On 10/7/2013 2:05 PM, Stephan T. Lavavej wrote:
If this is left out of a function specification, is not the default calling convention user-specified, with __cdecl being the default ?
Yes for ordinary functions (non-varargs, non-member). Member functions (non-varargs) always default to __thiscall.
So if the end-user changes the default to something other than __cdecl is that considered the end-user's fault with VC++ ?
Our goal for VC's STL is to be immune to all of the calling-convention switches (/Gd, /Gr, /Gz, /Gv). I believe that 2013's
has achieved this (except for /Gv, which we've fixed post-2013-RTM), although I am aware of bugs elsewhere (mostly in <functional>). Because varargs functions (member and non-member) are always __cdecl, regardless of the calling-convention switches, it doesn't matter if you mark them as such, or leave them unmarked.
I believe that clang wants to be compatible enough to VC++ on Windows to be able to compile Windows header files, but still wants to follow the C++ standard as closely as possible.
Well, calling conventions are non-Standard.
Agreed ! What I think clang should do, when emulating VC++ under Windows, is to do what VC++ does. In the area of calling conventions with varargs clang should accept anything the user specifies and silently treat the calling convention as __cdecl. As I understand your explanation that is what VC++ does. OTOH, when dealing with VC++ and other compilers emulating VC++ by defining _MSC_VER, I am going to suggest that Boost type_traits only present vararg overloads with no calling convention, as there is no need to specify vararg overloads with any calling convention because they all become __cdecl anyway. Thanks very much for your comments and help with how VC++ treats this topic.
STL
-----Original Message----- From: Boost [mailto:boost-bounces@lists.boost.org] On Behalf Of Edward Diener Sent: Sunday, October 06, 2013 4:02 PM To: boost@lists.boost.org Subject: Re: [boost] [type_traits] Variadic function pointer with __fastcall calling convention
On 10/5/2013 4:02 PM, Stephan T. Lavavej wrote:
[Edward Diener]
The problem from clang's perspective is that they must emulate this "feature" <g> to compile Boost type_traits. Their developers feel that Boost type_traits should not depend on this feature, ie. should not specify any VC++ calling conventions for varargs function or function template declarations.
I agree with clang's devs (because that's what VC's
does). Thanks for the encouragement. I will quote you when I propose to John Maddock, the main Boost type_traits implementor, that Boost should so the same.
What I mean is that in VC++ the declarations are redundant, being the same template redeclared.
Right. So if you want to absorb all old-style varargs, you need only one overload/specialization.
It looks as if the overload specialization for C++ should specify __cdecl. If this is left out of a function specification, is not the default calling convention user-specified, with __cdecl being the default ? So if the end-user changes the default to something other than __cdecl is that considered the end-user's fault with VC++ ?
In this case clang either has to change to be like VC++
Oh, I thought clang was attempting to be VC-compatible here (like our copy of EDG for Intellisense does).
I believe that clang wants to be compatible enough to VC++ on Windows to be able to compile Windows header files, but still wants to follow the C++ standard as closely as possible. Of course it is a bit of a tightrope act because 3rd party libraries for Windows which test for VC++ using _MSC_VER and other VC++ macros which clang on Windows supports, may want to be able to use clang to compile the code also.
If clang is supposed to be VC-compatible, then they should treat varargs with calling conventions like VC does, and Boost should do what VC
does (i.e. one overload/specialization with no calling convention - or __cdecl would work, I think). It seems as if __cdecl is better than no calling convention, because the end-user has the ability to change the default calling convention on VC++.
If clang wants to be different from VC, then Boost should do something different for clang and VC.
I do not work developing clang but I am just trying to compile Boost code with the clang toolset.