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Throughout the libc++ headers, there are a few instances where
_VSTD::move() is used to return a local variable. Howard commented in
r189039 that these were there "for non-obvious reasons such as to help
things limp along in C++03 language mode".
However, when compiling these headers with warnings on, and in C++11 or
higher mode (like we do in FreeBSD), they cause the following complaints
about pessimizing moves:
In file included from tests.cpp:26:
In file included from tests.hpp:29:
/usr/include/c++/v1/map:1368:12: error: moving a local object in a return
statement prevents copy elision [-Werror,-Wpessimizing-move]
return _VSTD::move(__h); // explicitly moved for C++03
^
/usr/include/c++/v1/__config:368:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
^
Attempt to fix this by adding a _LIBCPP_EXPLICIT_MOVE() macro to
__config, which gets defined to _VSTD::move for pre-C++11, and to
nothing for C++11 and later.
I am not completely satisfied with the macro name (I also considered
_LIBCPP_COMPAT_MOVE and some other variants), so suggestions are
welcome. :)
http://reviews.llvm.org/D11394
Files:
include/__config
include/__hash_table
include/__tree
include/algorithm
include/ext/hash_map
include/map
include/unordered_map
Index: include/unordered_map
===================================================================
--- include/unordered_map
+++ include/unordered_map
@@ -1511,7 +1511,7 @@
__h.get_deleter().__first_constructed = true;
__node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
__h.get_deleter().__second_constructed = true;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
Index: include/map
===================================================================
--- include/map
+++ include/map
@@ -1523,7 +1523,7 @@
__h.get_deleter().__first_constructed = true;
__node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
__h.get_deleter().__second_constructed = true;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
template <class _Key, class _Tp, class _Compare, class _Allocator>
Index: include/ext/hash_map
===================================================================
--- include/ext/hash_map
+++ include/ext/hash_map
@@ -681,7 +681,7 @@
__h.get_deleter().__first_constructed = true;
__node_traits::construct(__na, _VSTD::addressof(__h->__value_.second));
__h.get_deleter().__second_constructed = true;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
Index: include/algorithm
===================================================================
--- include/algorithm
+++ include/algorithm
@@ -851,7 +851,7 @@
{
for (; __first != __last; ++__first)
__f(*__first);
- return _VSTD::move(__f); // explicitly moved for (emulated) C++03
+ return _LIBCPP_EXPLICIT_MOVE(__f); // explicitly moved for (emulated) C++03
}
// find
Index: include/__tree
===================================================================
--- include/__tree
+++ include/__tree
@@ -1843,7 +1843,7 @@
__node_holder __h(__node_traits::allocate(__na, 1), _Dp(__na));
__node_traits::construct(__na, _VSTD::addressof(__h->__value_), __v);
__h.get_deleter().__value_constructed = true;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
#endif // _LIBCPP_HAS_NO_RVALUE_REFERENCES
Index: include/__hash_table
===================================================================
--- include/__hash_table
+++ include/__hash_table
@@ -2108,7 +2108,7 @@
__h.get_deleter().__value_constructed = true;
__h->__hash_ = hash_function()(__h->__value_);
__h->__next_ = nullptr;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
#endif // _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -2124,7 +2124,7 @@
__h.get_deleter().__value_constructed = true;
__h->__hash_ = __hash;
__h->__next_ = nullptr;
- return _VSTD::move(__h); // explicitly moved for C++03
+ return _LIBCPP_EXPLICIT_MOVE(__h); // explicitly moved for C++03
}
template <class _Tp, class _Hash, class _Equal, class _Alloc>
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -716,6 +716,12 @@
#define _LIBCPP_CONSTEXPR_AFTER_CXX11
#endif
+#if _LIBCPP_STD_VER < 11
+# define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)
+#else
+# define _LIBCPP_EXPLICIT_MOVE(x) (x)
+#endif
+
#ifndef _LIBCPP_HAS_NO_ASAN
extern "C" void __sanitizer_annotate_contiguous_container(
const void *, const void *, const void *, const void *);
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