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type_index::operator<=> should not call the comparison function twice #3600

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Merged
StephanTLavavej merged 1 commit into from
Apr 7, 2023
Merged

type_index::operator<=> should not call the comparison function twice #3600

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10 changes: 7 additions & 3 deletions stl/inc/typeindex
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@

#if _HAS_CXX20
#include <compare>
#include <cstring>
#endif // _HAS_CXX20

#pragma pack(push, _CRT_PACKING)
Expand Down Expand Up @@ -40,9 +41,12 @@ public:

#if _HAS_CXX20
_NODISCARD strong_ordering operator<=>(const type_index& _Right) const noexcept {
return *_Tptr == *_Right._Tptr ? strong_ordering::equal
: _Tptr->before(*_Right._Tptr) ? strong_ordering::less
: strong_ordering::greater;
// TRANSITION, DevCom-10326599, should rely on a stable interface
if (_Tptr == _Right._Tptr) {
return strong_ordering::equal;
}

return _CSTD strcmp(_Tptr->raw_name() + 1, _Right._Tptr->raw_name() + 1) <=> 0;
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@StephanTLavavej StephanTLavavej Mar 31, 2023

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No change requested, note to final reviewer: This raw_name() + 1 looks like 5 squirrels out of 5 🐿️ but it's exactly the correct technique.

  • __std_type_info_compare() in std_type_info.cpp (internal link) compares strcmp(lhs->_DecoratedName + 1, rhs->_DecoratedName + 1).
  • type_info::raw_name() in vcruntime_typeinfo.h (internal link) says return _Data._DecoratedName;.
  • The character at index 0 appears to be a dot; I'm not sure why that's stored, or why raw_name() exposes this, but the implementation here is correct.

}
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
_NODISCARD bool operator!=(const type_index& _Right) const noexcept {
Expand Down