Add a fixed-capacity alternative to std::function

core/CMakeLists.txt

@@ -21,6 +21,7 @@ set(PUBLIC_HEADERS
     include/join/condition.hpp
     include/join/semaphore.hpp
     include/join/futex.hpp
+    include/join/function.hpp
     include/join/thread.hpp
     include/join/threadpool.hpp
     include/join/macaddress.hpp

core/include/join/function.hpp

@@ -0,0 +1,247 @@
+/**
+ * MIT License
+ *
+ * Copyright (c) 2026 Mathieu Rabine
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#ifndef JOIN_CORE_FUNCTION_HPP
+#define JOIN_CORE_FUNCTION_HPP
+
+// C++.
+#include <type_traits>
+#include <functional>
+#include <utility>
+#include <new>
+
+// C.
+#include <cstddef>
+
+namespace join
+{
+    /**
+     * @brief fixed-capacity move-only allocation-free alternative to std::function.
+     */
+    template <typename Signature, std::size_t Capacity = 32, std::size_t Alignment = alignof (std::max_align_t)>
+    class Function;
+
+    /**
+     * @brief fixed-capacity move-only allocation-free alternative to std::function.
+     */
+    template <typename Return, typename... Args, std::size_t Capacity, std::size_t Alignment>
+    class Function<Return (Args...), Capacity, Alignment>
+    {
+    public:
+        /**
+         * @brief default constructor.
+         */
+        Function () noexcept = default;
+
+        /**
+         * @brief copy constructor.
+         * @param other other object to copy.
+         */
+        Function (const Function& other) = delete;
+
+        /**
+         * @brief copy assignment operator.
+         * @param other other object to copy.
+         * @return this.
+         */
+        Function& operator= (const Function& other) = delete;
+
+        /**
+         * @brief move constructor.
+         * @param other object to move from.
+         */
+        Function (Function&& other) noexcept
+        {
+            moveFrom (std::move (other));
+        }
+
+        /**
+         * @brief move assignment operator.
+         * @param other object to move from.
+         * @return *this.
+         */
+        Function& operator= (Function&& other) noexcept
+        {
+            clear ();
+            moveFrom (std::move (other));
+            return *this;
+        }
+
+        /**
+         * @brief construct from nullptr.
+         */
+        Function (std::nullptr_t) noexcept
+        : Function ()
+        {
+        }
+
+        /**
+         * @brief assign nullptr.
+         */
+        Function& operator= (std::nullptr_t) noexcept
+        {
+            clear ();
+            return *this;
+        }
+
+        /**
+         * @brief construct with a callable object.
+         * @param callable callable object.
+         */
+        template <
+            typename Func, typename DecayedFunc = std::decay_t<Func>,
+            typename = std::enable_if_t<!std::is_same<DecayedFunc, Function>::value &&
+                                        (std::is_void<Return>::value ||
+                                         std::is_convertible<std::result_of_t<DecayedFunc&(Args...)>, Return>::value)>>
+        Function (Func&& callable)
+        {
+            static_assert (sizeof (DecayedFunc) <= Capacity, "Callable size exceeds Function capacity.");
+            static_assert (alignof (DecayedFunc) <= Alignment, "Callable alignment exceeds Function alignment.");
+            static_assert (std::is_nothrow_move_constructible<DecayedFunc>::value,
+                           "Callable must be nothrow move constructible.");
+
+            new (&_storage) DecayedFunc (std::forward<Func> (callable));
+
+            _invoker = [] (void* storage, Args&&... args) -> Return {
+                return static_cast<Return> ((*static_cast<DecayedFunc*> (storage)) (std::forward<Args> (args)...));
+            };
+
+            _manager = [] (void* dst, void* src) noexcept {
+                DecayedFunc* source = static_cast<DecayedFunc*> (src);
+                new (dst) DecayedFunc (std::move (*source));
+                source->~DecayedFunc ();
+            };
+
+            _destructor = [] (void* storage) noexcept {
+                static_cast<DecayedFunc*> (storage)->~DecayedFunc ();
+            };
+        }
+
+        /**
+         * @brief destructor.
+         */
+        ~Function ()
+        {
+            clear ();
+        }
+
+        /**
+         * @brief invoke the stored callable.
+         * @param args arguments forwarded to the callable.
+         * @return result of the invocation.
+         * @throw std::bad_function_call if no callable is stored.
+         */
+        Return operator() (Args... args)
+        {
+            if (!_invoker)
+            {
+                throw std::bad_function_call ();
+            }
+            return _invoker (&_storage, std::forward<Args> (args)...);
+        }
+
+        /**
+         * @brief checks whether a callable is stored.
+         * @return true if a callable is present.
+         */
+        explicit operator bool () const noexcept
+        {
+            return _invoker != nullptr;
+        }
+
+        /**
+         * @brief clear the stored callable.
+         */
+        void reset () noexcept
+        {
+            clear ();
+        }
+
+        /**
+         * @brief swap two Function instances.
+         */
+        void swap (Function& other) noexcept
+        {
+            Function tmp (std::move (other));
+            other = std::move (*this);
+            *this = std::move (tmp);
+        }
+
+    private:
+        /// invocation function pointer type.
+        using InvokerFunc = Return (*) (void*, Args&&...);
+
+        /// relocation function pointer type.
+        using ManagerFunc = void (*) (void*, void*);
+
+        /// destructor function pointer type.
+        using DestructorFunc = void (*) (void*);
+
+        /**
+         * @brief destroy the stored callable and reset all pointers.
+         */
+        void clear () noexcept
+        {
+            if (_destructor)
+            {
+                _destructor (&_storage);
+                _invoker = nullptr;
+                _manager = nullptr;
+                _destructor = nullptr;
+            }
+        }
+
+        /**
+         * @brief move all state from another instance (assumes *this is empty).
+         */
+        void moveFrom (Function&& other) noexcept
+        {
+            _invoker = other._invoker;
+            _manager = other._manager;
+            _destructor = other._destructor;
+
+            if (_manager)
+            {
+                _manager (&_storage, &other._storage);
+                other._invoker = nullptr;
+                other._manager = nullptr;
+                other._destructor = nullptr;
+            }
+        }
+
+        /// fixed-capacity aligned storage for the callable target.
+        alignas (Alignment) unsigned char _storage[Capacity];
+
+        /// pointer to the static invocation wrapper.
+        InvokerFunc _invoker = nullptr;
+
+        /// pointer to the static move-relocate wrapper.
+        ManagerFunc _manager = nullptr;
+
+        /// pointer to the static destructor wrapper.
+        DestructorFunc _destructor = nullptr;
+    };
+}
+
+#endif

core/tests/CMakeLists.txt

@@ -157,6 +157,11 @@ target_link_libraries(sharedfutex.gtest ${JOIN_CORE} GTest::gtest_main)
 add_test(NAME sharedfutex.gtest COMMAND sharedfutex.gtest)
 install(TARGETS sharedfutex.gtest RUNTIME DESTINATION ${CMAKE_INSTALL_DATADIR}/${PROJECT_NAME}/test)
 
+add_executable(function.gtest function_test.cpp)
+target_link_libraries(function.gtest ${JOIN_CORE} GTest::gtest_main)
+add_test(NAME function.gtest COMMAND function.gtest)
+install(TARGETS function.gtest RUNTIME DESTINATION ${CMAKE_INSTALL_DATADIR}/${PROJECT_NAME}/test)
+
 add_executable(thread.gtest thread_test.cpp)
 target_link_libraries(thread.gtest ${JOIN_CORE} GTest::gtest_main)
 add_test(NAME thread.gtest COMMAND thread.gtest)