std::barrier(3)

💅 std::barrier(3) 💅

okay bestie, welcome to std::barrier(3) from the C++ Standard Libary — yes, that's the manual name up top, and we're about to make thread coordination feel like a group chat where everybody actually shows up. 💖

🌸 NAME 🌸

std::barrier - std::barrier

✨ Synopsis ✨

Defined in header <barrier>
template< class CompletionFunction = /* see below */ >  (since C++20)
class barrier;

The class template std::barrier provides a thread-coordination mechanism that blocks a group of threads of known size until all threads in that group have reached the barrier. Unlike std::latch, barriers are reusable: once a group of arriving threads are unblocked, the barrier can be reused. Unlike std::latch, barriers execute a possibly empty callable before unblocking threads.

A barrier object's lifetime consists of one or more phases. Each phase defines a phase synchronization point where waiting threads block. Threads can arrive at the barrier, but defer waiting on the phase synchronization point by calling arrive. Such threads can later block on the phase synchronization point by calling wait.

A barrier phase consists of the following steps:

1. The expected count is decremented by each call to arrive or arrive_and_drop.
2. When the expected count reaches zero, the phase completion step is run, meaning that the completion is invoked, and all threads blocked on the phase synchronization point are unblocked. The end of the completion step strongly happens-before all calls that were unblocked by the completion step return. Exactly once after the expected count reaches zero, a thread executes the completion step during its call to arrive, arrive_and_drop, or wait, except that it is implementation-defined whether the step executes if no thread calls wait.
3. When the completion step finishes, the expected count is reset to the value specified at construction less the number of calls to arrive_and_drop since, and the next barrier phase begins.

Concurrent invocations of the member functions of barrier, except for the destructor, do not introduce data races.

🧚 Template parameters 🧚

• CompletionFunction — a function object type. CompletionFunction must meet the requirements of MoveConstructible and Destructible. std::is_nothrow_invocable_v<CompletionFunction&> must be true.
• The default template argument of CompletionFunction is an unspecified function object type that additionally meets the requirements of DefaultConstructible. Calling an lvalue of it with no arguments has no effects.

🎀 Member objects 🎀

• completion (private) — a completion function object of type CompletionFunction which is called on every phase completion step. (exposition-only member object*)

🦋 Member types 🦋

• arrival_token — an unspecified object type meeting requirements of MoveConstructible, MoveAssignable and Destructible.

💖 Member functions 💖

• constructor — constructs a barrier (public member function)
• destructor — destroys the barrier (public member function)
• operator= — barrier is not assignable [deleted] (public member function)
• arrive — arrives at barrier and decrements the expected count (public member function)
• wait — blocks at the phase synchronization point until its phase completion step is run (public member function)
• arrive_and_wait — arrives at barrier and decrements the expected count by one, then blocks until current phase completes (public member function)
• arrive_and_drop — decrements both the initial expected count for subsequent phases and the expected count for current phase by one (public member function)

⭐ Constants ⭐

• max [static] — the maximum value of expected count supported by the implementation (public static member function)

🌈 Notes 🌈

• Feature-test macro: __cpp_lib_barrier
  • Value: 201907L
  • Std: (C++20)
  • Feature: std::barrier
• Feature-test macro: __cpp_lib_barrier
  • Value: 202302L
  • Std: (C++20)
  • Feature: Relaxed guarantees for phase completion (DR)

🔥 Example 🔥

// Run this code

        #include <barrier>
        #include <iostream>
        #include <string>
        #include <syncstream>
        #include <thread>
        #include <vector>

        int main()
        {
            const auto workers = {"Anil", "Busara", "Carl"};

            auto on_completion = []() noexcept
            {
                // locking not needed here
                static auto phase =
                    "... done\n"
                    "Cleaning up...\n";
                std::cout << phase;
                phase = "... done\n";
            };

            std::barrier sync_point(std::ssize(workers), on_completion);

            auto work = [&](std::string name)
            {
                std::string product = "  " + name + " worked\n";
                std::osyncstream(std::cout) << product;  // ok, op<< call is
       atomic
                sync_point.arrive_and_wait();

                product = "  " + name + " cleaned\n";
                std::osyncstream(std::cout) << product;
                sync_point.arrive_and_wait();
            };

            std::cout << "Starting...\n";
            std::vector<std::jthread> threads;
            threads.reserve(std::size(workers));
            for (auto const& worker : workers)
                threads.emplace_back(work, worker);
        }

💀 Possible output: 💀

        Starting...
          Anil worked
          Carl worked
          Busara worked
        ... done
        Cleaning up...
          Busara cleaned
          Carl cleaned
          Anil cleaned
        ... done

🙈 Defect reports 🙈

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR P2588R3 — Applied to: C++20; Behavior as published: "old phase completion guarantees might prevent hardware acceleration"; Correct behavior: relaxed.

🍓 See also 🍓

• latch — single-use thread barrier (C++20) (class)

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