Technically, yes, *get_if is enough, but most compilers can't recognize it yet. I think gcc can under specific circumstances, but the rest I've tested failed to do so.

On Sun, Oct 25, 2020 at 6:25 PM Tony V E <tvaneerd@gmail.com> wrote:
I don't think you need unreachable. 
*get_if should be enough. Unconditionally dereference the result.

Sent from my BlackBerry portable Babbage Device
From: Justin Bassett via Std-Proposals
Sent: Sunday, October 25, 2020 9:16 PM
To: Std-Proposals
Cc: Justin Bassett
Subject: Re: [std-proposals] std::as

I'm glad to see that clang can also optimize this get_if access; it used to be unable to do so. MSVC is also unable to optimize this when using __assume ( https://docs.microsoft.com/en-us/cpp/intrinsics/assume?view=vs-2019 ) (which is the same as Clang's __builtin_assume() https://clang.llvm.org/docs/LanguageExtensions.html#builtin-assume ) instead of __builtin_unreachable().

In theory, a portable __builtin_unreachable() would be:

[[noreturn]]
inline void unreachable() {} // basically, unconditionally trigger undefined behavior

In practice, only GCC seems to recognize this.

--Justin Bassett

On Sun, Oct 25, 2020 at 5:59 PM Ryan P. Nicholl via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
[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.)

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Ryan P. Nicholl
Tel: (678)-358-7765
Personal Email: rnicholl@protonmail.com
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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Sunday, October 25, 2020 8:50 PM, Barry Revzin <barry.revzin@gmail.com> wrote:



On Sun, Oct 25, 2020 at 7:41 PM Ryan P. Nicholl via Std-Proposals <std-proposals@lists.isocpp.org> 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.

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Ryan P. Nicholl
Tel: (678)-358-7765
Personal Email: rnicholl@protonmail.com
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

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