endl; }
>     ContainedObj(std::initializer_list<ContainedObj>) { std::cout << this
> << ": il ctor" << std::endl; }
> };
>
> struct WithBraces {
>     ContainedObj mContained;
>     explicit WithBraces(ContainedObj const& contained) :
> mContained{contained} {}
> };
>
> struct WithBrackets {
>     ContainedObj mContained;
>     explicit WithBrackets(ContainedObj const& contained) :
> mContained(contained) {}
> };
>
> ContainedObj a;
> WithBrackets b(a);
> WithBrackets c{a};
> WithBraces d(a);
> WithBraces e{a};
>
> Output on MSVC 2017 and g++ 6.3.0:
> 00F3FA6B: copy ctor
> 00F3FA5F: copy ctor
> 00F3F887: copy ctor
> 00F3FAF3: il ctor
> 00F3F887: copy ctor
> 00F3FA47: il ctor
>
> Clang output:
> 0x7ffeefbff580: copy ctor
> 0x7ffeefbff578: copy ctor
> 0x7ffeefbff570: copy ctor
> 0x7ffeefbff568: copy ctor
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