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1 : //
2 : // Copyright (c) 2025 Vinnie Falco (vinnie dot falco at gmail dot com)
3 : //
4 : // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 : // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 : //
7 : // Official repository: https://github.com/cppalliance/capy
8 : //
9 :
10 : #ifndef BOOST_CAPY_ANY_EXECUTOR_REF_HPP
11 : #define BOOST_CAPY_ANY_EXECUTOR_REF_HPP
12 :
13 : #include <boost/capy/detail/config.hpp>
14 : #include <boost/capy/ex/any_coro.hpp>
15 :
16 : #include <concepts>
17 : #include <coroutine>
18 : #include <type_traits>
19 : #include <utility>
20 :
21 : namespace boost {
22 : namespace capy {
23 :
24 : class execution_context;
25 :
26 : namespace detail {
27 :
28 : /** Virtual function table for type-erased executor operations. */
29 : struct executor_vtable
30 : {
31 : execution_context& (*context)(void const*) noexcept;
32 : void (*on_work_started)(void const*) noexcept;
33 : void (*on_work_finished)(void const*) noexcept;
34 : void (*post)(void const*, std::coroutine_handle<>);
35 : std::coroutine_handle<> (*dispatch)(void const*, std::coroutine_handle<>);
36 : bool (*equals)(void const*, void const*) noexcept;
37 : };
38 :
39 : /** Vtable instance for a specific executor type. */
40 : template<class Ex>
41 : inline constexpr executor_vtable vtable_for = {
42 : // context
43 0 : [](void const* p) noexcept -> execution_context& {
44 0 : return const_cast<Ex*>(static_cast<Ex const*>(p))->context();
45 : },
46 : // on_work_started
47 0 : [](void const* p) noexcept {
48 0 : const_cast<Ex*>(static_cast<Ex const*>(p))->on_work_started();
49 : },
50 : // on_work_finished
51 0 : [](void const* p) noexcept {
52 0 : const_cast<Ex*>(static_cast<Ex const*>(p))->on_work_finished();
53 : },
54 : // post
55 70 : [](void const* p, std::coroutine_handle<> h) {
56 35 : static_cast<Ex const*>(p)->post(h);
57 : },
58 : // dispatch
59 110 : [](void const* p, std::coroutine_handle<> h) -> std::coroutine_handle<> {
60 110 : return static_cast<Ex const*>(p)->dispatch(h);
61 : },
62 : // equals
63 4 : [](void const* a, void const* b) noexcept -> bool {
64 4 : return *static_cast<Ex const*>(a) == *static_cast<Ex const*>(b);
65 : }
66 : };
67 :
68 : } // detail
69 :
70 : /** A type-erased reference wrapper for executor objects.
71 :
72 : This class provides type erasure for any executor type, enabling
73 : runtime polymorphism without virtual functions or allocation.
74 : It stores a pointer to the original executor and a pointer to a
75 : static vtable, allowing executors of different types to be stored
76 : uniformly while satisfying the full `Executor` concept.
77 :
78 : @par Reference Semantics
79 : This class has reference semantics: it does not allocate or own
80 : the wrapped executor. Copy operations simply copy the internal
81 : pointers. The caller must ensure the referenced executor outlives
82 : all `any_executor_ref` instances that wrap it.
83 :
84 : @par Thread Safety
85 : The `any_executor_ref` itself is not thread-safe for concurrent
86 : modification, but its executor operations are safe to call
87 : concurrently if the underlying executor supports it.
88 :
89 : @par Executor Concept
90 : This class satisfies the `Executor` concept, making it usable
91 : anywhere a concrete executor is expected.
92 : */
93 : class any_executor_ref
94 : {
95 : void const* ex_ = nullptr;
96 : detail::executor_vtable const* vt_ = nullptr;
97 :
98 : public:
99 : /** Default constructor.
100 :
101 : Constructs an empty `any_executor_ref`. Calling any executor
102 : operations on a default-constructed instance results in
103 : undefined behavior.
104 : */
105 533 : any_executor_ref() = default;
106 :
107 : /** Copy constructor.
108 :
109 : Copies the internal pointers, preserving identity.
110 : This enables the same-executor optimization when passing
111 : any_executor_ref through coroutine chains.
112 : */
113 : any_executor_ref(any_executor_ref const&) = default;
114 :
115 : /** Copy assignment operator. */
116 : any_executor_ref& operator=(any_executor_ref const&) = default;
117 :
118 : /** Constructs from any executor type.
119 :
120 : Captures a reference to the given executor and stores a pointer
121 : to the type-specific vtable. The executor must remain valid for
122 : the lifetime of this `any_executor_ref` instance.
123 :
124 : @param ex The executor to wrap. Must satisfy the `Executor`
125 : concept. A pointer to this object is stored
126 : internally; the executor must outlive this wrapper.
127 : */
128 : template<class Ex>
129 : requires (!std::same_as<std::decay_t<Ex>, any_executor_ref>)
130 461 : any_executor_ref(Ex const& ex) noexcept
131 461 : : ex_(&ex)
132 461 : , vt_(&detail::vtable_for<Ex>)
133 : {
134 461 : }
135 :
136 : /** Returns true if this instance holds a valid executor.
137 :
138 : @return `true` if constructed with an executor, `false` if
139 : default-constructed.
140 : */
141 2 : explicit operator bool() const noexcept
142 : {
143 2 : return ex_ != nullptr;
144 : }
145 :
146 : /** Returns a reference to the associated execution context.
147 :
148 : @return A reference to the execution context.
149 :
150 : @pre This instance was constructed with a valid executor.
151 : */
152 : execution_context& context() const noexcept
153 : {
154 : return vt_->context(ex_);
155 : }
156 :
157 : /** Informs the executor that work is beginning.
158 :
159 : Must be paired with a subsequent call to `on_work_finished()`.
160 :
161 : @pre This instance was constructed with a valid executor.
162 : */
163 : void on_work_started() const noexcept
164 : {
165 : vt_->on_work_started(ex_);
166 : }
167 :
168 : /** Informs the executor that work has completed.
169 :
170 : @pre A preceding call to `on_work_started()` was made.
171 : @pre This instance was constructed with a valid executor.
172 : */
173 : void on_work_finished() const noexcept
174 : {
175 : vt_->on_work_finished(ex_);
176 : }
177 :
178 : /** Dispatches a coroutine handle through the wrapped executor.
179 :
180 : Invokes the executor's `dispatch()` operation with the given
181 : coroutine handle, returning a handle suitable for symmetric
182 : transfer.
183 :
184 : @param h The coroutine handle to dispatch for resumption.
185 :
186 : @return A coroutine handle that the caller may use for symmetric
187 : transfer, or `std::noop_coroutine()` if the executor
188 : posted the work for later execution.
189 :
190 : @pre This instance was constructed with a valid executor.
191 : */
192 110 : any_coro dispatch(any_coro h) const
193 : {
194 110 : return vt_->dispatch(ex_, h);
195 : }
196 :
197 : /** Posts a coroutine handle to the wrapped executor.
198 :
199 : Posts the coroutine handle to the executor for later execution
200 : and returns. The caller should transfer to `std::noop_coroutine()`
201 : after calling this.
202 :
203 : @param h The coroutine handle to post for resumption.
204 :
205 : @pre This instance was constructed with a valid executor.
206 : */
207 35 : void post(any_coro h) const
208 : {
209 35 : vt_->post(ex_, h);
210 35 : }
211 :
212 : /** Compares two executor references for equality.
213 :
214 : Two `any_executor_ref` instances are equal if they wrap
215 : executors of the same type that compare equal.
216 :
217 : @param other The executor reference to compare against.
218 :
219 : @return `true` if both wrap equal executors of the same type.
220 : */
221 4 : bool operator==(any_executor_ref const& other) const noexcept
222 : {
223 4 : if (vt_ != other.vt_)
224 0 : return false;
225 4 : if (!vt_)
226 0 : return true;
227 4 : return vt_->equals(ex_, other.ex_);
228 : }
229 : };
230 :
231 : } // capy
232 : } // boost
233 :
234 : #endif
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