Implement n3668 - std::exchange

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185863 91177308-0d34-0410-b5e6-96231b3b80d8

include/utility

@@ -138,6 +138,8 @@ template<size_t N>
 template<class... T>
   using index_sequence_for = make_index_sequence<sizeof...(T)>;
 
+template<class T, class U=T> 
+    T exchange(T& obj, U&& new_value);
 }  // std
 
 */
@@ -685,6 +687,17 @@ template<class... _Tp>
   
 #endif  // _LIBCPP_STD_VER > 11
 
+#if _LIBCPP_STD_VER > 11
+template<class _T1, class _T2 = _T1>
+_LIBCPP_INLINE_VISIBILITY inline
+_T1 exchange(_T1& __obj, _T2 && __new_value)
+{
+	_T1 __old_value = _VSTD::move(__obj);
+	__obj = _VSTD::forward<_T2>(__new_value);
+	return __old_value;
+}	
+#endif  // _LIBCPP_STD_VER > 11
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_UTILITY

test/utilities/utility/exchange/exchange.pass.cpp

@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// utilities
+
+// exchange
+
+#include <utility>
+#include <cassert>
+#include <string>
+
+int main()
+{
+#if _LIBCPP_STD_VER > 11
+	{
+	int v = 12;
+	assert ( std::exchange ( v, 23 ) == 12 );
+	assert ( v == 23 );
+	assert ( std::exchange ( v, 67.2 ) == 23 );
+	assert ( v = 67 );
+	}
+
+	{
+	bool b = false;
+	assert ( !std::exchange ( b, true ));
+	assert ( b );
+	}
+
+	{
+	const std::string s1 ( "Hi Mom!" );
+	const std::string s2 ( "Yo Dad!" );
+	std::string s3 = s1; // Mom
+	assert ( std::exchange ( s3, s2 ) == s1 );
+	assert ( s3 == s2 );
+	assert ( std::exchange ( s3, "Hi Mom!" ) == s2 );
+	assert ( s3 == s1 );
+	assert ( std::exchange ( s3, "" ) == s1 );
+	assert ( s3.size () == 0 );
+	}
+#endif
+}