< cpp‎ | thread
Concurrency support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and Barriers
Safe Reclamation
Hazard Pointers

Atomic types
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
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:

  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.


[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 objects

Name Definition
completion (private) A completion function object of type CompletionFunction which is called on every phase completion step.
(exposition-only member object*)

[edit] Member types

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

[edit] Member functions

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

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_barrier 201907L (C++20) std::barrier
202302L (C++20)
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
        product = "  " + name + " cleaned\n";
        std::osyncstream(std::cout) << product;
    std::cout << "Starting...\n";
    std::vector<std::jthread> threads;
    for (auto const& worker : workers)
        threads.emplace_back(work, worker);

Possible output:

  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

single-use thread barrier
(class) [edit]