Directory: make some code generic, move to VarSize.hxx

Makefile.am

@@ -251,6 +251,7 @@ libutil_a_SOURCES = \
 	src/util/Macros.hxx \
 	src/util/Cast.hxx \
 	src/util/Clamp.hxx \
+	src/util/VarSize.hxx \
 	src/util/Error.cxx src/util/Error.hxx \
 	src/util/Domain.hxx \
 	src/util/ReusableArray.hxx \

src/Directory.cxx

@@ -25,6 +25,7 @@
 #include "SongSort.hxx"
 #include "Song.hxx"
 #include "fs/Traits.hxx"
+#include "util/VarSize.hxx"
 #include "util/Error.hxx"
 
 extern "C" {
@@ -42,14 +43,9 @@ Directory::Allocate(const char *path)
 {
 	assert(path != nullptr);
 
-	const size_t path_size = strlen(path) + 1;
-	Directory *directory =
-		(Directory *)g_malloc(sizeof(*directory)
-				      - sizeof(directory->path)
-				      + path_size);
-	new(directory) Directory(path);
-
-	return directory;
+	return NewVarSize<Directory>(sizeof(Directory::path),
+				     strlen(path) + 1,
+				     path);
 }
 
 Directory::Directory()
@@ -96,8 +92,7 @@ Directory::NewGeneric(const char *path, Directory *parent)
 void
 Directory::Free()
 {
-	this->Directory::~Directory();
-	g_free(this);
+	DeleteVarSize(this);
 }
 
 void

src/Directory.hxx

@@ -49,6 +49,10 @@ class SongFilter;
 class Error;
 
 struct Directory {
+	template<class T, typename... Args> friend T *
+	NewVarSize(size_t declared_tail_size, size_t real_tail_size,
+		   Args&&... args);
+
 	/**
 	 * Pointers to the siblings of this directory within the
 	 * parent directory.  It is unused (undefined) in the root

src/util/VarSize.hxx

@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2008-2014 Max Kellermann <max@duempel.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef MPD_VAR_SIZE_HXX
+#define MPD_VAR_SIZE_HXX
+
+#include "Compiler.h"
+
+#include <type_traits>
+#include <utility>
+#include <new>
+
+#include <glib.h>
+
+/**
+ * Allocate and construct a variable-size object.  That is useful for
+ * example when you want to store a variable-length string as the last
+ * attribute without the overhead of a second allocation.
+ *
+ * @param T a struct/class with a variable-size last attribute
+ * @param declared_tail_size the declared size of the last element in
+ * #T
+ * @param real_tail_size the real required size of the last element in
+ * #T
+ */
+template<class T, typename... Args>
+gcc_malloc
+T *
+NewVarSize(size_t declared_tail_size, size_t real_tail_size, Args&&... args)
+{
+	static_assert(std::is_standard_layout<T>::value,
+		      "Not standard-layout");
+
+	/* determine the total size of this instance */
+	size_t size = sizeof(T) - declared_tail_size + real_tail_size;
+
+	/* allocate memory */
+	T *instance = (T *)g_malloc(size);
+
+	/* call the constructor */
+	new(instance) T(std::forward<Args>(args)...);
+
+	return instance;
+}
+
+template<typename T>
+gcc_nonnull_all
+void
+DeleteVarSize(T *instance)
+{
+	/* call the destructor */
+	instance->T::~T();
+
+	/* free memory */
+	g_free(instance);
+}
+
+#endif