[PATCH] D25042: [libc++] Clarify _LIBCPP_NEW_DELETE_VIS for Windows

[Shoaib Meenai via cfe-commits]

smeenai created this revision.
smeenai added reviewers: compnerd, EricWF, mclow.lists.
smeenai added a subscriber: cfe-commits.

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.

https://reviews.llvm.org/D25042

Files:
  include/new

Index: include/new
===================================================================
--- include/new
+++ include/new
@@ -125,8 +125,18 @@
 
 }  // std
 
-#if defined(_WIN32) && !defined(cxx_EXPORTS)
-# define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS_ONLY
+/*
+ * NOTE: When using the Microsoft CRT, the operator new and delete
+ * family of functions are defined statically in msvcrt.lib. Marking
+ * them as dllimport in this header is therefore undesirable: if we were
+ * to mark them dllimport and then link against libc++, source files
+ * which included <new> would end up linking against libc++'s new and
+ * delete, while source files which did not include <new> would end up
+ * still linking against msvcrt's new, which would be a confusing and
+ * potentially error-prone inconsistency.
+ */
+#if defined(_LIBCPP_MSVCRT) && !defined(_LIBCPP_BUILDING_LIBRARY)
+# define _LIBCPP_NEW_DELETE_VIS
 #else
 # define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS
 #endif


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