From d3353488e6243f8cdfae745946d8eeb0fd1ced43 Mon Sep 17 00:00:00 2001
From: Giel van Schijndel <giel@mortis.eu>
Date: Mon, 16 Sep 2024 19:55:12 +0200
Subject: [PATCH] feat(function_ref): add as_c_callback() member function

---
 function_ref.hpp | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/function_ref.hpp b/function_ref.hpp
index 4942ee7..fd7cebb 100644
--- a/function_ref.hpp
+++ b/function_ref.hpp
@@ -135,6 +135,19 @@ class function_ref<Signature, R(Args...)>
       assert(*this && "*this does not store a callable function target");
       return _erased_func(_invocable, std::forward<Args>(args)...);
     }
+
+    // Helper that exposes the guts of this class to make it easily usable in C-style callbacks.
+    // This avoids extra indirection & conversion overhead.
+    // Bit hackis right now, should probably perform some proper conversions to actually rely on defined behavior...
+    std::pair<void* /* c_cb_arg */, R (*)(void* c_cb_arg, Args...) noexcept(is_noexcept)> as_c_callback() const noexcept {
+      return {
+        _invocable.optr,
+        reinterpret_cast<R (*)(void* c_cb_arg, Args...) noexcept(is_noexcept)>(
+          // casting via intermediate most basic function pointer to suppress a warning about incompatible function pointer type.
+          reinterpret_cast<void (*)() noexcept(is_noexcept)>(_erased_func)
+        ),
+      };
+    }
 };
 
 template <typename R, typename... Args>