std::atomic_ref::wait(3)

💖 NAME 💖

Okay bestie, welcome to std::atomic_ref::wait — literally std::atomic_ref::wait — from the C++ Standard Libary (that spelling is the header's, not mine 😭). This is the manual page for atomic waiting, and it's giving patient-icon energy. ✨

📜 Synopsis 📜

void wait( T old, std::memory_order order =
           std::memory_order::seq_cst ) const noexcept;

(1) (since C++20)

void wait( T old, std::memory_order order =
           std::memory_order::seq_cst ) const volatile
noexcept;

(2) (since C++20)

Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:

• Compare the value representation of this->load(order) with that of old.
  • If those are equal, then blocks until *this is notified by notify_one() or notify_all(), or the thread is unblocked spuriously.
  • Otherwise, returns.

These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously. 🫶

🚨 If order is one of std::memory_order::release and std::memory_order::acq_rel, the behavior is undefined. Do not do that, bestie — that's not the vibe.

🎀 Parameters 🎀

• old — the value to check the atomic_ref's object no longer contains
• order — memory order constraints to enforce

🌸 Return value 🌸

(none) — she returns nothing. Iconic minimalism. 💅

🦋 Notes 🦋

This form of change-detection is often more efficient than simple polling or pure spinlocks. Slay.

Due to the ABA problem, transient changes from old to another value and back to old might be missed, and not unblock. 😭 The classic "you changed but also changed back" betrayal.

The comparison is bitwise (similar to std::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored. Fr fr, bitwise.

🍓 Example 🍓

This section is incomplete Reason: no example

👑 See also 👑

• notify_one — notifies at least one thread waiting on the atomic object (public member function)
• notify_all — notifies all threads blocked waiting on the atomic object (public member function)
• atomic_notify_one (C++20) — notifies a thread blocked in atomic_wait (function template)
• atomic_notify_all (C++20) — notifies all threads blocked in atomic_wait (function template)

🌈 Category 🌈

• Todo no example

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That's the whole page, bestie — source is http://cppreference.com, version/date 2024.06.10, and the page is std::atomic_ref::wait(3). Catch you in the group chat, mwah 💖✨