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Subject: Re: [std-proposals] Feature test macro for variant<bool, string> taking const char* fix?
From: Emile Cormier (emile.cormier.jr_at_[hidden])
Date: 2020-12-07 12:02:31


I'm going ahead and submitting an LWG issue unless someone points out that
I've overlooked something.

On Sun, Dec 6, 2020 at 6:44 PM Emile Cormier <emile.cormier.jr_at_[hidden]>
wrote:

> Okay, perhaps not the perfect example. But the problem is that the
> behavior of std::variant changed between C++17 and C++20 and there's no
> easy way to tell at compile time which behavior std::variant with exhibit.
> It's the principle of the thing.
>
> Your same "why not just leave the workaround code" argument could apply
> for many other language and standard library fixes which DO have a feature
> test macro (or an increased version of the macro).
>
> To be clear, I'm not asking for programming help (I'd go to Stackoverflow
> for that). I'm raising the flag that there's perhaps an omission in the
> C++20 spec.
>
> On Sun, Dec 6, 2020 at 6:34 PM Barry Revzin <barry.revzin_at_[hidden]>
> wrote:
>
>>
>>
>> On Sun, Dec 6, 2020 at 4:29 PM Emile Cormier <emile.cormier.jr_at_[hidden]>
>> wrote:
>>
>>> I do, Barry. Something like this (not tested):
>>>
>>> struct parse_event
>>> {
>>> template <typename T>
>>> parse_event(T&& value) : token(std::forward<T>(value)) {}
>>>
>>> #ifndef VARIANT_SAFELY_TAKES_CHAR_POINTER
>>> // Workaround
>>> parse_event(const char* s) : token(std::string(s)) {}
>>> #endif
>>>
>>> std::variant<bool, int, std::string> token;
>>> };
>>>
>>> The idea is to disable the workaround code when the C++ library
>>> implementation has the P0608/P1957 fixes.
>>>
>>
>> You could just write:
>>
>> struct parse_event
>> {
>> template <typename T>
>> parse_event(T&& value) : token(std::forward<T>(value)) {}
>>
>> // Workaround
>> parse_event(const char* s) : token(std::string(s)) {}
>>
>> std::variant<bool, int, std::string> token;
>> };
>>
>> This works regardless of whether the fix is implemented.
>>
>> Barry
>>
>



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