[libc++] Clarify _LIBCPP_NEW_DELETE_VIS for Windows
Replace a stale reference to cxx_EXPORTS with _LIBCPP_BUILDING_LIBRARY,
and clarify why the operator new and delete family of functions are
marked dllexport when building but *not* dllimport when including the
header externally.
The new code is identical to the intent of the old code (and would be
functionally equivalent were cxx_EXPORTS still defined when building
libc++). The overall behavior is not ideal, since Microsoft's operator
new and delete functions will get called instead of libc++'s, but I
think consistently calling msvcrt's functions is better than either
calling msvcrt's or libc++'s functions depending on header inclusion.
Differential Revision: https://reviews.llvm.org/D25042
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@282644 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/DesignDocs/VisibilityMacros.rst b/docs/DesignDocs/VisibilityMacros.rst
index f7d1400..f45d67b 100644
--- a/docs/DesignDocs/VisibilityMacros.rst
+++ b/docs/DesignDocs/VisibilityMacros.rst
@@ -108,6 +108,19 @@
versioning namespace. This allows throwing and catching some exception types
between libc++ and libstdc++.
+**_LIBCPP_NEW_DELETE_VIS**
+ Mark a symbol as being exported by the libc++ library. This macro must be
+ applied to all `operator new` and `operator delete` overloads.
+
+ **Windows Behavior**: When using the Microsoft CRT, all the `operator new` and
+ `operator delete` overloads are defined statically in `msvcrt.lib`. Marking
+ them as `dllimport` in the libc++ `<new>` header is therefore undesirable: if
+ we were to mark them as `dllimport` and then link against libc++, source files
+ which included `<new>` would end up linking against libc++'s `operator new`
+ and `operator delete`, while source files which did not include `<new>` would
+ end up linking against msvcrt's `operator new` and `operator delete`, which
+ would be a confusing and potentially error-prone inconsistency.
+
Links
=====
diff --git a/include/new b/include/new
index 936f9ee..16b3011 100644
--- a/include/new
+++ b/include/new
@@ -125,8 +125,8 @@
} // std
-#if defined(_WIN32) && !defined(cxx_EXPORTS)
-# define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS_ONLY
+#if defined(_LIBCPP_MSVCRT) && !defined(_LIBCPP_BUILDING_LIBRARY)
+# define _LIBCPP_NEW_DELETE_VIS
#else
# define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS
#endif