From 6986c73651729ae46244df5cd470c0ec4b52b38d Mon Sep 17 00:00:00 2001
From: Shaun Reed <shaunrd0@gmail.com>
Date: Sun, 3 Apr 2022 14:06:36 -0400
Subject: [PATCH] [cpp] Add example of using condition_variables

---
 cpp/multithreading/CMakeLists.txt            |  1 +
 cpp/multithreading/conditions/CMakeLists.txt | 26 ++++++++
 cpp/multithreading/conditions/driver.cpp     | 62 ++++++++++++++++++++
 3 files changed, 89 insertions(+)
 create mode 100644 cpp/multithreading/conditions/CMakeLists.txt
 create mode 100644 cpp/multithreading/conditions/driver.cpp

diff --git a/cpp/multithreading/CMakeLists.txt b/cpp/multithreading/CMakeLists.txt
index fc9e2b8..f04f838 100644
--- a/cpp/multithreading/CMakeLists.txt
+++ b/cpp/multithreading/CMakeLists.txt
@@ -15,6 +15,7 @@ project(
     LANGUAGES   CXX
 )
 
+add_subdirectory(conditions)
 add_subdirectory(deadlock)
 add_subdirectory(livelock)
 add_subdirectory(race-condition)
diff --git a/cpp/multithreading/conditions/CMakeLists.txt b/cpp/multithreading/conditions/CMakeLists.txt
new file mode 100644
index 0000000..af82c09
--- /dev/null
+++ b/cpp/multithreading/conditions/CMakeLists.txt
@@ -0,0 +1,26 @@
+################################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example of condition_variables in multithreaded C++              ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+
+cmake_minimum_required(VERSION 3.16)
+
+# std::scoped_lock requires C++17
+set(CMAKE_CXX_STANDARD 17)
+add_compile_options("-Wall")
+
+project(
+    #[[NAME]]   ConditionVariables
+    VERSION     1.0
+    DESCRIPTION "Example of condition_variables in multithreaded C++"
+    LANGUAGES   CXX
+)
+
+add_executable(
+    multithread-conditions driver.cpp
+)
+
+target_link_libraries(multithread-conditions pthread)
diff --git a/cpp/multithreading/conditions/driver.cpp b/cpp/multithreading/conditions/driver.cpp
new file mode 100644
index 0000000..1dd51e4
--- /dev/null
+++ b/cpp/multithreading/conditions/driver.cpp
@@ -0,0 +1,62 @@
+/*##############################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example of condition_variables in multithreaded C++              ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+*/
+
+#include <chrono>
+#include <condition_variable>
+#include <iostream>
+#include <mutex>
+#include <thread>
+
+static std::mutex mtx;
+std::condition_variable cv;
+bool processing = false;
+
+// Starts a job that waits for kick-off from main
+// + When job finishes, handoff result back to main via processing bool
+void job(int32_t & shared) {
+  std::unique_lock uniqueLock(mtx);
+  cv.wait(uniqueLock, []()->bool {return processing;});
+  std::cout << std::this_thread::get_id()
+            << " thread_A: Initial value of shared = " << shared << std::endl;
+  while (shared < INT32_MAX) {
+    shared++;
+  }
+  // We're no longer processing data
+  processing = false;
+  std::cout << std::this_thread::get_id()
+            << " thread_A: Done working." << std::endl;
+  uniqueLock.unlock(); // Important! Unlock uniqueLock before we notify
+  // Notify main that we've finished, so it can proceed
+  cv.notify_one();
+}
+
+int main(const int argc, const char * argv[]) {
+  std::cout << "main() thread id: " << std::this_thread::get_id() << std::endl;
+
+  int32_t share = 0;
+  std::thread thread_A(job, std::ref(share));
+
+  mtx.lock();
+  std::this_thread::sleep_for(std::chrono::seconds(3));
+  share = INT32_MAX / 2;
+  processing = true;
+  mtx.unlock();
+  // Notify thread_A that its work can begin
+  cv.notify_one();
+
+  // Wait until thread_A finishes its work
+  std::unique_lock uniqueLock(mtx);
+  // Block execution until we are not processing
+  cv.wait(uniqueLock, []()->bool { return !processing;});
+  std::cout << std::this_thread::get_id() << " main(): final value of shared = "
+            << share << std::endl;
+  thread_A.join();
+
+  return 0;
+}