std::barrier

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

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:

The expected count is decremented by each call to arrive or arrive_and_drop.
When the expected count reaches zero, the phase completion step is run, meaning that the CompletionFunction 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.
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.

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

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_barrier`	201907L	(C++20)	`std::barrier`

Example

Run this code

#include <barrier>
#include <iostream>
#include <string>
#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::cout << product;  // ok, op<< call is atomic
    sync_point.arrive_and_wait();
 
    product = "  " + name + " cleaned\n";
    std::cout << product;
    sync_point.arrive_and_wait();
  };
 
  std::cout << "Starting...\n";
  std::vector<std::thread> threads;
  for (auto const& worker : workers) {
    threads.emplace_back(work, worker);
  }
  for (auto& thread : threads) {
    thread.join();
  }
}

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	Applied to	Behavior as published	Correct behavior
P2588R3	C++20	phase completion guarantees might prevent hardware acceleration	relaxed

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Member functions
barrier::barrier
barrier::~barrier
barrier::arrive
barrier::wait
barrier::arrive_and_wait
barrier::arrive_and_drop
Constants
barrier::max

Name	Definition
`arrival_token`	an unspecified object type meeting requirements of MoveConstructible, MoveAssignable and Destructible

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std::barrier

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CompletionFunction	-	a function object type
- `CompletionFunction` must meet the requirements of MoveConstructible and Destructible. std::is_nothrow_invocable_v<CompletionFunction&> must be true.