C++ coroutines let you write high-performance async code that pauses and resumes naturally, eliminating the complexity of traditional multitasking approaches.Â
C++ coroutines make this possible by letting you pause and resume functions naturally, almost like hitting pause on a video and picking up exactly where you left off. What makes this exciting isn’t just cleaner code (though that’s huge), but the fact that your programs can handle thousands of tasks at once while using far less memory than traditional approaches—without drowning in callback hell or complicated state machines.
Think of it as getting the best of both worlds: code that’s easy to read and understand, plus performance that doesn’t make you choose between simplicity and speed.
With C++20, C++ Coroutines arrived to make asynchronous programming cleaner and more intuitive. They allow you to write code that looks sequential but runs asynchronously under the hood. It reduces boilerplate and keeps your logic easy to follow.
But what exactly are C++ Coroutines, and can they truly change the way we write async code? Or are they just another niche feature? Let’s break it down.
Table of Contents
What are C++ Coroutines, really?
At their core, C++ Coroutines are a C++20-added feature that lets you pause and resume a function’s execution without blocking the main thread. Think of them like hitting “pause” on a task. You stop at a certain point, go do something else, and then “resume” right where you left off.
For instance, when you’re preparing food, you stop and hop to other tasks. You may start boiling water, then while waiting for it to complete, you chop some onions, but then you also start marinating your meat—you get the point.
In programming terms, this means you can write asynchronous code that looks and feels like synchronous code. Instead of chaining callbacks or building complex state machines, coroutines allow your logic to flow naturally while the compiler handles the heavy lifting.
A coroutine in C++ is just a function that uses one or more of these special keywords:
- co_await – pauses execution until a task completes (great for async operations).
- co_yield – produces a value and pauses until the next request (ideal for generators).
- co_return – returns the final result and ends the coroutine.
Here’s a minimal coroutine example in C++:
#include <iostream>
#include <coroutine>
struct SimpleTask {
    struct promise_type {
        SimpleTask get_return_object() { return {}; }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() {}
    };
};
SimpleTask sayHello() {
    std::cout << “Hello, “;
    co_await std::suspend_always{};
    std::cout << “World!\n”;
}
int main() {
    auto task = sayHello(); // Starts coroutine
    // At this point, execution is paused after “Hello, “
    // In real use, you’d resume it when ready.
}
In this example, the function sayHello() starts execution, prints “Hello, “, and then suspends itself with co_await. It only resumes when explicitly told to, allowing other operations to run in between.
Unlike coroutines in C, which often require manual stack management or custom state handling, C++ coroutines are built into the language and supported directly by the compiler. This makes them far more ergonomic for modern async programming in C++.
C++14 vs C++20 side-by-side example
To really see the difference, let’s compare how the same async task is written using C++14 threads and futures versus a C++20 coroutine. The logic is identical, but the coroutine version reads more like straightforward, top-to-bottom code.
1) C++14 Threads/Futures (verbose wiring)
Goal: download ? process ? save without blocking the main thread.
// C++14: compile with -std=c++14
#include <iostream>
#include <thread>
#include <future>
#include <chrono>
using namespace std::chrono_literals;
// — Simulated work ———————————————————
std::string download() {
    std::this_thread::sleep_for(500ms);
    return “data”;
}
std::string process(const std::string& in) {
    std::this_thread::sleep_for(400ms);
    return “processed(“ + in + “)”;
}
void save(const std::string& in) {
    std::this_thread::sleep_for(300ms);
    std::cout << “Saved: “ << in << “\n”;
}
// — Pipeline using threads + futures ————————————–
int main() {
    // 1) Set up futures/promises to shuttle results between stages
    std::promise<std::string> p_download;
    std::future<std::string> f_download = p_download.get_future();
    std::promise<std::string> p_process;
    std::future<std::string> f_process = p_process.get_future();
    // 2) Each stage runs on its own thread, waiting on the previous stage
    std::thread t1([&] { p_download.set_value(download()); });
    std::thread t2([&] {
        auto d = f_download.get();     // wait for download
        p_process.set_value(process(d));  // pass to processing
    });
    std::thread t3([&] {
        auto r = f_process.get();     // wait for processing
        save(r);
    });
    // 3) Don’t forget to join—easy to leak or forget in real code
    t1.join(); t2.join(); t3.join();
}
Takeaways
- Works, but there’s a lot of glue code (promises/futures, joins).
- Logic is scattered across three lambdas.
- Easy to make synchronization mistakes as it scales.
2) C++20 Coroutine (linear, top?to?bottom)
Same workflow, written as one sequential coroutine. Readers can run this as-is.
// C++20: compile with -std=c++20
#include <iostream>
#include <coroutine>
#include <thread>
#include <chrono>
#include <semaphore>
using namespace std::chrono_literals;
// — Tiny awaitable to “sleep” without blocking the coroutine —————
struct sleep_awaitable {
    std::chrono::milliseconds dur;
    bool await_ready() const noexcept { return dur.count() == 0; }  // run-through?
    void await_resume() const noexcept {}               // nothing to return
    // Schedule resume on a background thread after ‘dur’
    void await_suspend(std::coroutine_handle<> h) const {
        std::thread([h, d = dur]{
            std::this_thread::sleep_for(d);
            h.resume();                        // continue pipeline()
        }).detach();
    }
};
// — Minimal Task so main() can wait for completion ————————-
struct Task {
    struct promise_type {
        std::binary_semaphore done{0};
        Task get_return_object() {
            return Task{ std::coroutine_handle<promise_type>::from_promise(*this) };
        }
        std::suspend_never initial_suspend() noexcept { return {}; } // start immediately
        std::suspend_always final_suspend() noexcept {         // signal on finish
            done.release();
            return {};
        }
        void return_void() {}
        void unhandled_exception() { std::terminate(); }
    };
    std::coroutine_handle<promise_type> h;
    void wait() { h.promise().done.acquire(); h.destroy(); }
};
// — The async pipeline, but written linearly ——————————–
Task pipeline() {
    std::cout << “Downloading…\n”;
    co_await sleep_awaitable{500ms};  // pause here, resume later
    std::cout << “Processing…\n”;
    co_await sleep_awaitable{400ms};
    std::cout << “Saving…\n”;
    co_await sleep_awaitable{300ms};
    std::cout << “Saved: processed(data)\n”;
}
int main() {
    auto t = pipeline(); // starts and runs until first co_await
    t.wait();       // wait for coroutine to finish
}
Takeaways
- One function expresses the whole flow.
- The compiler manages suspend/resume; you manage business logic.
- No manual futures/promises/joins to wire up.
What pipeline() does
As you can see, coroutines let us express the same workflow with far less boilerplate, setting the stage for why modern C++ needed them in the first place.
In the next section, we’ll explore why C++ needed coroutines in the first place and how they improve the developer experience over older async approaches like callbacks and goto-style state jumps.
Why Async Development Needed C++ Coroutines – and Their Key Benefits
Before C++20, asynchronous programming in C++ was often messy. Developers relied on callback hell, long promise chains, or manual state machines to keep tasks running without blocking the main thread. A seemingly simple workflow—like reading a file, processing it, and sending the results over a network—required multiple callback functions, manual state passing, and careful thread-safety management. The result was verbose, error-prone code scattered across files, making debugging feel like watching a movie out of order.
C++ Coroutines change this by letting you write asynchronous logic in a clean, sequential style. The compiler automatically handles the suspend and resume points, so you can focus on what should happen instead of how to wire it all together.
In practice, this brings big benefits:
- Smoother Gameplay – Spread heavy computations over multiple frames to avoid frame drops in game engines like Unreal.
- Cleaner Animation Sequences – Write natural-looking code for actions that pause and resume.
- Better Networking – Manage multiplayer communications without blocking the game loop.
- Readable Async Code – Keep logic easy to follow.
- Memory Efficiency – Lower overhead compared to threads.
- Composability – Build complex workflows from smaller coroutine tasks.
The result: more maintainable code, fewer bugs, and a better developer experience.
Current Alternatives and Workarounds
Before C++ Coroutines, developers had to rely on other tools and patterns to achieve asynchronous behavior. While each has its place, they often come with trade-offs that coroutines aim to solve.
- Callbacks – Like giving someone your phone number and saying, “Call me when you’re done.” This works, but over time, you end up with deeply nested, hard-to-follow code—often referred to as callback hell.
- Threads – Like hiring a separate worker for each task. Threads are powerful, but they’re expensive in terms of memory, and coordinating them quickly becomes complex.
- State Machines – Manually tracking where you are in a process. It’s like keeping detailed cooking notes so you know exactly where to pick up later. Functional, but tedious to maintain and easy to break.
- Promises/Futures – Available in some C++ libraries, they improve structure but still follow callback-style thinking and lack the elegance of coroutines.
C++ Coroutines streamline all of these approaches by making asynchronous flows look and feel like synchronous code—without the overhead or complexity of these older patterns.
Visual Studio C++ Tooling
Microsoft Visual Studio has steadily improved its support for C++ Coroutines, making it easier for developers to adopt this modern asynchronous feature.
- IntelliSense – Autocomplete and error detection now understand coroutine-specific syntax like co_await, co_yield, and co_return, reducing the chance of syntax errors.
- Debugging – Stepping through suspension points is more intuitive, with improved visibility into coroutine states and variable values.
- Code Analysis – Static analysis tools are coroutine-aware, helping detect logic issues in coroutine control flow.
- Project Templates – Certain project templates now come preconfigured for coroutine use, streamlining setup for new projects.
That said, coroutine tooling is still catching up to the maturity of traditional C++ features. While debugging has improved, it can remain challenging, especially when coroutines suspend and resume across multiple points in complex workflows. As coroutine adoption grows, further tooling enhancements are expected to make the development and debugging experience even smoother.
Why you may not be as enthusiastic about coroutines
C++ Coroutines provide a better method to create asynchronous code, yet they present difficulties that mainly affect new teams learning them.
The main obstacle developers face when working with C++ Coroutines is the complexity of the learning process. The introduction of coroutines brings new keywords and control flow patterns and concepts, which make them difficult to understand even for developers who already know C++. The process of mastering suspension points, promise types, and coroutine handles requires both time and practice.
The main restriction stems from the limited support of libraries and STL. The support for coroutines in popular C++ libraries remains incomplete despite recent improvements. Developers must create bridging code or depend on third-party libraries to implement coroutines within their current systems.
The complexity of debugging represents another significant challenge. The improved IDE tools do not fully address the difficulty of tracking execution flow between multiple suspension and resumption points in complex applications.
The adoption of coroutines faces challenges at the last point. The C++ ecosystem has only recently adopted coroutines, which results in limited production examples, less community guidance, and slower codebase integration.
The situation is getting better despite current challenges. The development of compiler support and IDE features, and library compatibility continues to advance, which enables developers to test and accept coroutines in their work processes.
Where C++ Coroutines Shine
While C++ Coroutines are still maturing, they already excel in performance-critical domains where asynchronous tasks need to be both efficient and predictable.
1. High-Performance Game Loops
Game engines like Unreal can use coroutines to spread heavy computations—such as AI pathfinding, physics calculations, or asset streaming—across multiple frames. This avoids frame drops and keeps gameplay smooth without blocking the main loop.
2. Network Servers
In high-throughput servers, coroutines enable non-blocking I/O operations that handle thousands of concurrent connections without spawning thousands of threads. This reduces memory overhead while maintaining responsiveness.
3. Embedded Systems
For resource-constrained environments, coroutines provide concurrency without the cost of full threads. They’re ideal for managing sensor polling, device communication, or real-time control loops with minimal footprint.
4. Real-Time Data Processing
In systems that process live streams of data—such as financial tick feeds or telemetry—coroutines allow processing to be paused and resumed seamlessly, ensuring timely updates without unnecessary blocking.
In all these areas, the main advantage is structured concurrency: writing async workflows in a linear, maintainable style that still delivers the performance and control C++ is known for.
Conclusion
C++ Coroutines offer a cleaner, more intuitive way to write asynchronous code in C++. By making async flows look like straightforward, sequential code, they reduce the complexity of callbacks, promise chains, and manual state tracking.
Challenges remain—such as the learning curve, evolving tooling, and partial library support—but these are steadily improving. With better compiler integration, enhanced IDE features, and growing community adoption, coroutines are becoming more practical for real-world use.
In performance-critical fields like game development, network servers, embedded systems, and real-time analytics, coroutines already show clear benefits: smoother execution, reduced overhead, and more maintainable code.
While they may still feel niche to some, coroutines represent a meaningful step forward in modern C++ development. For teams willing to invest the time, the gains in clarity, maintainability, and performance can be well worth it.
FAQ
1. What is a coroutine in C++?
A coroutine in C++ is a special function that can pause (co_await, co_yield) and resume execution later, enabling easier asynchronous programming and generators.
2. What are C++ Coroutines used for?
C++ Coroutines are mainly used for asynchronous programming, generators, and cooperative multitasking. They help simplify complex async flows like networking, game loops, or real-time data processing.
3. Are C++ Coroutines better than threads?
Not always. Coroutines are lighter and more memory-efficient than threads, making them great for many concurrent tasks. However, threads are still needed when true parallel execution across multiple CPU cores is required.
4. Do all compilers support C++ Coroutines?
Most modern compilers like MSVC, GCC, and Clang support coroutines with C++20 enabled, but library support is still maturing. It’s best to check your compiler version and documentation.
5. How do C++ Coroutines compare to Rust or Kotlin coroutines?
Rust and Kotlin offer more concise syntax and richer async libraries out of the box. C++ coroutines require more setup but give developers greater control and performance in low-level, high-performance scenarios.
