[PATCH] D11394: Fix warnings about pessimizing return moves for C++11 and higher

[Eric Fiselier]

EricWF added a comment.

Thanks for the patch. I ran into this issue the other day and I'm glad to see it fixed.

A little rational: The explicit move's are needed in order to "move" a `unique_ptr` in C++03. There is a special definition of `std::move` in memory at line 3100 that performs some hacks to make `unique_ptr` movable. I don't think any other classes benefit from the "explicit move" in C++03.

> I am not completely satisfied with the macro name (I also considered

>  _LIBCPP_COMPAT_MOVE and some other variants), so suggestions are

>  welcome. :)


I think the name is fine and there is no need to change it. However maybe the name should reflect the fact that it is only needed for `unique_ptr`? Something like `_LIBCPP_UNIQUE_PTR_MOVE` would better convey the weirdness this macro is doing.


================
Comment at: include/__config:719
@@ -718,1 +718,3 @@
 
+#if _LIBCPP_STD_VER < 11
+#  define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)
----------------
We need the explicit move whenever `_LIBCPP_HAS_NO_RVALUE_REFERENCES` is defined. Please use that instead.

================
Comment at: include/algorithm:854
@@ -853,3 +853,3 @@
         __f(*__first);
-    return _VSTD::move(__f);  // explicitly moved for (emulated) C++03
+    return _LIBCPP_EXPLICIT_MOVE(__f);  // explicitly moved for (emulated) C++03
 }
----------------
This one seems suspect. I don't really know if we need the move here. Do you know why this here?


http://reviews.llvm.org/D11394