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==================
MSVC compatibility
==================
When Clang compiles C++ code for Windows, it attempts to be compatible with
MSVC. There are multiple dimensions to compatibility.
First, Clang attempts to be ABI-compatible, meaning that Clang-compiled code
should be able to link against MSVC-compiled code successfully. However, C++
ABIs are particularly large and complicated, and Clang's support for MSVC's C++
ABI is a work in progress. If you don't require MSVC ABI compatibility or don't
want to use Microsoft's C and C++ runtimes, the mingw32 toolchain might be a
better fit for your project.
Second, Clang implements many MSVC language extensions, such as
``__declspec(dllexport)`` and a handful of pragmas. These are typically
controlled by ``-fms-extensions``.
Third, MSVC accepts some C++ code that Clang will typically diagnose as
invalid. When these constructs are present in widely included system headers,
Clang attempts to recover and continue compiling the user's program. Most
parsing and semantic compatibility tweaks are controlled by
``-fms-compatibility`` and ``-fdelayed-template-parsing``, and they are a work
in progress.
Finally, there is :ref:`clang-cl`, a driver program for clang that attempts to
be compatible with MSVC's cl.exe.
ABI features
============
The status of major ABI-impacting C++ features:
* Record layout: :good:`Complete`. We've tested this with a fuzzer and have
fixed all known bugs.
* Class inheritance: :good:`Mostly complete`. This covers all of the standard
OO features you would expect: virtual method inheritance, multiple
inheritance, and virtual inheritance. Every so often we uncover a bug where
our tables are incompatible, but this is pretty well in hand. This feature
has also been fuzz tested.
* Name mangling: :good:`Ongoing`. Every new C++ feature generally needs its own
mangling. For example, member pointer template arguments have an interesting
and distinct mangling. Fortunately, incorrect manglings usually do not result
in runtime errors. Non-inline functions with incorrect manglings usually
result in link errors, which are relatively easy to diagnose. Incorrect
manglings for inline functions and templates result in multiple copies in the
final image. The C++ standard requires that those addresses be equal, but few
programs rely on this.
* Member pointers: :good:`Mostly complete`. Standard C++ member pointers are
fully implemented and should be ABI compatible. Both `#pragma
pointers_to_members`_ and the `/vm`_ flags are supported. However, MSVC
supports an extension to allow creating a `pointer to a member of a virtual
base class`_. Clang does not yet support this.
.. _#pragma pointers_to_members:
https://msdn.microsoft.com/en-us/library/83cch5a6.aspx
.. _/vm: https://msdn.microsoft.com/en-us/library/yad46a6z.aspx
.. _pointer to a member of a virtual base class: https://llvm.org/PR15713
* Debug info: :good:`Mostly complete`. Clang emits relatively complete CodeView
debug information if ``/Z7`` or ``/Zi`` is passed. Microsoft's link.exe will
transform the CodeView debug information into a PDB that works in Windows
debuggers and other tools that consume PDB files like ETW. Work to teach lld
about CodeView and PDBs is ongoing.
* RTTI: :good:`Complete`. Generation of RTTI data structures has been
finished, along with support for the ``/GR`` flag.
* C++ Exceptions: :good:`Mostly complete`. Support for
C++ exceptions (``try`` / ``catch`` / ``throw``) have been implemented for
x86 and x64. Our implementation has been well tested but we still get the
odd bug report now and again.
C++ exception specifications are ignored, but this is `consistent with Visual
C++`_.
.. _consistent with Visual C++:
https://msdn.microsoft.com/en-us/library/wfa0edys.aspx
* Asynchronous Exceptions (SEH): :partial:`Partial`.
Structured exceptions (``__try`` / ``__except`` / ``__finally``) mostly
work on x86 and x64.
LLVM does not model asynchronous exceptions, so it is currently impossible to
catch an asynchronous exception generated in the same frame as the catching
``__try``.
* Thread-safe initialization of local statics: :good:`Complete`. MSVC 2015
added support for thread-safe initialization of such variables by taking an
ABI break.
We are ABI compatible with both the MSVC 2013 and 2015 ABI for static local
variables.
* Lambdas: :good:`Mostly complete`. Clang is compatible with Microsoft's
implementation of lambdas except for providing overloads for conversion to
function pointer for different calling conventions. However, Microsoft's
extension is non-conforming.
Template instantiation and name lookup
======================================
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 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:
https://clang.llvm.org/compatibility.html#dep_lookup
.. code-block:: c++
template <typename T>
struct Foo : T {
void f() {
/*typename*/ T::UnknownType x = /*this->*/unknownMember;
}
};
After recovery, Clang warns the user that this code is non-standard and issues
a hint suggesting how to fix the problem.
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.