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Implement P3860R1 Make atomic_ref<T> Convertible To atomic_ref<const T> #5846

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Implement P3860R1 Make atomic_ref<T> Convertible To atomic_ref<const T> #5846

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70f4265
Implement P3860R1
frederick-vs-ja Nov 11, 2025
741f2a3
Merge branch 'main' into p3860r1
frederick-vs-ja Nov 15, 2025
3211a8a
LWG-4472 is filed
frederick-vs-ja Nov 15, 2025
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24 changes: 24 additions & 0 deletions stl/inc/atomic
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Original file line number Diff line number Diff line change
Expand Up @@ -533,6 +533,11 @@ inline bool __stdcall _Atomic_wait_compare_16_bytes(const void* _Storage, void*
#endif // _HAS_CXX20
#endif // TRANSITION, ABI

#if _HAS_CXX20
_EXPORT_STD template <class>
struct atomic_ref;
#endif // _HAS_CXX20

template <class _Ty, size_t /* = ... */>
struct _Atomic_storage {
// Provides operations common to all specializations of std::atomic, load, store, exchange, and CAS.
Expand Down Expand Up @@ -666,6 +671,11 @@ public:
_Ty _Storage;
mutable _Smtx_t _Mutex{};
#endif // ^^^ break ABI ^^^

#if _HAS_CXX20
template <class>
friend struct atomic_ref;
#endif // _HAS_CXX20
};

template <class _Ty>
Expand Down Expand Up @@ -2319,8 +2329,22 @@ public:
}
}

explicit atomic_ref(_Ty&&) = delete; // per LWG-4472

atomic_ref(const atomic_ref&) noexcept = default;

template <class _Uty>
requires is_convertible_v<_Uty (*)[], _Ty (*)[]>
atomic_ref(const atomic_ref<_Uty>& _Ref) noexcept : _Base(_Ref._Storage) {
if constexpr (!is_always_lock_free) {
#if 1 // TRANSITION, ABI
this->_Spinlock = _Ref._Spinlock;
#else // ^^^ don't break ABI / break ABI vvv
this->_Mutex = _Ref._Mutex;
#endif // ^^^ break ABI ^^^
}
}

atomic_ref& operator=(const atomic_ref&) = delete;

static constexpr bool is_always_lock_free =
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1 change: 1 addition & 0 deletions stl/inc/yvals_core.h
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Expand Up @@ -320,6 +320,7 @@
// P3323R1 Forbid atomic<cv T>, Specify atomic_ref<cv T>
// (for atomic_ref<cv T>)
// P3349R1 Converting Contiguous Iterators To Pointers
// P3860R1 Make atomic_ref<T> Convertible To atomic_ref<const T>

// _HAS_CXX20 indirectly controls:
// P0619R4 Removing C++17-Deprecated Features
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