Ensure correctness of unique_frame_ptr ↔ unique_payload_ptr conversion

include/mpi_support.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cassert>
+#include <functional>
 #include <memory>
 #include <utility>
 
@@ -104,15 +105,40 @@ class unique_payload_ptr : private std::unique_ptr<void, std::function<void(void
 	unique_payload_ptr() noexcept = default;
 
 	template <typename T>
-	explicit unique_payload_ptr(allocate_uninitialized_tag<T>, size_t count) : impl(operator new(count * sizeof(T)), [](void* p) { operator delete(p); }) {}
+	explicit unique_payload_ptr(allocate_uninitialized_tag<T>, size_t count) : impl(allocate_uninitialized_payload<T>(count)) {}
 
 	template <typename Frame>
-	explicit unique_payload_ptr(unique_frame_ptr<Frame> frame) : impl(frame.release() + 1, [](void* p) { delete(static_cast<Frame*>(p) - 1); }) {}
+	explicit unique_payload_ptr(unique_frame_ptr<Frame> frame) : impl(unique_frame_to_payload(std::move(frame))) {}
 
 	void* get_pointer() { return impl::get(); }
 	const void* get_pointer() const { return impl::get(); }
 
 	using impl::operator bool;
+
+  private:
+	template <typename Frame>
+	static void delete_frame_from_payload(void* const type_erased_payload) {
+		const auto payload = static_cast<typename Frame::payload_type*>(type_erased_payload);
+		const auto frame = reinterpret_cast<Frame*>(payload) - 1; // frame header is located at -sizeof(Frame) bytes (-1 Frame object)
+		delete frame;
+	}
+
+	template <typename Frame>
+	static impl unique_frame_to_payload(unique_frame_ptr<Frame> unique_frame) {
+		deleter_type deleter{delete_frame_from_payload<Frame>}; // allocate deleter (aka std::function) first so `impl` construction is noexcept
+		const auto frame = unique_frame.release();
+		const auto payload = reinterpret_cast<typename Frame::payload_type*>(frame + 1); // payload is located at +sizeof(Frame) bytes (+1 Frame object)
+		return impl{payload, std::move(deleter)};
+	}
+
+	static void delete_uninitialized_payload(void* const p) { operator delete(p); }
+
+	template <typename T>
+	static impl allocate_uninitialized_payload(size_t count) {
+		deleter_type deleter{delete_uninitialized_payload}; // allocate deleter (aka std::function) first so `impl` construction is noexcept
+		const auto payload = operator new(count * sizeof(T));
+		return impl{payload, std::move(deleter)};
+	}
 };
 
 } // namespace celerity::detail