older thread pool impl with perfect forwarding, to recall later

thread-pool/pool.cpp

@@ -0,0 +1,84 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class ThreadPool {
+   private:
+    vector<thread> workers;
+    queue<function<void()>> tasks;
+    mutex mtx;
+    condition_variable cv;
+    bool stop = false;
+
+   public:
+    ThreadPool(int num_threads) {
+        while (num_threads-- > 0) {
+            workers.emplace_back([this] {
+                while (true) {
+                    function<void()> task;
+                    {
+                        unique_lock<mutex> lock(mtx);
+                        cv.wait(lock,
+                                [this] { return stop || !tasks.empty(); });
+
+                        if (stop && tasks.empty()) return;
+
+                        task = std::move(tasks.front());
+                        tasks.pop();
+                    }
+                    task();
+                }
+            });
+        }
+    }
+
+    template <typename F, typename... Args>
+    future<typename result_of<F(Args...)>::type> enqueue(F&& f,
+                                                         Args&&... args) {
+        if (stop) throw std::runtime_error("enqueue on stopped ThreadPool");
+
+        using return_type = typename std::result_of<F(Args...)>::type;
+        // this is not calling the return type its a func with no args and
+        // returning return type value as in int(int) this is int()
+        auto task = make_shared<packaged_task<return_type()>>(
+            bind(forward<F>(f), forward<Args>(args)...));
+        future<return_type> fut = task->get_future();
+        // since its a share pointer so the task lives as long as future lives
+        // you could not make it shared but then pool might die with future
+        // being invalid now the desctructor isn't called now unless the future
+        // also goes out of scope
+
+        {
+            unique_lock<mutex> lock(mtx);
+            tasks.emplace([task] { (*task)(); });
+        }
+
+        cv.notify_one();
+        return fut;
+    }
+
+    ~ThreadPool() {
+        {
+            unique_lock<mutex> lock(mtx);
+            stop = true;
+        }
+
+        cv.notify_all();
+        for (auto& worker : workers) worker.join();
+    }
+};
+
+int main() {
+    ThreadPool pool(4);
+
+    auto taskFunction = [](int a, int& b, std::string&& c) {
+        std::cout << "Task executed with: " << a << ", " << b << ", " << c
+                  << std::endl;
+        b *= 2;  // Modify the lvalue reference
+        return a + b;
+    };
+
+    int lval = 10;
+    auto future = pool.enqueue(taskFunction, 5, ref(lval), "hello");
+
+    cout << "result: " << future.get() << endl;
+}