How to get a reasonably well working generator class template in C++20?

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I'm just a simple coder who used Python a lot and got addicted to its generators. As far as I understand the current situation, they can be cleanly implemented in C++20 with coroutines, however, at least until C++23, it's not a trivial task since one needs to write a generator class (template). How do I get one that

  1. Runs reasonably fast (at least not slower than the good old generators hack with macros)
  2. I can use ranged-based for, the ranges library and some equivalent of Python's next on it. It would also be great if there was a method to test whether the generator is exhausted.
  3. When (if?) an equivalent is added to the standard library, I (with high probability) won't need to change my code too much

Is this possible at all?

c++20 generator c++
2 Answers

As mentioned in the comment, libcoro provides a higher level of abstraction and might solve some of your issues.

For point2, if you really need a public method that tells you the generator is exhausted, I guess enhancing libcoro generator would make it somehow easy. Here is a (not tested) possible sample. But is checking against generator.end() a problem for you?

namespace libcoro {
    template<typename T>
    class [[nodiscard]] generator
    {
    public:

    using reference_type = std::conditional_t<std::is_reference_v<T>, T, T&>;

    //.. libcoro stuff

    // ADDED
    bool done() const {
      m_coroutine.done();
    }

    reference_type value() const {
      return m_coroutine.promise().value();
    }

    void resume() {
      m_coroutine.resume();
    }

    // ...
};

}

Then you can do:

while (true) {
        gen.resume();
        if(gen.done()) {
            std::cout << "this is the end!" << std::endl;
            break;
        }
        std::cout << "new value: " << gen.value() << std::endl;
}
user_129780130

Here you can find some reasonably fast generator implementation which prefers a std implementation if available (like in MSVC):

In my real code it is splitted in multiple files, but to show it here I merged it...

It works with all major comilers (gcc, clang, msvc).

//------------------
//general coroutine support
//------------------
#if defined(__clang__)
    //see: https://developercommunity.visualstudio.com/content/problem/502513/unable-to-use-clang-cl-coroutines-due-to-unresolve.html
    namespace std {
        namespace experimental {
            inline namespace coroutines_v1 {

                template <typename R, typename...>
                struct coroutine_traits {
                    using promise_type = typename R::promise_type;
                };

                template <typename Promise = void> 
                struct coroutine_handle;

                template <> struct coroutine_handle<void> {
                    static coroutine_handle from_address(void* addr) noexcept {
                        coroutine_handle me;
                        me.ptr = addr;
                        return me;
                    }
                    void operator()() { resume(); }
                    void* address() const { return ptr; }
                    void resume() const { __builtin_coro_resume(ptr); }
                    void destroy() const { __builtin_coro_destroy(ptr); }
                    bool done() const { return __builtin_coro_done(ptr); }
                    coroutine_handle& operator=(decltype(nullptr)) {
                        ptr = nullptr;
                        return *this;
                    }
                    coroutine_handle(decltype(nullptr)) : ptr(nullptr) {}
                    coroutine_handle() : ptr(nullptr) {}
                    //  void reset() { ptr = nullptr; } // add to P0057?
                    explicit operator bool() const { return ptr; }

                protected:
                    void* ptr;
                };

                template <typename Promise> 
                struct coroutine_handle : coroutine_handle<> {
                    using coroutine_handle<>::operator=;
                    using coroutine_handle<>::coroutine_handle;

                    static coroutine_handle from_address(void* addr) noexcept {
                        coroutine_handle me;
                        me.ptr = addr;
                        return me;
                    }

                    Promise& promise() const {
                        return *reinterpret_cast<Promise*>(
                            __builtin_coro_promise(ptr, alignof(Promise), false));
                    }
                    static coroutine_handle from_promise(Promise& promise) {
                        coroutine_handle p;
                        p.ptr = __builtin_coro_promise(&promise, alignof(Promise), true);
                        return p;
                    }
                };

                template <typename _PromiseT>
                bool operator==(coroutine_handle<_PromiseT> const& _Left,
                    coroutine_handle<_PromiseT> const& _Right) noexcept
                {
                    return _Left.address() == _Right.address();
                }

                template <typename _PromiseT>
                bool operator!=(coroutine_handle<_PromiseT> const& _Left,
                    coroutine_handle<_PromiseT> const& _Right) noexcept
                {
                    return !(_Left == _Right);
                }

                struct suspend_always {
                    bool await_ready() noexcept { return false; }
                    void await_suspend(coroutine_handle<>) noexcept {}
                    void await_resume() noexcept {}
                };
                struct suspend_never {
                    bool await_ready() noexcept { return true; }
                    void await_suspend(coroutine_handle<>) noexcept {}
                    void await_resume() noexcept {}
                };
            }
        }
    }
    #define NATIVE_COROUTINE_IMPL_NS std::experimental
#elif __has_include(<coroutine>)
    #include <coroutine>
    #define NATIVE_COROUTINE_IMPL_NS std    
#elif __has_include(<experimental/coroutine>)
    #include <experimental/coroutine>
    #define NATIVE_COROUTINE_IMPL_NS std::experimental
#endif

//------------------
//Generator defintion
//-----------------

#if __has_include(<generator>)
    #include <generator>
    namespace CORE_NATIVE_NS
    {
        template<typename T>
        using YieldEnum = std::generator<T>;
    }
#elif __has_include(<experimental/generator>) && !defined(__clang__)
    #include <experimental/generator>
    namespace CORE_NATIVE_NS
    {
        template<typename T>
        using YieldEnum = std::experimental::generator<T>;
    }
#else
#include <memory>
#include <cstddef>
#include <type_traits>
#include <utility>
#include <exception>
#include <iterator>
#include <functional>

namespace CORE_NATIVE_NS
{
    template<typename T>
    class generator;

    namespace detail
    {
        template<typename T>
        class generator_promise
        {
        public:
            using value_type = std::remove_reference_t<T>;
            using reference_type = std::conditional_t<std::is_reference_v<T>, T, T&>;
            using pointer_type = value_type*;

            generator_promise() = default;

            generator<T> get_return_object() noexcept;

            constexpr NATIVE_COROUTINE_IMPL_NS::suspend_always initial_suspend() const noexcept 
            { return {}; }
            constexpr NATIVE_COROUTINE_IMPL_NS::suspend_always final_suspend() const noexcept
            { return {}; }

            template<
                typename U = T,
                std::enable_if_t<!std::is_rvalue_reference<U>::value, int> = 0
            >
            NATIVE_COROUTINE_IMPL_NS::suspend_always yield_value(std::remove_reference_t<T>& value) noexcept
            {
                m_value = std::addressof(value);
                return {};
            }

            NATIVE_COROUTINE_IMPL_NS::suspend_always yield_value(std::remove_reference_t<T>&& value) noexcept
            {
                m_value = std::addressof(value);
                return {};
            }

            void unhandled_exception() noexcept
            {
                m_exception = std::current_exception();
            }

            void return_void() const noexcept
            {
            }

            reference_type value() const noexcept
            {
                return static_cast<reference_type>(*m_value);
            }

            // Don't allow any use of 'co_await' inside the generator coroutine.
            template<typename U>
            NATIVE_COROUTINE_IMPL_NS::suspend_never await_transform(U&& value) = delete;

            void rethrow_if_exception()
            {
                if (m_exception)
                {
                    std::rethrow_exception(m_exception);
                }
            }
        private:
            pointer_type m_value;
            std::exception_ptr m_exception;
        };

        struct generator_sentinel {};

        template<typename T>
        class generator_iterator
        {
            using coroutine_handle = NATIVE_COROUTINE_IMPL_NS::coroutine_handle<generator_promise<T>>;
        public:

            using iterator_category = std::input_iterator_tag;
            // What type should we use for counting elements of a potentially infinite sequence?
            using difference_type = std::ptrdiff_t;
            using value_type = typename generator_promise<T>::value_type;
            using reference = typename generator_promise<T>::reference_type;
            using pointer = typename generator_promise<T>::pointer_type;

            // Iterator needs to be default-constructible to satisfy the Range concept.
            generator_iterator() noexcept
                : m_coroutine(nullptr)
            {}

            explicit generator_iterator(coroutine_handle coroutine) noexcept
                : m_coroutine(coroutine)
            {}

            friend bool operator==(const generator_iterator& it, generator_sentinel) noexcept
            {
                return !it.m_coroutine || it.m_coroutine.done();
            }

            friend bool operator!=(const generator_iterator& it, generator_sentinel s) noexcept
            {
                return !(it == s);
            }

            friend bool operator==(generator_sentinel s, const generator_iterator& it) noexcept
            {
                return (it == s);
            }

            friend bool operator!=(generator_sentinel s, const generator_iterator& it) noexcept
            {
                return it != s;
            }

            generator_iterator& operator++()
            {
                m_coroutine.resume();
                if (m_coroutine.done())
                {
                    m_coroutine.promise().rethrow_if_exception();
                }

                return *this;
            }

            // Need to provide post-increment operator to implement the 'Range' concept.
            void operator++(int)
            {
                (void)operator++();
            }

            void next(){
                (void)operator++();
            }

            reference operator*() const noexcept
            {
                return m_coroutine.promise().value();
            }

            pointer operator->() const noexcept
            {
                return std::addressof(operator*());
            }

        private:
            coroutine_handle m_coroutine;
        };
    }

    template<typename T>
    class [[nodiscard]] generator
    {
    public:

        using promise_type = detail::generator_promise<T>;
        using iterator = detail::generator_iterator<T>;

        generator() noexcept
            : m_coroutine(nullptr)
        {}

        generator(generator&& other) noexcept
            : m_coroutine(other.m_coroutine)
        {
            other.m_coroutine = nullptr;
        }

        generator(const generator& other) = delete;

        ~generator()
        {
            if (m_coroutine)
            {
                m_coroutine.destroy();
            }
        }

        generator& operator=(generator other) noexcept
        {
            swap(other);
            return *this;
        }

        iterator begin()
        {
            if (m_coroutine)
            {
                m_coroutine.resume();
                if (m_coroutine.done())
                {
                    m_coroutine.promise().rethrow_if_exception();
                }
            }

            return iterator{ m_coroutine };
        }

        detail::generator_sentinel end() noexcept
        {
            return detail::generator_sentinel{};
        }

        void swap(generator& other) noexcept
        {
            std::swap(m_coroutine, other.m_coroutine);
        }

    private:

        friend class detail::generator_promise<T>;

        explicit generator(NATIVE_COROUTINE_IMPL_NS::coroutine_handle<promise_type> coroutine) noexcept
            : m_coroutine(coroutine)
        {}

        NATIVE_COROUTINE_IMPL_NS::coroutine_handle<promise_type> m_coroutine;

    };

    template<typename T>
    void swap(generator<T>& a, generator<T>& b)
    {
        a.swap(b);
    }

    namespace detail
    {
        template<typename T>
        generator<T> generator_promise<T>::get_return_object() noexcept
        {
            using coroutine_handle = NATIVE_COROUTINE_IMPL_NS::coroutine_handle<generator_promise<T>>;
            return generator<T>{ coroutine_handle::from_promise(*this) };
        }
    }

    template<typename FUNC, typename T>
    generator<std::invoke_result_t<FUNC&, typename generator<T>::iterator::reference>> fmap(FUNC func, generator<T> source)
    {
        for (auto&& value : source)
        {
            co_yield std::invoke(func, static_cast<decltype(value)>(value));
        }
    }

    template<typename T>
    using YieldEnum = CORE_NATIVE_NS::generator<T>;
}
#endif


inline CORE_NATIVE_NS::YieldEnum<int> GetNumbers() noexcept
{
    for(int i=0; i<= 10; ++i){
        co_yield 10;
    }
}

#include <iostream>

int main()
{
    int sum = 0;
    for(auto x : GetNumbers()){
        sum += x;
    }
    std::cout << sum;
    return 0;
}
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