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	Wed, 30 Oct 2024 09:30:15 -0700 (PDT)
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Subject:	Re: g++ 12 -std=gnu++20
In-Reply-To:	<764d17ab-a660-4e89-8fd3-175e93f11507@gmail.com>
Date:	Wed, 30 Oct 2024 17:30:14 +0100
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From:	Dimitry Andric via Cygwin <cygwin AT cygwin DOT com>
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On 30 Oct 2024, at 17:14, Csaba Ráduly via Cygwin <cygwin AT cygwin DOT com> wrote:
> 
> On 30/10/2024 17:00, Dimitry Andric via Cygwin wrote:
>> #include<string>
>> #include<cassert>
>> 
>> constexpr bool foo()
>> {
>>   std::string str2{"abcwe"};
>>   return str2.size()==5;
>> }
>> 
>> static_assert(foo());
>> 
>> int main()
>> {
>>   assert(foo());
>> }
> 
> Seems like _GLIBCXX_USE_CXX11_ABI is not defined by default.

Indeed, /usr/lib/gcc/x86_64-pc-cygwin/12/include/c++/x86_64-pc-cygwin/bits/c++config.h shows:

   317  # define _GLIBCXX_USE_DUAL_ABI 1
   318
   319  #if ! _GLIBCXX_USE_DUAL_ABI
   320  // Ignore any pre-defined value of _GLIBCXX_USE_CXX11_ABI
   321  # undef _GLIBCXX_USE_CXX11_ABI
   322  #endif
   323
   324  #ifndef _GLIBCXX_USE_CXX11_ABI
   325  # define _GLIBCXX_USE_CXX11_ABI 0
   326  #endif
 
Also, g++ -v shows --with-default-libstdcxx-abi=gcc4-compatible, so I guess this was deliberate on the part of the packagers. Probably for backwards compatibility reasons?

-Dimitry


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