“warning C4350: behavior change” when including <string> and no precompiled header

Question

Warning C4350 says "A non-const reference may only be bound to an lvalue". I think since C4350 is off by default I haven't found much out there on it. I have read this post and it makes sense: Non-const reference may only be bound to an lvalue

Anyway, I'm asking about it because I'm trying to update VS2008 VC++ projects to VS2012. When I do I get hundreds of C4350 warnings which I have turned on for some legacy reason (that admittedly may need to be reassessed). I have whittled it down to this simple bit of code that can reproduce the warning, but only if I have precompiled headers turned off:

#pragma warning(default:4350)

#include "stdafx.h"
#include <string>

int _tmain(int argc, char* argv[])
{
   return 0;
}

If I turn precomiled headers on I get no warning. I also can't get it to happen in VS2008.

Is this a Visual Studio 2012 bug in std:string? Why does the precompiled header change the behavior? How do I fix the problem, as opposed to just ignoring the warning? Thanks!

The warning is (sorry for the poor formatting, it was the best I could figure):

c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(689): warning C4350: behavior change: 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc(const std::_Wrap_alloc<_Alloc> &) throw()' called instead of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc>(_Other &) throw()'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(838) : see declaration of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(850) : see declaration of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> A non-const reference may only be bound to an lvalue

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(688) : while compiling class template member function 'std::_Wrap_alloc<_Alloc> std::_String_alloc<_Al_has_storage,_Alloc_types>::_Getal(void) const'

1> with

1> [

1> _Alloc=std::allocator,

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(898) : see reference to function template instantiation 'std::_Wrap_alloc<_Alloc> std::_String_alloc<_Al_has_storage,_Alloc_types>::_Getal(void) const' being compiled

1> with

1> [

1> _Alloc=std::allocator,

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(700) : see reference to class template instantiation 'std::_String_alloc<_Al_has_storage,_Alloc_types>' being compiled

1> with

1> [

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdexcept(31) : see reference to class template instantiation 'std::basic_string<_Elem,_Traits,_Alloc>' being compiled

1> with

1> [

1> _Elem=char,

1> _Traits=std::char_traits,

1> _Alloc=std::allocator

1> ]

1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(689): warning C4350: behavior change: 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc(const std::_Wrap_alloc<_Alloc> &) throw()' called instead of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc>(_Other &) throw()'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(838) : see declaration of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(850) : see declaration of 'std::_Wrap_alloc<_Alloc>::_Wrap_alloc'

1> with

1> [

1> _Alloc=std::allocator

1> ]

1> A non-const reference may only be bound to an lvalue

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(688) : while compiling class template member function 'std::_Wrap_alloc<_Alloc> std::_String_alloc<_Al_has_storage,_Alloc_types>::_Getal(void) const'

1> with

1> [

1> _Alloc=std::allocator,

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(898) : see reference to function template instantiation 'std::_Wrap_alloc<_Alloc> std::_String_alloc<_Al_has_storage,_Alloc_types>::_Getal(void) const' being compiled

1> with

1> [

1> _Alloc=std::allocator,

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(700) : see reference to class template instantiation 'std::_String_alloc<_Al_has_storage,_Alloc_types>' being compiled

1> with

1> [

1> _Al_has_storage=false,

1> _Alloc_types=std::_String_base_types>

1> ]

1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\string(689) : see reference to class template instantiation 'std::basic_string<_Elem,_Traits,_Alloc>' being compiled

1> with

1> [

1> _Elem=wchar_t,

1> _Traits=std::char_traits,

1> _Alloc=std::allocator

1> ]

Answer 1

Microsoft answered my question here:

http://connect.microsoft.com/VisualStudio/feedback/details/767960/warning-c4350-behavior-change-when-including-string-and-no-precompiled-header

Summary:

Is this a Visual Studio 2012 bug in std:string? -They don't count it as a bug if if cleanly builds at warning level 4, which excludes this warning

Why does the precompiled header change the behavior? -The compiler ignores everything that comes before a precompiled header, which in this case was my pragma statement that was enabling the warning. Enabling precompiled headers in the property settings caused the pragma to be ignored. Who knew?

How do I fix the problem, as opposed to just ignoring the warning? -No fix it would seem, just ignore the warning