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 ================== 14 MSVC compatibility 15 ================== 16 17 When Clang compiles C++ code for Windows, it attempts to be compatible with 18 MSVC. There are multiple dimensions to compatibility. 19 20 First, Clang attempts to be ABI-compatible, meaning that Clang-compiled code 21 should be able to link against MSVC-compiled code successfully. However, C++ 22 ABIs are particularly large and complicated, and Clang's support for MSVC's C++ 23 ABI is a work in progress. If you don't require MSVC ABI compatibility or don't 24 want to use Microsoft's C and C++ runtimes, the mingw32 toolchain might be a 25 better fit for your project. 26 27 Second, Clang implements many MSVC language extensions, such as 28 ``__declspec(dllexport)`` and a handful of pragmas. These are typically 29 controlled by ``-fms-extensions``. 30 31 Third, MSVC accepts some C++ code that Clang will typically diagnose as 32 invalid. When these constructs are present in widely included system headers, 33 Clang attempts to recover and continue compiling the user's program. Most 34 parsing and semantic compatibility tweaks are controlled by 35 ``-fms-compatibility`` and ``-fdelayed-template-parsing``, and they are a work 36 in progress. 37 38 Finally, there is :ref:`clang-cl`, a driver program for clang that attempts to 39 be compatible with MSVC's cl.exe. 40 41 ABI features 42 ============ 43 44 The 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 CodeView line tables into the 76 object file, similar to what MSVC emits when given the ``/Z7`` flag. 77 Microsoft's link.exe will read this information and use it to create a PDB, 78 enabling stack traces in all modern Windows debuggers. Clang does not emit 79 any type info or description of variable layout. 80 81 * RTTI: :good:`Complete`. Generation of RTTI data structures has been 82 finished, along with support for the ``/GR`` flag. 83 84 * Exceptions and SEH: :none:`Unstarted`. Clang can parse both constructs, but 85 does not know how to emit compatible handlers. 86 87 * Thread-safe initialization of local statics: :none:`Unstarted`. We are ABI 88 compatible with MSVC 2013, which does not support thread-safe local statics. 89 MSVC "14" changed the ABI to make initialization of local statics thread safe, 90 and we have not yet implemented this. 91 92 * Lambdas: :good:`Mostly complete`. Clang is compatible with Microsoft's 93 implementation of lambdas except for providing overloads for conversion to 94 function pointer for different calling conventions. However, Microsoft's 95 extension is non-conforming. 96 97 Template instantiation and name lookup 98 ====================================== 99 100 MSVC allows many invalid constructs in class templates that Clang has 101 historically rejected. In order to parse widely distributed headers for 102 libraries such as the Active Template Library (ATL) and Windows Runtime Library 103 (WRL), some template rules have been relaxed or extended in Clang on Windows. 104 105 The first major semantic difference is that MSVC appears to defer all parsing 106 an analysis of inline method bodies in class templates until instantiation 107 time. By default on Windows, Clang attempts to follow suit. This behavior is 108 controlled by the ``-fdelayed-template-parsing`` flag. While Clang delays 109 parsing of method bodies, it still parses the bodies *before* template argument 110 substitution, which is not what MSVC does. The following compatibility tweaks 111 are necessary to parse the the template in those cases. 112 113 MSVC allows some name lookup into dependent base classes. Even on other 114 platforms, this has been a `frequently asked question`_ for Clang users. A 115 dependent base class is a base class that depends on the value of a template 116 parameter. Clang cannot see any of the names inside dependent bases while it 117 is parsing your template, so the user is sometimes required to use the 118 ``typename`` keyword to assist the parser. On Windows, Clang attempts to 119 follow the normal lookup rules, but if lookup fails, it will assume that the 120 user intended to find the name in a dependent base. While parsing the 121 following program, Clang will recover as if the user had written the 122 commented-out code: 123 124 .. _frequently asked question: 125 http://clang.llvm.org/compatibility.html#dep_lookup 126 127 .. code-block:: c++ 128 129 template <typename T> 130 struct Foo : T { 131 void f() { 132 /*typename*/ T::UnknownType x = /*this->*/unknownMember; 133 } 134 }; 135 136 After recovery, Clang warns the user that this code is non-standard and issues 137 a hint suggesting how to fix the problem. 138 139 As of this writing, Clang is able to compile a simple ATL hello world 140 application. There are still issues parsing WRL headers for modern Windows 8 141 apps, but they should be addressed soon. 142
