[CPP20]Add std::barrier examples

CMakeLists.txt

@@ -56,6 +56,7 @@ add_executable(cpp_weekly
 
         cpp20_features/comparisons.cc
         cpp20_features/latches.cc
+        cpp20_features/barrier_test.cc
 
         cpp_challenges/chap1_math_problems.cc
 

cpp20_features/barrier_test.cc

@@ -0,0 +1,56 @@
+#include <barrier>
+#include <cmath>
+#include <format>
+#include <gtest/gtest.h>
+#include <iostream>
+#include <stop_token>
+#include <thread>
+#include <vector>
+
+namespace {
+
+void barrier_test() {
+  // initialize and print a collection of floating-point values:
+  std::vector values{1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0};
+
+  // define a lambda function that prints all values.
+  // -NOTE: has to be noexcept to be used as barrier callback
+  auto print_values = [&values]() noexcept {
+    for (auto val : values) {
+      std::cout << std::format("{:<7.5}", val);
+    }
+    std::cout << '\n';
+  };
+
+  // print initial values:
+  print_values();
+
+  // initialize a barrier that prints the values when all threads have done
+  // their computations:
+  std::barrier all_done{
+      int(values.size()), // initial value of the counter
+      print_values};      // callback to call whenever the counter is 0
+
+  // initialize a thread for each value to compute its square root in a loop:
+  std::vector<std::thread> threads;
+  threads.reserve(values.size());
+
+  for (auto &value : values) {
+    threads.emplace_back([&value, &values, &all_done] {
+      // repeatedly:
+      for (int i = 0; i < 5; ++i) {
+        // compute square root:
+        value = std::sqrt(value);
+        // and synchronize with other threads to print values:
+        all_done.arrive_and_wait();
+      }
+    });
+  }
+
+  for (auto &t : threads)
+    t.join();
+}
+
+} // namespace
+
+TEST(barrier_test, basic_test) { barrier_test(); }