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
arriveorarrive_and_drop. - When the expected count reaches zero, the phase completion step is run, meaning that the
completionis 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 toarrive,arrive_and_drop, orwait, except that it is implementation-defined whether the step executes if no thread callswait. - When the completion step finishes, the expected count is reset to the value specified at construction less the number of calls to
arrive_and_dropsince, and the next barrier phase begins.
Concurrent invocations of the member functions of barrier, except for the destructor, do not introduce data races.
Contents |
[edit] 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.
[edit] Member types
| Name | Definition |
arrival_token
|
an unspecified object type meeting requirements of MoveConstructible, MoveAssignable and Destructible |
[edit] Data members
| Member | Definition |
CompletionFunction completion
|
a completion function object which is called on every phase completion step (exposition-only member object*) |
[edit] Member functions
constructs a barrier (public member function) | |
destroys the barrier (public member function) | |
| operator= [deleted] |
barrier is not assignable (public member function) |
| arrives at barrier and decrements the expected count (public member function) | |
| blocks at the phase synchronization point until its phase completion step is run (public member function) | |
| arrives at barrier and decrements the expected count by one, then blocks until current phase completes (public member function) | |
| decrements both the initial expected count for subsequent phases and the expected count for current phase by one (public member function) | |
Constants | |
| [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
|
| 202302L | (C++20) (DR) |
Relaxed guarantees for phase completion |
[edit] Example
#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
[edit] 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 | old phase completion guarantees might prevent hardware acceleration | relaxed |
[edit] See also
| (C++20) |
single-use thread barrier (class) |
