标准库标头＜barrier＞（C++20）学习-编程新知

此头文件是线程支持库的一部分。

类模板 std::barrier 提供一种线程协调机制，阻塞已知大小的线程组直至该组中的所有线程到达该屏障。不同于 std::latch，屏障是可重用的：一旦到达的线程组被解除阻塞，即可重用同一屏障。与 std::latch 不同，会在线程解除阻塞前执行一个可能为空的可调用对象。

屏障对象的生存期由一个或多个屏障阶段组成。每个阶段定义一个阻塞线程的阶段同步点。线程可以抵达屏障，但通过调用 arrive 来推迟它在阶段同步点上的等待。这样的线程可以随后再通过调用 wait 在阶段同步点上阻塞。

屏障阶段由以下步骤组成：

每次调用 arrive 或 arrive_and_drop 减少期待计数。
期待计数抵达零时，运行阶段完成步骤，即调用 completion，并解除所有在阶段同步点上阻塞的线程。完成步骤的结束强先发生于所有从完成步骤所除阻的调用的返回。
在期待计数抵达零后，一个线程会在其调用 arrive、arrive_and_drop 或 wait 的过程中执行完成步骤恰好一次，但如果没有线程调用 wait 则是否执行完成步骤为实现定义。
完成步骤结束时，重置期待计数为构造中指定的值，它可能为 arrive_and_drop 调用所调整，自此开始下一阶段。

并发调用barrier 除了析构函数外的成员函数不会引起数据竞争。

vs2022 类模板如下：

class barrier {
public:static_assert(
#ifndef __cpp_noexcept_function_typeis_function_v<remove_pointer_t<_Completion_function>> ||
#endif // !defined(__cpp_noexcept_function_type)is_nothrow_invocable_v<_Completion_function&>,"N4950 [thread.barrier.class]/5: is_nothrow_invocable_v<CompletionFunction&> shall be true");using arrival_token = _Arrival_token<_Completion_function>;constexpr explicit barrier(const ptrdiff_t _Expected, _Completion_function _Fn = _Completion_function()) noexcept /* strengthened */: _Val(_One_then_variadic_args_t{}, _STD move(_Fn), _Expected << _Barrier_value_shift) {_STL_VERIFY(_Expected >= 0 && _Expected <= (max) (),"Precondition: expected >= 0 and expected <= max() (N4950 [thread.barrier.class]/9)");}barrier(const barrier&)            = delete;barrier& operator=(const barrier&) = delete;_NODISCARD static constexpr ptrdiff_t(max)() noexcept {return _Barrier_max;}_NODISCARD_BARRIER_TOKEN arrival_token arrive(ptrdiff_t _Update = 1) noexcept /* strengthened */ {_STL_VERIFY(_Update > 0 && _Update <= (max) (), "Precondition: update > 0 (N4950 [thread.barrier.class]/12)");_Update <<= _Barrier_value_shift;// TRANSITION, GH-1133: should be memory_order_releaseptrdiff_t _Current = _Val._Myval2._Current.fetch_sub(_Update) - _Update;_STL_VERIFY(_Current >= 0, "Precondition: update is less than or equal to the expected count ""for the current barrier phase (N4950 [thread.barrier.class]/12)");if ((_Current & _Barrier_value_mask) == 0) {// TRANSITION, GH-1133: should have this fence:// atomic_thread_fence(memory_order_acquire);_Completion(_Current);}// Embedding this into the token to provide an additional correctness check that the token is from the same// barrier and wasn't used. All bits of this fit, as barrier should be aligned to at least the size of an// atomic counter.return arrival_token{(_Current & _Barrier_arrival_token_mask) | reinterpret_cast<intptr_t>(this)};}void wait(arrival_token&& _Arrival) const noexcept /* strengthened */ {_STL_VERIFY((_Arrival._Value & _Barrier_value_mask) == reinterpret_cast<intptr_t>(this),"Preconditions: arrival is associated with the phase synchronization point for the current phase ""or the immediately preceding phase of the same barrier object (N4950 [thread.barrier.class]/19)");const ptrdiff_t _Arrival_value = _Arrival._Value & _Barrier_arrival_token_mask;_Arrival._Value                = _Barrier_invalid_token;for (;;) {// TRANSITION, GH-1133: should be memory_order_acquireconst ptrdiff_t _Current = _Val._Myval2._Current.load();_STL_VERIFY(_Current >= 0, "Invariant counter >= 0, possibly caused by preconditions violation ""(N4950 [thread.barrier.class]/12)");if ((_Current & _Barrier_arrival_token_mask) != _Arrival_value) {break;}_Val._Myval2._Current.wait(_Current, memory_order_relaxed);}}void arrive_and_wait() noexcept /* strengthened */ {// TRANSITION, GH-1133: should be memory_order_acq_relptrdiff_t _Current       = _Val._Myval2._Current.fetch_sub(_Barrier_value_step) - _Barrier_value_step;const ptrdiff_t _Arrival = _Current & _Barrier_arrival_token_mask;_STL_VERIFY(_Current >= 0, "Precondition: update is less than or equal to the expected count ""for the current barrier phase (N4950 [thread.barrier.class]/12)");if ((_Current & _Barrier_value_mask) == 0) {_Completion(_Current);return;}for (;;) {_Val._Myval2._Current.wait(_Current, memory_order_relaxed);// TRANSITION, GH-1133: should be memory_order_acquire_Current = _Val._Myval2._Current.load();_STL_VERIFY(_Current >= 0, "Invariant counter >= 0, possibly caused by preconditions violation ""(N4950 [thread.barrier.class]/12)");if ((_Current & _Barrier_arrival_token_mask) != _Arrival) {break;}}}void arrive_and_drop() noexcept /* strengthened */ {const ptrdiff_t _Rem_count =_Val._Myval2._Total.fetch_sub(_Barrier_value_step, memory_order_relaxed) - _Barrier_value_step;_STL_VERIFY(_Rem_count >= 0, "Precondition: The expected count for the current barrier phase ""is greater than zero (N4950 [thread.barrier.class]/24) ""(checked initial expected count, which is not less than the current)");(void) arrive(1);}private:void _Completion(const ptrdiff_t _Current) noexcept {const ptrdiff_t _Rem_count = _Val._Myval2._Total.load(memory_order_relaxed);_STL_VERIFY(_Rem_count >= 0, "Invariant: initial expected count less than zero, ""possibly caused by preconditions violation ""(N4950 [thread.barrier.class]/24)");_Val._Get_first()();const ptrdiff_t _New_phase_count = _Rem_count | ((_Current + 1) & _Barrier_arrival_token_mask);// TRANSITION, GH-1133: should be memory_order_release_Val._Myval2._Current.store(_New_phase_count);_Val._Myval2._Current.notify_all();}struct _Counter_t {constexpr explicit _Counter_t(ptrdiff_t _Initial) : _Current(_Initial), _Total(_Initial) {}// wait(arrival_token&&) accepts a token from the current phase or the immediately preceding phase; this means// we can track which phase is the current phase using 1 bit which alternates between each phase. For this// purpose we use the low order bit of _Current.atomic<ptrdiff_t> _Current;atomic<ptrdiff_t> _Total;};_Compressed_pair<_Completion_function, _Counter_t> _Val;
};

成员对象

名称	定义
`completion` (私有)	`CompletionFunction` 类型的完成函数对象，在每个阶段完成步骤调用。 (仅用于阐述的成员对象*)

成员类型

名称	定义
`arrival_token`	未指定的对象类型，满足可移动构造 (MoveConstructible) 、可移动赋值 (MoveAssignable) 及可析构 (Destructible)

成员函数

(构造函数)	构造 `barrier` (公开成员函数)
(析构函数)	销毁 `barrier` (公开成员函数)
operator= [弃置]	`barrier` 不可赋值 (公开成员函数)
arrive	到达屏障并减少期待计数 (公开成员函数)
wait	在阶段同步点阻塞，直至运行其阶段完成步骤 (公开成员函数)
arrive_and_wait	到达屏障并把期待计数减少一，然后阻塞直至当前阶段完成 (公开成员函数)
arrive_and_drop	将后继阶段的初始期待计数和当前阶段的期待计数均减少一 (公开成员函数)
常量
max [静态]	实现所支持的期待计数的最大值 (公开静态成员函数)

示例代码：

#include <barrier>
#include <iostream>
#include <string>
#include <syncstream>
#include <thread>
#include <vector>int main()
{const auto workers = { "Anil", "Busara", "Carl" };auto on_completion = []() noexcept{// 此处无需锁定static auto phase ="... 完成\n""清理...\n";std::cout << phase;phase = "... 完成\n";};std::barrier sync_point(std::ssize(workers), on_completion);auto work = [&](std::string name){std::string product = "  " + name + " 已工作\n";std::osyncstream(std::cout) << product;  // OK, op<< 的调用是原子的sync_point.arrive_and_wait();product = "  " + name + " 已清理\n";std::osyncstream(std::cout) << product;sync_point.arrive_and_wait();};std::cout << "启动...\n";std::vector<std::jthread> threads;threads.reserve(std::size(workers));for (auto const& worker : workers)threads.emplace_back(work, worker);return 0;
}

运行结果：

另一个示例：

#include <barrier>
#include <iostream>
#include <string>
#include <syncstream>
#include <thread>
#include <vector>std::barrier bar(4); //创建一个barrier，需要4个线程到达同步点void thread_func(int id) {// 线程执行一些任务//std::cout << "Thread ID: " << std::this_thread::get_id() << " is doing some work." << std::endl;std::cout << "Thread ID=======start===== is doing some work.\n";// 等待所有线程到达栅栏bar.arrive_and_wait();// 所有线程到达栅栏后，继续执行后续任务//std::cout << "Thread ID: " << std::this_thread::get_id() << " continues working after barrier." << std::endl;std::cout << "Thread ID=======end: ===== continues working after barrier.\n";
}int main() {constexpr int num_threads = 4;std::vector<std::thread> threads;// 创建线程并执行线程函数for (int i = 0; i < num_threads; ++i) {threads.emplace_back(thread_func, i);}// 等待所有线程执行完毕for (auto& t : threads) {t.join();}return 0;
}

示例代码：

#include <barrier>
#include <iostream>
#include <string>
#include <syncstream>
#include <thread>
#include <vector>std::barrier bar(4); //创建一个barrier，需要4个线程到达同步点void thread_func(int id) {// 线程执行一些任务//std::cout << "Thread ID: " << std::this_thread::get_id() << " is doing some work." << std::endl;std::cout << "Thread ID=======start===== is doing some work.\n";// 等待所有线程到达栅栏bar.arrive_and_wait();// 所有线程到达栅栏后，继续执行后续任务//std::cout << "Thread ID: " << std::this_thread::get_id() << " continues working after barrier." << std::endl;std::cout << "Thread ID=======end: ===== continues working after barrier.\n";
}int main() {constexpr int num_threads = 4;std::vector<std::thread> threads;// 创建线程并执行线程函数for (int i = 0; i < num_threads; ++i) {threads.emplace_back(thread_func, i);}// 等待所有线程执行完毕for (auto& t : threads) {t.join();}return 0;
}

运行结果：