Support variant direct storage and inline operation state

.github/workflows/build.yml

@@ -94,7 +94,7 @@ jobs:
       - name: Archive Single-Header as is
         uses: kittaakos/upload-artifact-as-is@v0
         with:
-          path: build/source/single-header/futures/futures.h
+          path: build/include/single-header/futures/futures.hpp
       - name: Archive Installer Packages as is
         uses: kittaakos/upload-artifact-as-is@v0
         with:

docs/adaptors/continuations.md

@@ -197,18 +197,18 @@ The following table describes all unwrapping functions by their priority:
 
 | Future output                                                 | Continuation input                             | Inputs |
 |---------------------------------------------------------------|------------------------------------------------|--------|
-| `future<T>`                                                   | `future<T>`                                    | 1      |
-| `future<T>`                                                   | ``                                             | 0      |
-| `future<T>`                                                   | `T`                                            | 1      |
+| `future<R>`                                                   | `future<R>`                                    | 1      |
+| `future<R>`                                                   | ``                                             | 0      |
+| `future<R>`                                                   | `R`                                            | 1      |
 | `future<tuple<future<T1>, future<T2>, ...>>`                  | `future<T1>`, `future<T2>` ...                 | N      |
 | `future<tuple<future<T1>, future<T2>, ...>>`                  | `T1`, `T2` ...                                 | N      |
-| `future<vector<future<T>>>`                                   | `vector<T>`                                    | 1      |
+| `future<vector<future<R>>>`                                   | `vector<R>`                                    | 1      |
 | `future<when_any_result<tuple<future<T1>, future<T2>, ...>>>` | `size_t`, `tuple<future<T1>, future<T2>, ...>` | 2      |
 | `future<when_any_result<tuple<future<T1>, future<T2>, ...>>>` | `size_t`, `future<T1>`, `future<T2>`, ...      | N + 1  |
-| `future<when_any_result<tuple<future<T>, future<T>, ...>>>`   | `future<T>`                                    | 1      |
-| `future<when_any_result<vector<future<T>>>>`                  | `future<T>`                                    | 1      |
-| `future<when_any_result<tuple<future<T>, future<T>, ...>>>`   | `T`                                            | 1      |
-| `future<when_any_result<vector<future<T>>>>`                  | `T`                                            | 1      |
+| `future<when_any_result<tuple<future<R>, future<R>, ...>>>`   | `future<R>`                                    | 1      |
+| `future<when_any_result<vector<future<R>>>>`                  | `future<R>`                                    | 1      |
+| `future<when_any_result<tuple<future<R>, future<R>, ...>>>`   | `R`                                            | 1      |
+| `future<when_any_result<vector<future<R>>>>`                  | `R`                                            | 1      |
 
 Note that types are very important here. Whenever the continuation has the same number of arguments for the same future
 output, a template function or a lambda using `auto` would be ambiguous.
@@ -219,7 +219,7 @@ In this case, the continuation function will attempt to use the unwrapping with
 be `cfuture<int>`. However, this is not always possible if the unwrapping overloads are ambiguous enough.
 
 The continuation with the highest priority is always the safer and usually more verbose continuation. This means a
-template continuation will usually unwrap to `future<T>` over `T` continuation input variants. On the other hand, this
+template continuation will usually unwrap to `future<R>` over `R` continuation input variants. On the other hand, this
 is also useful since the most verbose continuation patterns are the ones that could benefit the most from `auto`.
 
 ## Return type unwrapping

include/futures/adaptor/detail/make_ready_future.hpp

@@ -0,0 +1,62 @@
+//
+// Copyright (c) 2021 alandefreitas (alandefreitas@gmail.com)
+//
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+
+#ifndef FUTURES_ADAPTOR_DETAIL_MAKE_READY_FUTURE_HPP
+#define FUTURES_ADAPTOR_DETAIL_MAKE_READY_FUTURE_HPP
+
+#include <futures/futures/basic_future.hpp>
+#include <futures/futures/promise.hpp>
+#include <futures/futures/traits/is_future.hpp>
+#include <future>
+
+namespace futures::detail {
+    struct make_ready_future_impl
+    {
+        template <typename T>
+        basic_future<typename std::decay_t<T>, future_options<>>
+        make_ready_future(T &&value) {
+            basic_future<std::decay_t<T>, future_options<>> result(
+                std::forward<T>(value));
+            return result;
+        }
+
+        template <typename T>
+        basic_future<T &, future_options<>>
+        make_ready_future(std::reference_wrapper<T> value) {
+            promise<T &, future_options<>> p;
+            basic_future<T &, future_options<>> result = p.get_future();
+            p.set_value(value);
+            return result;
+        }
+
+        basic_future<void, future_options<>>
+        make_ready_future() {
+            promise<void, future_options<>> p;
+            basic_future<void, future_options<>> result = p.get_future();
+            p.set_value();
+            return result;
+        }
+
+        template <typename T = void>
+        basic_future<T, future_options<>>
+        make_exceptional_future(std::exception_ptr ex) {
+            promise<T, future_options<>> p;
+            p.set_exception(ex);
+            return p.get_future();
+        }
+
+        template <class T = void, class E>
+        basic_future<T, future_options<>>
+        make_exceptional_future(E ex) {
+            promise<T, future_options<>> p;
+            p.set_exception(std::make_exception_ptr(ex));
+            return p.get_future();
+        }
+    };
+} // namespace futures::detail
+
+#endif // FUTURES_ADAPTOR_DETAIL_MAKE_READY_FUTURE_HPP

include/futures/adaptor/detail/unwrap_and_continue.hpp

@@ -11,6 +11,7 @@
 #include <futures/futures/future_options.hpp>
 #include <futures/adaptor/detail/unwrap_and_continue_traits.hpp>
 #include <futures/detail/algorithm/tuple_algorithm.hpp>
+#include <futures/detail/container/small_vector.hpp>
 #include <futures/detail/traits/is_single_type_tuple.hpp>
 #include <futures/detail/traits/is_tuple_invocable.hpp>
 #include <futures/detail/traits/is_when_any_result.hpp>
@@ -207,20 +208,24 @@ namespace futures::detail {
                         std::tuple<PrefixArgs...>,
                         unwrapped_elements>>;
                 if constexpr (tuple_explode_unwrap) {
-                    return transform_and_apply(
-                        continuation,
-                        [](auto &&el) {
+                    auto future_to_value = [](auto &&el) {
                         if constexpr (!is_future_v<std::decay_t<decltype(el)>>)
                         {
                             return el;
                         } else {
                             return el.get();
                         }
-                        },
+                    };
+                    auto prefix_as_tuple = std::make_tuple(
+                        std::forward<PrefixArgs>(prefix_args)...);
+                    auto futures_tuple = before_future.get();
+                    // transform each tuple with future_to_value
+                    return transform_and_apply(
+                        continuation,
+                        future_to_value,
                         std::tuple_cat(
-                            std::make_tuple(
-                                std::forward<PrefixArgs>(prefix_args)...),
-                            before_future.get()));
+                            prefix_as_tuple,
+                            std::move(futures_tuple)));
                 } else {
                     detail::throw_exception<std::logic_error>(
                         "Continuation unwrapping not possible");
@@ -537,26 +542,24 @@ namespace futures::detail {
     template <class Executor, class Function, class Future>
     struct continuation_traits_helper
     {
-        // The return type of unwrap and continue function
+        // The possible return types of unwrap and continue function
         using unwrap_result = result_of_unwrap_t<Future, Function>;
-
         using unwrap_result_with_token_prefix
             = result_of_unwrap_with_token_t<Future, Function>;
 
         // Whether the continuation expects a token
         static constexpr bool is_valid_without_stop_token
             = !std::is_same_v<unwrap_result, unwrapping_failure_t>;
-
         static constexpr bool is_valid_with_stop_token = !std::is_same_v<
             unwrap_result_with_token_prefix,
             unwrapping_failure_t>;
 
-        // Whether the continuation is valid
+        // Whether the continuation is valid at all
         static constexpr bool is_valid = is_valid_without_stop_token
                                          || is_valid_with_stop_token;
 
-        // The result type of unwrap and continue for the valid version, with or
-        // without token
+        // The result type of unwrap and continue for the valid overload
+        // (with or without the token)
         using next_value_type = std::conditional_t<
             is_valid_with_stop_token,
             unwrap_result_with_token_prefix,
@@ -565,7 +568,7 @@ namespace futures::detail {
         // Stop token for the continuation function
         constexpr static bool expects_stop_token = is_valid_with_stop_token;
 
-        // Check if the stop token should be inherited from previous future
+        // Check if the stop token can be inherited from to next future
         constexpr static bool previous_future_has_stop_token = has_stop_token_v<
             Future>;
         constexpr static bool previous_future_is_shared = is_shared_future_v<
@@ -574,33 +577,45 @@ namespace futures::detail {
             = previous_future_has_stop_token && (!previous_future_is_shared);
 
         // Continuation future should have stop token
+        // note: this is separate from `expects_stop_token` because (in the
+        // future), the continuation might reuse the stop source without
+        // actually containing a function that expects the token.
         constexpr static bool after_has_stop_token = expects_stop_token;
 
-        // The result type of unwrap and continue for the valid version, with or
-        // without token
-        using base_future_options = std::conditional_t<
+        // The result type of unwrap and continue for the valid unwrap overload
+        // (with or without token)
+
+        // Next needs to inherit the constructor from previous future
+        // Next needs continuation source if previous is eager
+        using next_maybe_continuable_future_options = std::conditional_t<
             !is_always_deferred_v<Future>,
             future_options<executor_opt<Executor>, continuable_opt>,
             future_options<executor_opt<Executor>>>;
 
-        using eager_future_options = conditional_append_future_option_t<
-            after_has_stop_token,
-            stoppable_opt,
-            base_future_options>;
-
-        using maybe_deferred_future_options = conditional_append_future_option_t<
-            is_always_deferred_v<Future>,
-            always_deferred_opt,
-            eager_future_options>;
-
-        using maybe_function_type_future_options = conditional_append_future_option_t<
-            is_always_deferred_v<Future>,
-            deferred_function_opt<detail::unwrap_and_continue_task<Future, Function>>,
-            maybe_deferred_future_options>;
-
-        // The result type of unwrap and continue for the valid version, with or
-        // without token
-        using next_future_options = maybe_function_type_future_options;
+        // Next is stoppable if we identified the function expects a token
+        using next_maybe_stoppable_future_options
+            = conditional_append_future_option_t<
+                after_has_stop_token,
+                stoppable_opt,
+                next_maybe_continuable_future_options>;
+
+        // Next needs the always_deferred_opt if it's deferred
+        using next_maybe_deferred_future_options
+            = conditional_append_future_option_t<
+                is_always_deferred_v<Future>,
+                always_deferred_opt,
+                next_maybe_stoppable_future_options>;
+
+        // Next needs the continuation function type if it's deferred
+        using next_maybe_function_type_future_options
+            = conditional_append_future_option_t<
+                is_always_deferred_v<Future>,
+                deferred_function_opt<
+                    detail::unwrap_and_continue_task<Future, Function>>,
+                next_maybe_deferred_future_options>;
+
+        // The result options type of unwrap and continue
+        using next_future_options = next_maybe_function_type_future_options;
     };
 
     template <class Executor, class Function, class Future>

include/futures/adaptor/make_ready_future.hpp

@@ -8,10 +8,7 @@
 #ifndef FUTURES_ADAPTOR_MAKE_READY_FUTURE_HPP
 #define FUTURES_ADAPTOR_MAKE_READY_FUTURE_HPP
 
-#include <futures/futures/basic_future.hpp>
-#include <futures/futures/promise.hpp>
-#include <futures/futures/traits/is_future.hpp>
-#include <future>
+#include <futures/adaptor/detail/make_ready_future.hpp>
 
 namespace futures {
     /** @addtogroup adaptors Adaptors
@@ -26,10 +23,8 @@ namespace futures {
     template <typename T>
     basic_future<typename std::decay_t<T>, future_options<>>
     make_ready_future(T &&value) {
-        promise<std::decay_t<T>, future_options<>> p;
-        basic_future<std::decay_t<T>, future_options<>> result = p.get_future();
-        p.set_value(std::forward<T>(value));
-        return result;
+        return detail::make_ready_future_impl{}.template make_ready_future<T>(
+            std::forward<T>(value));
     }
 
     /// Make a placeholder future object that is ready from a reference
@@ -40,10 +35,8 @@ namespace futures {
     template <typename T>
     basic_future<T &, future_options<>>
     make_ready_future(std::reference_wrapper<T> value) {
-        promise<T &, future_options<>> p;
-        basic_future<T &, future_options<>> result = p.get_future();
-        p.set_value(value);
-        return result;
+        return detail::make_ready_future_impl{}.template make_ready_future<T>(
+            value);
     }
 
     /// Make a placeholder void future object that is ready
@@ -53,10 +46,7 @@ namespace futures {
     /// @return A future associated with the shared state that is created.
     inline basic_future<void, future_options<>>
     make_ready_future() {
-        promise<void, future_options<>> p;
-        basic_future<void, future_options<>> result = p.get_future();
-        p.set_value();
-        return result;
+        return detail::make_ready_future_impl{}.make_ready_future();
     }
 
     /// Make a placeholder future object that is ready with an exception
@@ -69,9 +59,8 @@ namespace futures {
     template <typename T = void>
     basic_future<T, future_options<>>
     make_exceptional_future(std::exception_ptr ex) {
-        promise<T, future_options<>> p;
-        p.set_exception(ex);
-        return p.get_future();
+        return detail::make_ready_future_impl{}
+            .template make_exceptional_future<T>(ex);
     }
 
     /// Make a placeholder future object that is ready with from any
@@ -84,10 +73,10 @@ namespace futures {
     template <class T = void, class E>
     basic_future<T, future_options<>>
     make_exceptional_future(E ex) {
-        promise<T, future_options<>> p;
-        p.set_exception(std::make_exception_ptr(ex));
-        return p.get_future();
+        return detail::make_ready_future_impl{}
+            .template make_exceptional_future<T, E>(ex);
     }
+
     /** @} */
 } // namespace futures
 

include/futures/detail/container/small_vector.hpp

@@ -69,7 +69,7 @@ namespace futures::detail {
     template <
         class T,
         size_t N
-        = std::max(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
+        = (std::max)(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
         class Allocator = std::allocator<T>,
         class AllowHeap = std::true_type,
         class SizeType = size_t,
@@ -1426,7 +1426,7 @@ namespace futures::detail {
             const auto old_capacity = capacity();
             // Set the initial capacity
             if (old_capacity == 0) {
-                return std::max(64 / sizeof(value_type), size_type(5));
+                return (std::max)(64 / sizeof(value_type), size_type(5));
             }
             // Blocks larger than or equal to 4096 bytes can be expanded in place
             constexpr size_t min_in_place_expansion = 4096;
@@ -1791,8 +1791,8 @@ namespace futures::detail {
     template <
         class T,
         size_t N_INPUT,
-        size_t N_OUTPUT = std::max(
-            std::max(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
+        size_t N_OUTPUT = (std::max)(
+            (std::max)(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
             N_INPUT)>
     constexpr small_vector<std::remove_cv_t<T>, N_OUTPUT>
     to_small_vector(T (&a)[N_INPUT]) {
@@ -1804,8 +1804,8 @@ namespace futures::detail {
     template <
         class T,
         size_t N_INPUT,
-        size_t N_OUTPUT = std::max(
-            std::max(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
+        size_t N_OUTPUT = (std::max)(
+            (std::max)(std::size_t(5), (sizeof(T *) + sizeof(size_t)) / sizeof(T)),
             N_INPUT)>
     constexpr small_vector<std::remove_cv_t<T>, N_OUTPUT>
     to_small_vector(T(&&a)[N_INPUT]) {
@@ -1815,7 +1815,7 @@ namespace futures::detail {
     template <
         class T,
         size_t N
-        = std::max((sizeof(std::vector<T>) * 2) / sizeof(T), std::size_t(5)),
+        = (std::max)((sizeof(std::vector<T>) * 2) / sizeof(T), std::size_t(5)),
         class Allocator = std::allocator<T>,
         class SizeType = size_t>
     using max_size_small_vector
@@ -1824,7 +1824,7 @@ namespace futures::detail {
     template <
         class T,
         size_t N
-        = std::max((sizeof(std::vector<T>) * 2) / sizeof(T), std::size_t(5)),
+        = (std::max)((sizeof(std::vector<T>) * 2) / sizeof(T), std::size_t(5)),
         class Allocator = std::allocator<T>,
         class SizeType = size_t>
     using inline_small_vector