[build] C:\Users\Ryan\rpnx-core\private\sources\all\test3.cpp(119,5): error C3861: '__builtin_unreachable': identifier not found [C:\Users\Ryan\rpnx-core\build\rpnx-core-test3.vcxproj]

Great idea, except that __builtin_unreachable() is a GCC specific extension, and is not part of standard C++. (It would be nice to be able to do this in a cross platform way though! But that is for another discussion.)

--
Ryan P. Nicholl
Tel: (678)-358-7765
Personal Email: rnicholl_at_[hidden]
Work Email: ryan.nicholl_at_[hidden]rosoft.com
Tox: 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Sunday, October 25, 2020 8:50 PM, Barry Revzin <barry.revzin_at_[hidden]> wrote:
> On Sun, Oct 25, 2020 at 7:41 PM Ryan P. Nicholl via Std-Proposals <std-proposals_at_[hidden]> wrote:
>
>> I decided to implement a new class based on std::variant. I call it "rpnx::derivator", but it's basically "allocating_variant". I tried to make it as similar to std::variant as possible. When looking at this, I noticed something weird about std::variant. There is no "zero overhead" way to get the element contained by the variant, as std::get<I> checks for invalid access and throws an exception if invalid. To solve this issue, I would like to propose std::as, which works the same as std::get, but accessing the wrong type is undefined behavior instead of throwing an exception.
>>
>> --
>> Ryan P. Nicholl
>> Tel: (678)-358-7765
>> Personal Email: rnicholl_at_[hidden]
>> Work Email: ryan.nicholl_at_[hidden]
>> Tox: 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>
> You can achieve this by using std::get_if() and marking the nullptr case as unreachable. For example:
>
> auto f(std::variant<int, double>& v) -> int* {
> return std::get_if<int>(&v);
> }
>
> auto g(std::variant<int, double>& v) -> int* {
> auto p = std::get_if<int>(&v);
> if (not p) __builtin_unreachable();
> return p;
> }
>
> On -O1, f emits a check but g does not: https://godbolt.org/z/9G9fd5.
>
> Barry