Introduction to C++ Multithreading and Asynchronous Programming (std::async)

Sat, 12 Sep 2026 08:00:00 +0900

In modern software development, multithreading programming is essential to maximize the performance of multi-core CPUs. Starting with C++11, C++ introduced multithreading and asynchronous processing APIs (<thread>, <mutex>, <condition_variable>, <future>) as part of its standard library, making it possible to implement portable and safe concurrent processing without writing platform-dependent code (such as POSIX threads or the Windows API). Furthermore, with each version upgrade—C++14, C++17, and C++20—safer and more advanced features like std::scoped_lock and std::jthread have been added.

In this article, we will thoroughly explain the basics of C++ multithreading programming, synchronization mechanisms to prevent data races, and modern asynchronous processing (std::async) as well as the concept of thread pools, with detailed code examples.

1. Basics of Concurrency and Amdahl’s Law

The primary goal of multithreading is “performance improvement”, but it is not possible to parallelize an entire program. This is where Amdahl’s Law becomes important.

Amdahl’s Law is a model used to predict the extent to which overall system performance will improve when a part of the program is parallelized and optimized.

$$ S(N) = \frac{1}{(1 - P) + \frac{P}{N}} $$

$S(N)$ : Theoretical maximum speedup ratio
$P$ : The proportion of the program that can be parallelized (0 ≤ $P$ ≤ 1)
$N$ : The number of processors (threads)

An important fact indicated by this formula is that “no matter how much you increase the number of processors $N$, the non-parallelizable serial portion $(1 - P)$ becomes a bottleneck, and there is an upper limit to the speedup”. For example, even if $90\%$ of the program can be parallelized ($P = 0.9$), as long as the remaining $10\%$ is processed serially, the maximum speedup will be only $10$ times ($S(\infty) = 1 / 0.1$), even with an infinite number of processors.

Therefore, when doing multithreaded programming in C++, it is required not just to increase the number of threads, but to adopt a design that minimizes the serial processing parts (such as lock contention and synchronization overhead) as much as possible.

2. Thread Basics: `std::thread` and `std::jthread` (C++20)

The Traditional `std::thread` (C++11)

std::thread, introduced in C++11, is the most fundamental class for executing functions or lambda expressions in a new thread.

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#include <iostream>
#include <thread>

void workerFunction(int id) {
 std::cout << "Worker " << id << " is running on thread "
 << std::this_thread::get_id() << std::endl;
}

int main() {
 std::cout << "Main thread id: " << std::this_thread::get_id() << std::endl;

 // Create and start the thread
 std::thread t1(workerFunction, 1);

 // Create a thread using a lambda expression
 std::thread t2([](int id) {
 std::cout << "Lambda Worker " << id << " is running." << std::endl;
 }, 2);

 // Wait for the thread to finish (join)
 t1.join();
 t2.join();

 std::cout << "All threads completed." << std::endl;
 return 0;
}

A point to note about std::thread is that you must always call either join() or detach() before it is destroyed. If the destructor of std::thread is called without either of them having been invoked, std::terminate() is called and the program crashes. To ensure exception safety, it was necessary to create your own wrapper class using the RAII pattern.

The Modern `std::jthread` (C++20)

In C++20, std::jthread (joining thread) was introduced to resolve these shortcomings. Because std::jthread automatically calls join() in its destructor, you can safely wait for the thread to terminate even when exceptions occur. It also provides a cooperative cancellation feature for threads via std::stop_token.

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#include <iostream>
#include <thread>
#include <chrono>

int main() {
 // C++20: std::jthread
 // Can detect cancellation requests by receiving std::stop_token as the first argument
 std::jthread jt([](std::stop_token stoken) {
 while (!stoken.stop_requested()) {
 std::cout << "Working..." << std::endl;
 std::this_thread::sleep_for(std::chrono::milliseconds(500));
 }
 std::cout << "Stop requested. Exiting thread." << std::endl;
 });

 std::this_thread::sleep_for(std::chrono::seconds(2));

 // Explicitly request cancellation
 jt.request_stop();

 // Since join is automatically called by the jthread destructor, manual join() is unnecessary
 return 0;
}

3. Avoiding Data Races and Synchronization: Mutexes and Locks

When multiple threads access the same memory area (such as a variable) simultaneously, and at least one of them writes to it, a Data Race occurs. In the C++ standard, a data race causes Undefined Behavior. To prevent this, exclusive control using std::mutex is required.

`std::mutex` and `std::lock_guard`

Calling raw std::mutex::lock() and unlock() manually is not recommended because if an exception is thrown, unlock() may not be called, risking a deadlock. In C++, std::lock_guard (C++11) or std::scoped_lock (C++17), which use the RAII pattern, are used.

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#include <iostream>
#include <vector>
#include <thread>
#include <mutex>

std::mutex g_mutex;
int g_counter = 0;

void incrementCounter(int iterations) {
 for (int i = 0; i < iterations; ++i) {
 // Automatically unlocked when leaving the scope
 std::lock_guard<std::mutex> lock(g_mutex);
 ++g_counter;
 }
}

int main() {
 std::vector<std::thread> threads;
 for (int i = 0; i < 10; ++i) {
 threads.emplace_back(incrementCounter, 10000);
 }

 for (auto& t : threads) {
 t.join();
 }

 std::cout << "Final counter value: " << g_counter << std::endl;
 // Becomes 100000 as expected
 return 0;
}

`std::unique_lock`

std::lock_guard is a simple scope-based lock, but if you need more flexible control (such as deferred locking, time-constrained locking, or unlocking mid-way), you use std::unique_lock. std::unique_lock is required for the std::condition_variable explained next.

4. Inter-thread Communication: `std::condition_variable`

To implement patterns such as the “Producer-Consumer Pattern”, where one thread waits until a specific condition is met, and another thread sends a notification when that condition is fulfilled, you use std::condition_variable.

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#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <queue>

std::mutex g_mtx;
std::condition_variable g_cv;
std::queue<int> g_dataQueue;
bool g_isFinished = false;

void producer() {
 for (int i = 1; i <= 5; ++i) {
 std::this_thread::sleep_for(std::chrono::milliseconds(200));
 {
 std::lock_guard<std::mutex> lock(g_mtx);
 g_dataQueue.push(i);
 std::cout << "Produced: " << i << std::endl;
 }
 g_cv.notify_one(); // Notify consumer
 }

 {
 std::lock_guard<std::mutex> lock(g_mtx);
 g_isFinished = true;
 }
 g_cv.notify_one(); // Notify completion
}

void consumer() {
 while (true) {
 std::unique_lock<std::mutex> lock(g_mtx);
 // Wait until the condition is met (queue is not empty, or finished flag is set)
 // Specify the condition with a lambda expression to prevent spurious wakeups
 g_cv.wait(lock, []{ return !g_dataQueue.empty() || g_isFinished; });

 while (!g_dataQueue.empty()) {
 int val = g_dataQueue.front();
 g_dataQueue.pop();
 // Unlock and perform heavy processing (here, just output)
 lock.unlock();
 std::cout << "Consumed: " << val << std::endl;
 lock.lock(); // Acquire lock again
 }

 if (g_isFinished && g_dataQueue.empty()) {
 break;
 }
 }
}

int main() {
 std::thread t1(producer);
 std::thread t2(consumer);
 t1.join();
 t2.join();
 return 0;
}

In this example, std::condition_variable::wait puts the thread to sleep until the condition is met, preventing unnecessary consumption of CPU resources (busy waiting).

5. High-level Asynchronous Processing: `std::future`, `std::promise`, `std::async`

The std::thread and std::mutex introduced so far are powerful, but they bring the OS’s low-level thread mechanisms directly into C++, which often leads to verbose code when handling result retrieval and exception propagation. If you want to perform concurrent processing that returns a value or higher-level asynchronous processing, you use the features of the <future> header.

`std::promise` and `std::future`

std::promise represents the side that “sets” the result, and std::future represents the side that “receives” the result. These function as safe channels for passing results and exceptions between threads.

Task-based Concurrency with `std::async`

The most recommended way to execute asynchronous tasks in C++ is to use std::async. std::async executes a task asynchronously and returns a std::future to retrieve the result.

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#include <iostream>
#include <future>
#include <chrono>

int complexCalculation(int x) {
 std::cout << "Calculation started on thread: "
 << std::this_thread::get_id() << std::endl;
 std::this_thread::sleep_for(std::chrono::seconds(2));
 if (x < 0) {
 throw std::invalid_argument("x must be positive");
 }
 return x * 42;
}

int main() {
 std::cout << "Main thread id: " << std::this_thread::get_id() << std::endl;

 // Explicitly run in a separate thread by specifying std::launch::async
 std::future<int> resultFuture = std::async(std::launch::async, complexCalculation, 10);

 std::cout << "Main thread is doing other work..." << std::endl;

 try {
 // Calling get() blocks the current thread and waits until the calculation is complete
 int result = resultFuture.get();
 std::cout << "Result: " << result << std::endl;
 } catch (const std::exception& e) {
 std::cerr << "Exception caught: " << e.what() << std::endl;
 }

 return 0;
}

The behavior of std::async is shown in the sequence diagram below.

sequenceDiagram participant Main as "Main Thread" participant Async as "std::async Worker" Main->>Async: "std::async(std::launch::async, ...)" activate Async Note over Main: "Do other work..." Main->>Async: "future.get() (Blocks until ready)" Note right of Async: "Computations..." Async-->>Main: "Return Result or Throw Exception" deactivate Async

There are two types of Launch Policies for the first argument of std::async:

std::launch::async: Always creates a new thread (or allocates from a thread pool) and executes asynchronously.
std::launch::deferred: Lazy evaluation. Executes synchronously on the calling thread when future.get() or future.wait() is called.

The default (if not specified) is implementation-defined, and either one is chosen depending on the system load. If you definitely want asynchronous execution, explicitly specify std::launch::async.

6. Concept of a Thread Pool

Calling std::async every time, or creating and destroying a std::thread in a loop, causes the overhead of thread context switching and OS resource allocation to become non-negligible. Especially when processing a large number of fine-grained tasks, using a Thread Pool is essential.

A thread pool is an architecture where a certain number of worker threads are created in advance when the application starts, tasks are queued up, and available worker threads process the tasks one by one.

The C++ standard library (as of C++23) does not have a standard thread pool class, but it is possible to implement an efficient thread pool in a few dozen lines by combining std::thread, std::mutex, std::condition_variable, std::function, and std::packaged_task. In actual operations, it is also common to use Boost.Asio’s asynchronous I/O or third-party libraries.

7. Considerations for Performance and Scalability

To extract the maximum performance in multithreaded programming, it is necessary to pay attention not only to the parallelization of the code but also to the hardware architecture.

False Sharing: Even if multiple threads update different variables, if those variables are placed in the same CPU cache line (typically 64 bytes), unnecessary memory synchronization occurs to maintain cache coherency, resulting in a dramatic drop in performance. To prevent this, it is necessary to align variables on cache line boundaries using the alignas specifier.
Lock-Free and std::atomic: To avoid the overhead of locking/unlocking mutexes, introducing atomic operations (like Compare-And-Swap) using <atomic> or lock-free data structures is considered. However, this requires a correct understanding of memory ordering (std::memory_order) and is very difficult to implement. Therefore, it is usually introduced only when deemed necessary after careful performance measurements.

8. Conclusion

We have explained multithreading and asynchronous programming in C++, from the basics to the latest C++20 features. The key points are as follows:

Basically use std::async: For single asynchronous tasks or concurrent processing that returns a result, utilize std::async and std::future, which are safer than managing threads manually.
Use std::jthread for thread management: For threads running in the background long-term, use C++20’s std::jthread to guarantee safe termination processing.
Leverage RAII for synchronization: Always use std::lock_guard or std::unique_lock for locking mutexes to prevent data races.
Be aware of overhead: Avoid creating an excessive number of threads, and introduce a thread pool architecture as needed.

Concurrency bugs (deadlocks, data races) have low reproducibility and fall into the most difficult category to debug. By always being aware of thread safety and choosing appropriate standard library tools, let’s achieve robust and fast system development with modern C++.

Async on kenji.blog

Introduction to C++ Multithreading and Asynchronous Programming (std::async)

1. Basics of Concurrency and Amdahl’s Law

2. Thread Basics: std::thread and std::jthread (C++20)

The Traditional std::thread (C++11)

The Modern std::jthread (C++20)

3. Avoiding Data Races and Synchronization: Mutexes and Locks

std::mutex and std::lock_guard

std::unique_lock

4. Inter-thread Communication: std::condition_variable

5. High-level Asynchronous Processing: std::future, std::promise, std::async

std::promise and std::future

Task-based Concurrency with std::async