[libcxx] r185863 - Implement n3668 - std::exchange

[Richard Smith]

On Mon, Jul 8, 2013 at 1:54 PM, Marshall Clow <mclow.lists at gmail.com> wrote:
> Author: marshall
> Date: Mon Jul  8 15:54:40 2013
> New Revision: 185863
>
> URL: http://llvm.org/viewvc/llvm-project?rev=185863&view=rev
> Log:
> Implement n3668 - std::exchange
>
> Added:
>     libcxx/trunk/test/utilities/utility/exchange/
>     libcxx/trunk/test/utilities/utility/exchange/exchange.pass.cpp
> Modified:
>     libcxx/trunk/include/utility
>
> Modified: libcxx/trunk/include/utility
> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/utility?rev=185863&r1=185862&r2=185863&view=diff
> ==============================================================================
> --- libcxx/trunk/include/utility (original)
> +++ libcxx/trunk/include/utility Mon Jul  8 15:54:40 2013
> @@ -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
>
> Added: libcxx/trunk/test/utilities/utility/exchange/exchange.pass.cpp
> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/utilities/utility/exchange/exchange.pass.cpp?rev=185863&view=auto
> ==============================================================================
> --- libcxx/trunk/test/utilities/utility/exchange/exchange.pass.cpp (added)
> +++ libcxx/trunk/test/utilities/utility/exchange/exchange.pass.cpp Mon Jul  8 15:54:40 2013
> @@ -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
> +}

It'd be useful to also test std::exchange's default template argument
trick, for the case where U is not deducible. Something like:

  std::exchange( s3, {} )