From 2f1150703636b2b06d84bb66853fce7382bf6f29 Mon Sep 17 00:00:00 2001
From: Mark Rowe <mark@vector35.com>
Date: Thu, 31 Jul 2025 13:28:58 -0700
Subject: Introduce bn::base::function_ref, a non-owning function wrapper
 inspired by C++26's std::function_ref

This is to `std::function` as `std::string_view` is to `std::string`.
---
 base/function_ref.h | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 base/function_ref.h

(limited to 'base/function_ref.h')

diff --git a/base/function_ref.h b/base/function_ref.h
new file mode 100644
index 00000000..015cd8f2
--- /dev/null
+++ b/base/function_ref.h
@@ -0,0 +1,90 @@
+// Copyright (c) 2025 Vector 35 Inc
+//
+// 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.
+
+#pragma once
+
+#include <type_traits>
+#include <utility>
+#include <functional>
+
+namespace bn::base {
+
+template <typename Sig>
+class function_ref;
+
+// A non-owning reference to a callable object, inspired by C++26's std::function_ref.
+// If the callable needs to be stored or copied, use std::function instead.
+template <typename R, typename... Args>
+class function_ref<R(Args...)>
+{
+private:
+    union Storage
+    {
+        const void* object = nullptr;
+        R (*func)(Args...);
+    };
+
+    Storage m_storage;
+    R (*m_invoker)(const Storage&, Args...) = nullptr;
+    
+    static R invoke_function(const Storage& storage, Args... args)
+    {
+        auto fn = storage.func;
+        return fn(std::forward<Args>(args)...);
+    }
+    
+    template <typename T>
+    static R invoke_callable(const Storage& storage, Args... args)
+    {
+        return std::invoke(*static_cast<const T*>(storage.object), std::forward<Args>(args)...);
+    }
+    
+public:
+    function_ref() = delete;
+
+    // Constructor that accepts a function pointer
+    function_ref(R (*f)(Args...)) noexcept
+        : m_storage{.func = f}
+        , m_invoker(&invoke_function)
+    {
+    }
+
+    // Constructor that accepts any callable object that is not function_ref
+    template<typename F>
+        requires (!std::is_same_v<std::remove_cvref_t<F>, function_ref>) &&
+                 std::is_invocable_r_v<R, F&, Args...>
+    function_ref(const F& f) noexcept
+        : m_storage{.object = &f}
+        , m_invoker(&invoke_callable<std::remove_cvref_t<F>>)
+    {
+    }
+    
+    R operator()(Args... args) const
+    {
+        return m_invoker(m_storage, std::forward<Args>(args)...);
+    }
+
+    function_ref(const function_ref&) noexcept = default;
+    function_ref(function_ref&&) noexcept = default;
+    function_ref& operator=(const function_ref&) noexcept = default;
+    function_ref& operator=(function_ref&&) noexcept = default;
+};
+
+} // namespace bn::base
-- 
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