diff options
| -rw-r--r-- | clang/docs/MSVCCompatibility.rst | 51 |
1 files changed, 35 insertions, 16 deletions
diff --git a/clang/docs/MSVCCompatibility.rst b/clang/docs/MSVCCompatibility.rst index 8e20cb4b5e7..32efb76db25 100644 --- a/clang/docs/MSVCCompatibility.rst +++ b/clang/docs/MSVCCompatibility.rst @@ -97,26 +97,45 @@ The status of major ABI-impacting C++ features: Template instantiation and name lookup ====================================== -In addition to the usual `dependent name lookup FAQs`_, Clang is often unable to -parse certain invalid C++ constructs that MSVC allows. As of this writing, -Clang will reject code with missing ``typename`` annotations: - -.. _dependent name lookup FAQs: +MSVC allows many invalid constructs in class templates that Clang has +historically rejected. In order to parse widely distributed headers for +libraries such as the Active Template Library (ATL) and Windows Runtime Library +(WRL), some template rules have been relaxed or extended in Clang on Windows. + +The first major semantic difference is that MSVC appears to defer all parsing +an analysis of inline method bodies in class templates until instantiation +time. By default on Windows, Clang attempts to follow suit. This behavior is +controlled by the ``-fdelayed-template-parsing`` flag. While Clang delays +parsing of method bodies, it still parses the bodies *before* template argument +substitution, which is not what MSVC does. The following compatibility tweaks +are necessary to parse the the template in those cases. + +MSVC allows some name lookup into dependent base classes. Even on other +platforms, this has been a `frequently asked question`_ for Clang users. A +dependent base class is a base class that depends on the value of a template +parameter. Clang cannot see any of the names inside dependent bases while it +is parsing your template, so the user is sometimes required to use the +``typename`` keyword to assist the parser. On Windows, Clang attempts to +follow the normal lookup rules, but if lookup fails, it will assume that the +user intended to find the name in a dependent base. While parsing the +following program, Clang will recover as if the user had written the +commented-out code: + +.. _frequently asked question: http://clang.llvm.org/compatibility.html#dep_lookup .. code-block:: c++ - struct X { - typedef int type; + template <typename T> + struct Foo : T { + void f() { + /*typename*/ T::UnknownType x = /*this->*/unknownMember; + } }; - template<typename T> int f() { - // missing typename keyword - return sizeof(/*typename*/ T::type); - } - template void f<X>(); -Accepting code like this is ongoing work. Ultimately, it may be cleaner to -`implement a token-based template instantiation mode`_ than it is to add -compatibility hacks to the existing AST-based instantiation. +After recovery, Clang warns the user that this code is non-standard and issues +a hint suggesting how to fix the problem. -.. _implement a token-based template instantiation mode: http://llvm.org/PR18714 +As of this writing, Clang is able to compile a simple ATL hello world +application. There are still issues parsing WRL headers for modern Windows 8 +apps, but they should be addressed soon. |
