From 9c16603da6389de2bb08f8ac09bde53b50944fb7 Mon Sep 17 00:00:00 2001
From: Peter Nelson <peter1138@openttd.org>
Date: Wed, 11 Jun 2025 20:13:49 +0100
Subject: [PATCH] Codechange: Add AutoRelease helper to use function as
 unique_ptr deleter. (#14353)

This allows passing the function as a template parameter instead of requiring a custom deleter functor.
---
 src/fileio_type.h        | 11 ++---------
 src/misc/CMakeLists.txt  |  1 +
 src/misc/autorelease.hpp | 27 +++++++++++++++++++++++++++
 src/soundloader_opus.cpp | 13 +++----------
 4 files changed, 33 insertions(+), 19 deletions(-)
 create mode 100644 src/misc/autorelease.hpp

diff --git a/src/fileio_type.h b/src/fileio_type.h
index 6e35596e87..ab1fee9c9c 100644
--- a/src/fileio_type.h
+++ b/src/fileio_type.h
@@ -11,6 +11,7 @@
 #define FILEIO_TYPE_H
 
 #include "core/enum_type.hpp"
+#include "misc/autorelease.hpp"
 
 /** The different abstract types of files that the system knows about. */
 enum AbstractFileType : uint8_t {
@@ -142,15 +143,7 @@ public:
 	}
 
 private:
-	/** Helper to close a FILE * with a \c std::unique_ptr. */
-	struct FileDeleter {
-		void operator ()(FILE *f)
-		{
-			if (f != nullptr) fclose(f);
-		}
-	};
-
-	std::unique_ptr<FILE, FileDeleter> f;
+	AutoRelease<FILE, fclose> f;
 
 	FileHandle(FILE *f) : f(f) { assert(this->f != nullptr); }
 };
diff --git a/src/misc/CMakeLists.txt b/src/misc/CMakeLists.txt
index 031ad3e000..f6df9b38eb 100644
--- a/src/misc/CMakeLists.txt
+++ b/src/misc/CMakeLists.txt
@@ -1,5 +1,6 @@
 add_files(
     alternating_iterator.hpp
+    autorelease.hpp
     binaryheap.hpp
     dbg_helpers.cpp
     dbg_helpers.h
diff --git a/src/misc/autorelease.hpp b/src/misc/autorelease.hpp
new file mode 100644
index 0000000000..78f5d87226
--- /dev/null
+++ b/src/misc/autorelease.hpp
@@ -0,0 +1,27 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file autorelease.hpp Helper for std::unique_ptr to use an arbitrary function as the deleter. */
+
+#ifndef AUTORELEASE_HPP
+#define AUTORELEASE_HPP
+
+/** Deleter that calls a function rather than deleting the pointer. */
+template <auto Tfunc>
+struct DeleterFromFunc {
+	template <typename T>
+	constexpr void operator()(T *arg) const
+	{
+		if (arg != nullptr) Tfunc(arg);
+	}
+};
+
+/** Specialisation of std::unique_ptr for objects which must be deleted by calling a function. */
+template <typename T, auto Tfunc>
+using AutoRelease = std::unique_ptr<T, DeleterFromFunc<Tfunc>>;
+
+#endif /* AUTORELEASE_HPP */
diff --git a/src/soundloader_opus.cpp b/src/soundloader_opus.cpp
index 5a9645759b..5cf92c571c 100644
--- a/src/soundloader_opus.cpp
+++ b/src/soundloader_opus.cpp
@@ -8,6 +8,8 @@
 /** @file sound_opus.cpp Loading of opus sounds. */
 
 #include "stdafx.h"
+
+#include "misc/autorelease.hpp"
 #include "random_access_file_type.h"
 #include "sound_type.h"
 #include "soundloader_type.h"
@@ -51,7 +53,7 @@ public:
 		sound.file->ReadBlock(tmp.data(), tmp.size());
 
 		int error = 0;
-		auto of = std::unique_ptr<OggOpusFile, OggOpusFileDeleter>(op_open_memory(tmp.data(), tmp.size(), &error));
+		auto of = AutoRelease<OggOpusFile, op_free>(op_open_memory(tmp.data(), tmp.size(), &error));
 		if (error != 0) return false;
 
 		size_t datapos = 0;
@@ -81,15 +83,6 @@ public:
 
 		return true;
 	}
-
-private:
-	/** Helper class to RAII release an OggOpusFile. */
-	struct OggOpusFileDeleter {
-		void operator()(OggOpusFile *of)
-		{
-			if (of != nullptr) op_free(of);
-		}
-	};
 };
 
 static SoundLoader_Opus s_sound_loader_opus;