VS2015 supports magic statics, so why this warning?

Question

I'm upgrading from VS2013 to VS2015 and am getting the following warning. I thought VS2015 implemented magic statics so that the local static object should be thread-safe, so what is going on?

C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\system_error(698): error C2220: warning treated as error - no 'object' file generated
  C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\system_error(704): note: see reference to function template instantiation '_Ty &std::_Immortalize<std::_Generic_error_category>(void)' being compiled
          with
          [
              _Ty=std::_Generic_error_category
          ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\system_error(698): warning C4640: '_Static': construction of local static object is not thread-safe

The error is in the system_error header from the VS installation folder. The error is in this function:

template<class _Ty> inline
    _Ty& _Immortalize()
    {   // return a reference to an object that will live forever
    static _Immortalizer<_Ty> _Static;
    return (*reinterpret_cast<_Ty *>(&_Static._Storage));
    }

That's all the context there is in the error, and I can't see where system_error actually gets included.

Compiler flags are:

/Yu"stdafx.h" /GS /analyze /W3 /wd"4481" /wd"4251" /Zc:wchar_t 
/Zi /Gm- /O2 /sdl /Fd"x64\Release\\Release_vc140.pdb" /Zc:inline /fp:precise
/errorReport:prompt /WX /Zc:forScope /Gd /MT /Fa"x64\Release\" /EHsc /nologo /Fo"x64\Release\" /Fp"x64\Release\MyProj.pch"

Update

Sorry, it seems fixed now. It looks like I was on the v140_xp toolset and wrong TargetPlatformVersion. This one must have slipped through the net when I thought I'd replaced them all. I'm not quite sure why getting those wrong would result in this error though. Anyway, thanks for the help so far.

it is required by standard, and there is nothing magic about it. If MSVC in 2015 fails to follow C++ 11 in such a basic feature, I don't know what to say. — SergeyA, Apr 18 '16 at 13:28
That local static objects are initialized only once and in thread-safe manner. — SergeyA, Apr 18 '16 at 13:36
There is something magic about it, its name! The feature name is "magic statics" — paulm, Apr 18 '16 at 13:37

score 2 · Answer 1 · edited May 23 '17 at 12:17

2

It looks like its caused by the bogus warning which is turned into an Error, read this connect bug:

https://connect.microsoft.com/VisualStudio/feedback/details/2539759/c4640-warning-cannot-be-disabled

The error C2220 indicates you have enables /WX switch, which tells the compiler to treat all warnings as errors.

you can turn this warning off with:

#pragma warning (disable : 4640)

btw. unrelated, but might be usefull to You. You use _Immortalizer as a name for your class which starts with underscore followed by uppercase latter. This is prohibited by standard: read here: What are the rules about using an underscore in a C++ identifier?.

edited May 23 '17 at 12:17

Community

1
1

answered Apr 18 '16 at 13:36

marcinj

48,511
9
79
100

2

That's not my class, that's from the standard headers which is where the error occurs. – Scott Langham Apr 18 '16 at 13:42
@ScottLangham Oh, I understand - If your problem is no longer reproducible then delete question – marcinj Apr 18 '16 at 13:44
1

I can reproduce it, so I would think this question might still be useful if somebody else has their project incorrectly configured and gets the same error. – Scott Langham Apr 18 '16 at 13:46

Sergey Shambir · Answer 2 · 2017-05-04T06:11:01.340

It's caused by changes in MSBuild's toolset v140_xp. Issue appears when:

project builds with VS2015 Update 2, toolset v140_xp
project configuration is Dynamic Library
project was created with ATL App Wizard, so *.vcxproj contains tag <Keyword>AtlProj</Keyword>

There is workaround: open *.vcxproj file, find tag <Keyword>AtlProj</Keyword> and replace it with <Keyword>Win32Proj</Keyword>. Nothing will be changed while compiling, but warning will disappear.

If you want to know details, navigate to the directory C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V140\Platforms\Win32\PlatformToolsets\v140_xp, open Toolset.props and find comment . The line below it enables warning and disables "magic static" feature from C++11.

P.S. Please note that warning isn't useless: 'magic static' uses thread-local storage which suffers from ugly WindowsXP kernel bug: access to TLS from DLL causes crash when application built without TLS support. Any DLL plugins with WinXP support affected by this bug.

VS2015 supports magic statics, so why this warning?

2 Answers2