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Merged
StephanTLavavej merged 2 commits into from
Jan 28, 2021
Merged

feature/spaceship: Clause 32: Thread support #1590

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feature/spaceship: Clause 32: Thread support
statementreply Jan 26, 2021
85c01c2
Comment the assumption that thread::id{} occurs first.
StephanTLavavej Jan 28, 2021
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11 changes: 11 additions & 0 deletions stl/inc/thread
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@
#include <tuple>
#include <xthreads.h>
#if _HAS_CXX20
#include <compare>
#include <stop_token>
#endif // _HAS_CXX20

Expand Down Expand Up @@ -215,7 +216,11 @@ private:
friend thread::id thread::get_id() const noexcept;
friend thread::id this_thread::get_id() noexcept;
friend bool operator==(thread::id _Left, thread::id _Right) noexcept;
#if _HAS_CXX20
friend strong_ordering operator<=>(thread::id _Left, thread::id _Right) noexcept;
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
friend bool operator<(thread::id _Left, thread::id _Right) noexcept;
#endif // !_HAS_CXX20
Comment on lines 218 to +223
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@miscco miscco Jan 26, 2021

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Would it make sense to default the operator== for C++20 and make them hidden friends?

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@CaseyCarter CaseyCarter Jan 26, 2021

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They can't be hidden friends; the difference is observable. Arguably operator== also cannot be defaulted because doing so would make it constexpr and we're forbidden to make things constexpr where the Standard doesn't depict them as such.

template <class _Ch, class _Tr>
friend basic_ostream<_Ch, _Tr>& operator<<(basic_ostream<_Ch, _Tr>& _Str, thread::id _Id);
friend hash<thread::id>;
Expand All @@ -237,6 +242,11 @@ _NODISCARD inline bool operator==(thread::id _Left, thread::id _Right) noexcept
return _Left._Id == _Right._Id;
}

#if _HAS_CXX20
_NODISCARD inline strong_ordering operator<=>(thread::id _Left, thread::id _Right) noexcept {
return _Left._Id <=> _Right._Id;
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@CaseyCarter CaseyCarter Jan 26, 2021 •
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Fun fact: the Standard doesn't require the total ordering over thread ids that this function observes to be consistent with operator==, so we could have the two functions order them differently ;)

(I've submitted an LWG issue to fix this.)

}
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
_NODISCARD inline bool operator!=(thread::id _Left, thread::id _Right) noexcept {
return !(_Left == _Right);
}
Expand All @@ -256,6 +266,7 @@ _NODISCARD inline bool operator>(thread::id _Left, thread::id _Right) noexcept {
_NODISCARD inline bool operator>=(thread::id _Left, thread::id _Right) noexcept {
return !(_Left < _Right);
}
#endif // !_HAS_CXX20

template <class _Ch, class _Tr>
basic_ostream<_Ch, _Tr>& operator<<(basic_ostream<_Ch, _Tr>& _Str, thread::id _Id) {
Expand Down
9 changes: 9 additions & 0 deletions tests/std/tests/P1614R2_spaceship/test.cpp
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Expand Up @@ -21,6 +21,7 @@
#include <stack>
#include <string>
#include <system_error>
#include <thread>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
Expand Down Expand Up @@ -481,6 +482,14 @@ void ordering_test_cases() {

spaceship_test<std::strong_ordering>(c_mem[0], c_mem[0], c_mem[1]);
}
{ // thread::id
std::thread::id id1;
std::thread::id id1_equal;
std::thread::id id2 = std::this_thread::get_id();

// Implementation-specific assumption: std::thread::id{} occurs first in the unspecified total ordering.
spaceship_test<std::strong_ordering>(id1, id1_equal, id2);
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@StephanTLavavej StephanTLavavej Jan 28, 2021

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I believe there's an implementation assumption here - nothing in the Standard appears to require that std::thread::id{} occurs first in the unspecified total ordering. (It's a safe assumption for us, since we use 0 and store unsigned int.) I'll push a comment.

}
}

template <class Element, class Ordering>
Expand Down