From a291bf0d2a750d7c64c1151d335f5ce3a8af67de Mon Sep 17 00:00:00 2001
From: "an.tsouchlos" <an.tsouchlos@gmail.com>
Date: Fri, 25 Apr 2025 22:04:42 +0000
Subject: [PATCH] Add inplace-function.cpp

---
 cpp/inplace-function.md | 144 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 144 insertions(+)
 create mode 100644 cpp/inplace-function.md

diff --git a/cpp/inplace-function.md b/cpp/inplace-function.md
new file mode 100644
index 0000000..208bf0f
--- /dev/null
+++ b/cpp/inplace-function.md
@@ -0,0 +1,144 @@
+```c++
+#pragma once
+
+
+#include <cstddef>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+
+namespace util {
+
+
+///
+/// @brief Class storing a type-erased function object/pointer.
+/// @details This class does not employ dynamic memory allocation; instead, it
+/// stores the function object/pointer in a fixed-size buffer, the size of which
+/// can be specified as a template parameter.
+/// @warning This class does not take ownership of any resources it is
+/// constructed from. It is the responsibility of the user to ensure that the
+/// function object/pointer outlives the InplaceFunction object.
+///
+/// @tparam func_sig_t Signature of the function.
+/// @tparam t_storage_size Size of the storage buffer, in bytes.
+///
+template <typename func_sig_t, std::size_t t_storage_size = 8>
+class InplaceFunction;
+
+template <typename return_t, typename... args_t, std::size_t storage_size>
+class InplaceFunction<return_t(args_t...), storage_size> {
+
+    using storage_t      = char[storage_size];
+    using wrapper_func_t = return_t (*)(storage_t, args_t...);
+
+public:
+    InplaceFunction() = default;
+
+    ///
+    /// @brief Constructor for function objects.
+    ///
+    template <typename F>
+    InplaceFunction(F f) {
+        bind(f);
+    }
+
+    ///
+    /// @brief Construct from free function pointer.
+    ///
+    template <auto F>
+    static InplaceFunction construct() {
+        InplaceFunction<return_t(args_t...)> func;
+        func.template bind<F>();
+        return func;
+    }
+
+    ///
+    /// @brief Construct form member function pointer.
+    ///
+    template <auto F, typename class_t>
+    static InplaceFunction construct(class_t& c) {
+        InplaceFunction<return_t(args_t...)> func;
+        func.template bind<F>(c);
+        return func;
+    }
+
+    ///
+    /// @brief Construct from function object.
+    ///
+    template <typename F>
+    static InplaceFunction construct(F f) {
+        InplaceFunction<return_t(args_t...)> func;
+        func.bind(f);
+        return func;
+    }
+
+    ///
+    /// @warning If no callable has been bound, calling this function will
+    /// result in undefined behavior.
+    ///
+    return_t operator()(args_t... args) {
+        return mWrapper(mStorage, args...);
+    }
+
+    operator bool() const {
+        return !(mWrapper == nullptr);
+    }
+
+    ///
+    /// @brief Bind a free function.
+    ///
+    /// @tparam F Pointer to free function
+    ///
+    template <auto F>
+        requires std::is_invocable_r_v<return_t, decltype(F), args_t...>
+    void bind() {
+        mWrapper = [](storage_t, args_t... args) {
+            return F(std::forward<args_t>(args)...);
+            // return std::invoke(F, std::forward<args_t>(args)...);
+        };
+    }
+
+    ///
+    /// @brief Bind a member function.
+    ///
+    /// @tparam class_t Class the member belongs to.
+    /// @param c Reference to object the member function should be called on.
+    /// @tparam F Member function pointer.
+    ///
+    template <auto F, typename class_t>
+        requires std::is_invocable_r_v<return_t, decltype(F), class_t&,
+                                       args_t...> &&
+                 (sizeof(class_t*) <= storage_size)
+    void bind(class_t& c) {
+        new (mStorage)(class_t*){&c};
+
+        mWrapper = [](storage_t storage, args_t... args) {
+            return F(*reinterpret_cast<class_t**>(storage),
+                     std::forward<args_t>(args)...);
+        };
+    }
+
+    ///
+    /// @brief Bind a function object.
+    ///
+    template <typename F>
+        requires std::is_invocable_r_v<return_t, F, args_t...> &&
+                 (sizeof(F) <= storage_size) &&
+                 std::is_trivially_destructible_v<F>
+    void bind(F f) {
+        new (mStorage) F{std::move(f)};
+        mWrapper = [](storage_t storage, args_t... args) {
+            return (*reinterpret_cast<F*>(storage))(
+                std::forward<args_t>(args)...);
+        };
+    }
+
+private:
+    storage_t      mStorage;
+    wrapper_func_t mWrapper = nullptr;
+};
+
+
+} // namespace util
+```
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