1.. raw:: html 2 3 <style type="text/css"> 4 .none { background-color: #FFCCCC } 5 .partial { background-color: #FFFF99 } 6 .good { background-color: #CCFF99 } 7 </style> 8 9.. role:: none 10.. role:: partial 11.. role:: good 12 13================== 14MSVC compatibility 15================== 16 17When Clang compiles C++ code for Windows, it attempts to be compatible with 18MSVC. There are multiple dimensions to compatibility. 19 20First, Clang attempts to be ABI-compatible, meaning that Clang-compiled code 21should be able to link against MSVC-compiled code successfully. However, C++ 22ABIs are particularly large and complicated, and Clang's support for MSVC's C++ 23ABI is a work in progress. If you don't require MSVC ABI compatibility or don't 24want to use Microsoft's C and C++ runtimes, the mingw32 toolchain might be a 25better fit for your project. 26 27Second, Clang implements many MSVC language extensions, such as 28``__declspec(dllexport)`` and a handful of pragmas. These are typically 29controlled by ``-fms-extensions``. 30 31Third, MSVC accepts some C++ code that Clang will typically diagnose as 32invalid. When these constructs are present in widely included system headers, 33Clang attempts to recover and continue compiling the user's program. Most 34parsing and semantic compatibility tweaks are controlled by 35``-fms-compatibility`` and ``-fdelayed-template-parsing``, and they are a work 36in progress. 37 38Finally, there is :ref:`clang-cl`, a driver program for clang that attempts to 39be compatible with MSVC's cl.exe. 40 41ABI features 42============ 43 44The status of major ABI-impacting C++ features: 45 46* Record layout: :good:`Complete`. We've tested this with a fuzzer and have 47 fixed all known bugs. 48 49* Class inheritance: :good:`Mostly complete`. This covers all of the standard 50 OO features you would expect: virtual method inheritance, multiple 51 inheritance, and virtual inheritance. Every so often we uncover a bug where 52 our tables are incompatible, but this is pretty well in hand. This feature 53 has also been fuzz tested. 54 55* Name mangling: :good:`Ongoing`. Every new C++ feature generally needs its own 56 mangling. For example, member pointer template arguments have an interesting 57 and distinct mangling. Fortunately, incorrect manglings usually do not result 58 in runtime errors. Non-inline functions with incorrect manglings usually 59 result in link errors, which are relatively easy to diagnose. Incorrect 60 manglings for inline functions and templates result in multiple copies in the 61 final image. The C++ standard requires that those addresses be equal, but few 62 programs rely on this. 63 64* Member pointers: :good:`Mostly complete`. Standard C++ member pointers are 65 fully implemented and should be ABI compatible. Both `#pragma 66 pointers_to_members`_ and the `/vm`_ flags are supported. However, MSVC 67 supports an extension to allow creating a `pointer to a member of a virtual 68 base class`_. Clang does not yet support this. 69 70.. _#pragma pointers_to_members: 71 http://msdn.microsoft.com/en-us/library/83cch5a6.aspx 72.. _/vm: http://msdn.microsoft.com/en-us/library/yad46a6z.aspx 73.. _pointer to a member of a virtual base class: http://llvm.org/PR15713 74 75* Debug info: :partial:`Minimal`. Clang emits both CodeView line tables 76 (similar to what MSVC emits when given the ``/Z7`` flag) and DWARF debug 77 information into the object file. 78 Microsoft's link.exe will transform the CodeView line tables into a PDB, 79 enabling stack traces in all modern Windows debuggers. Clang does not emit 80 any CodeView-compatible type info or description of variable layout. 81 Binaries linked with either binutils' ld or LLVM's lld should be usable with 82 GDB however sophisticated C++ expressions are likely to fail. 83 84* RTTI: :good:`Complete`. Generation of RTTI data structures has been 85 finished, along with support for the ``/GR`` flag. 86 87* C++ Exceptions: :good:`Mostly complete`. Support for 88 C++ exceptions (``try`` / ``catch`` / ``throw``) have been implemented for 89 x86 and x64. Our implementation has been well tested but we still get the 90 odd bug report now and again. 91 C++ exception specifications are ignored, but this is `consistent with Visual 92 C++`_. 93 94.. _consistent with Visual C++: 95 https://msdn.microsoft.com/en-us/library/wfa0edys.aspx 96 97* Asynchronous Exceptions (SEH): :partial:`Partial`. 98 Structured exceptions (``__try`` / ``__except`` / ``__finally``) mostly 99 work on x86 and x64. 100 LLVM does not model asynchronous exceptions, so it is currently impossible to 101 catch an asynchronous exception generated in the same frame as the catching 102 ``__try``. 103 104* Thread-safe initialization of local statics: :good:`Complete`. MSVC 2015 105 added support for thread-safe initialization of such variables by taking an 106 ABI break. 107 We are ABI compatible with both the MSVC 2013 and 2015 ABI for static local 108 variables. 109 110* Lambdas: :good:`Mostly complete`. Clang is compatible with Microsoft's 111 implementation of lambdas except for providing overloads for conversion to 112 function pointer for different calling conventions. However, Microsoft's 113 extension is non-conforming. 114 115Template instantiation and name lookup 116====================================== 117 118MSVC allows many invalid constructs in class templates that Clang has 119historically rejected. In order to parse widely distributed headers for 120libraries such as the Active Template Library (ATL) and Windows Runtime Library 121(WRL), some template rules have been relaxed or extended in Clang on Windows. 122 123The first major semantic difference is that MSVC appears to defer all parsing 124an analysis of inline method bodies in class templates until instantiation 125time. By default on Windows, Clang attempts to follow suit. This behavior is 126controlled by the ``-fdelayed-template-parsing`` flag. While Clang delays 127parsing of method bodies, it still parses the bodies *before* template argument 128substitution, which is not what MSVC does. The following compatibility tweaks 129are necessary to parse the template in those cases. 130 131MSVC allows some name lookup into dependent base classes. Even on other 132platforms, this has been a `frequently asked question`_ for Clang users. A 133dependent base class is a base class that depends on the value of a template 134parameter. Clang cannot see any of the names inside dependent bases while it 135is parsing your template, so the user is sometimes required to use the 136``typename`` keyword to assist the parser. On Windows, Clang attempts to 137follow the normal lookup rules, but if lookup fails, it will assume that the 138user intended to find the name in a dependent base. While parsing the 139following program, Clang will recover as if the user had written the 140commented-out code: 141 142.. _frequently asked question: 143 http://clang.llvm.org/compatibility.html#dep_lookup 144 145.. code-block:: c++ 146 147 template <typename T> 148 struct Foo : T { 149 void f() { 150 /*typename*/ T::UnknownType x = /*this->*/unknownMember; 151 } 152 }; 153 154After recovery, Clang warns the user that this code is non-standard and issues 155a hint suggesting how to fix the problem. 156 157As of this writing, Clang is able to compile a simple ATL hello world 158application. There are still issues parsing WRL headers for modern Windows 8 159apps, but they should be addressed soon. 160