An Infinite Data Stream with Coroutines in C++20

In this post, I analyze the new keyword co_yield. Thanks to co_yield, you can create an infinite data stream in C++20.

This is what happened so far in my pragmatical journey through the new coroutine keywords co_return, co_yield, and co_await.

co_return:

A Generator

As a starting point for further variations, I want to start with a generator which I only ask for three values. This simplification and visualization should help understand the generator’s control flow.

// infiniteDataStreamComments.cpp

#include <coroutine>
#include <memory>
#include <iostream>

template<typename T>
struct Generator {
    
    struct promise_type;
    using handle_type = std::coroutine_handle<promise_type>;
    
    Generator(handle_type h): coro(h) {
        std::cout << "        Generator::Generator" << '\n';
    }                

    handle_type coro;
    
    ~Generator() {
        std::cout << "        Generator::~Generator" << '\n';
        if ( coro ) coro.destroy();
    }
    Generator(const Generator&) = delete;
    Generator& operator = (const Generator&) = delete;
    Generator(Generator&& oth): coro(oth.coro) {
        oth.coro = nullptr;
    }
    Generator& operator = (Generator&& oth) {
        coro = oth.coro;
        oth.coro = nullptr;
        return *this;
    }
    T getNextValue() {
        std::cout << "        Generator::getNextValue" << '\n';
        coro.resume();                                                  // (13) 
        return coro.promise().current_value;
    }
    struct promise_type {
        promise_type() {                                                // (2)
            std::cout << "            promise_type::promise_type" << '\n';
        }                              
          
        ~promise_type() {
            std::cout << "            promise_type::~promise_type" << '\n';
        }
        
        std::suspend_always initial_suspend() {                         // (5)
            std::cout << "            promise_type::initial_suspend" << '\n';  
            return {};                                                  // (6)
        }
        std::suspend_always final_suspend() noexcept {
            std::cout << "            promise_type::final_suspend" << '\n';
            return {};
        }
        auto get_return_object() {                                       // (3)
            std::cout << "            promise_type::get_return_object" << '\n';
            return Generator{handle_type::from_promise(*this)};          // (4)
        }
      
        std::suspend_always yield_value(int value) {                     // (8)
            std::cout << "            promise_type::yield_value" << '\n';
            current_value = value;                                       // (9)
            return {};                                                   // (10)
        }
        void return_void() {}
        void unhandled_exception() {
            std::exit(1);
        }

        T current_value;
    };

};

Generator<int> getNext(int start = 10, int step = 10) {
    std::cout << "    getNext: start" << '\n';
    auto value = start;
    for (true) {                                                         // (11)
        std::cout << "    getNext: before co_yield" << '\n';
        co_yield value;                                                  // (7)
        std::cout << "    getNext: after co_yield" << '\n';
        value += step;
    }
}

int main() {
  
    auto gen = getNext();                                                // (1)
    for (int i = 0; i <= 2; ++i) {
        auto val = gen.getNextValue();                                   // (12)
        std::cout << "main: " << val << '\n';                            // (14)
    }
    
}

Executing the program on the Compiler Explorer makes the control flow transparent.

Let’s analyze the control flow.

The call getNext() (line 1) triggers the creation of the Generator<int>. First, promise_type (line 2) is created, and the following get_return_object call (line 3) creates the generator (line 4) and stores it in a local variable. The result of this call is returned to the caller when the coroutine is suspended the first time. The initial suspension happens immediately (line 5). Because the member function call initial_suspend returns an Awaitable std::suspend_always (line 6), the control flow continues with the coroutine getNext until the instruction co_yield value (line 7). This call is mapped to the call yield_value(int value) (line 8), and the current value is prepared current_value = value (line 9). The member function yield_value(int value) returns the Awaitable std::suspend_always (line 10). Consequently, the execution of the coroutine pauses, the control flow goes back to the main function, and the for loop starts (line 11). The call gen.getNextValue() (line 12) starts the execution of the coroutine by resuming the coroutine using coro.resume() (line 13). Further, the function getNextValue() returns the current value prepared using the previously invoked member function yield_value(int value) (line 8). Finally, the generated number is displayed in line 14 and the for loop continues. In the end, the generator and the promise are destroyed.

After this detailed analysis, I want first to modify the control flow.

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Modifications

My code snippets and line numbers are all based on the previous program infiniteDataStreamComments.cpp. I only show the modifications.

The Coroutine is Not Resumed

When I disable the resumption of the coroutine (gen.getNextValue() in line 12) and the display of its value (line 14), the coroutine immediately pauses.

int main() {
  
    auto gen = getNext();
    for (int i = 0; i <= 2; ++i) {
        // auto val = gen.getNextValue();
        // std::cout << "main: " << val << '\n';                   
    }
    
}

The coroutine never runs. Consequently, the generator and its promise are created and destroyed.

`initial_suspend` Never Suspends

In the program, the member function initial_suspend returns the Awaitable std::suspend_always (line 5). As its name suggests, the Awaitable std::suspends_always causes the coroutine to pause immediately. Let me return std::suspend_never instead of std::suspend_always.

std::suspend_never initial_suspend() {  
    std::cout << "            promise_type::initial_suspend" << '\n';
    return {};
}

In this case, the coroutine runs immediately and pauses when the function yield_value (line 8) is invoked. A subsequent call gen.getNextValue() (line 12) resumes the coroutine and triggers the execution of the member function yield_value once more. The start value 10 is ignored, and the coroutine returns 20, 30, and 40.

infiniteDataStreamInitialSuspendSuspendsNever

`yield_value` Never Suspends

The call triggers the member function (line 8) co_yield value and prepares the current_value (line 9). The function returns the Awaitable std::suspend_always (line 10) and pauses the coroutine. Consequently, a subsequent call gen.getNextValue (line 12) has to resume the coroutine. When I change the return value of the member function yield_value to std::suspend_never, let me see what happens.

std::suspend_never yield_value(int value) {    
    std::cout << "            promise_type::yield_value" << '\n';
    current_value = value;
    return {};
}

As you may guess, the while loop (line 1) runs forever, and the coroutine returns nothing.

infiniteDataStreamCommentsYieldValueSuspendsNever

@Matt Godbolt: this was not a denial-of-service attack.

What’s next?

So far, I have never used the fact that the coroutine is a class template. In my next post, I restructure the generator to produce a finite number of arbitrary values.

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