Author: ericwf Date: Sun Dec 4 15:37:37 2016 New Revision: 288623 URL: http://llvm.org/viewvc/llvm-project?rev=288623&view=rev Log: Choose better hash values for std::monostate and valueless variants.
Previously these hashes were 0 and -1 respectively. These seem like common sentinel values and should be avoided to prevent needless collisions. This patch changes those values to different arbitrary numbers, which should hopefully cause less collisions. Because I couldn't help myself I choose the fundamental constants for gravity and the speed of light. Modified: libcxx/trunk/include/variant Modified: libcxx/trunk/include/variant URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/variant?rev=288623&r1=288622&r2=288623&view=diff ============================================================================== --- libcxx/trunk/include/variant (original) +++ libcxx/trunk/include/variant Sun Dec 4 15:37:37 2016 @@ -1538,7 +1538,7 @@ struct _LIBCPP_TYPE_VIS_ONLY hash<varian using __variant_detail::__visitation::__variant; size_t __res = __v.valueless_by_exception() - ? __v.index() + ? 299792458 // Random value chosen by the universe upon creation : __variant::__visit_alt( [](const auto& __alt) { using __alt_type = decay_t<decltype(__alt)>; @@ -1556,7 +1556,9 @@ struct _LIBCPP_TYPE_VIS_ONLY hash<monost using result_type = size_t; inline _LIBCPP_INLINE_VISIBILITY - result_type operator()(const argument_type&) const { return 0; } + result_type operator()(const argument_type&) const { + return 66740831; // return a fundamentally attractive random value. + } }; #endif // _LIBCPP_STD_VER > 14 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits