Codechange: use RAII to automatically restore _cur_dpi after use

src/core/backup_type.hpp

@@ -144,4 +144,46 @@ private:
 	const int line;
 };
 
+/**
+ * Class to backup a specific variable and restore it upon destruction of this object to prevent
+ * stack values going out of scope before resetting the global to its original value. Contrary to
+ * #Backup this restores the variable automatically and there is no manual option to restore.
+ */
+template <typename T>
+struct AutoRestoreBackup {
+	/*
+	 * There is explicitly no only original constructor version, as that would make it possible
+	 * for the new value to go out of scope before this object goes out of scope, thus defeating
+	 * the whole goal and reason for existing of this object.
+	 */
+
+	/**
+	 * Backup variable and switch to new value.
+	 * @param original Variable to backup.
+	 * @param new_value New value for variable.
+	 */
+	AutoRestoreBackup(T &original, T new_value) : original(original), original_value(original)
+	{
+		original = new_value;
+	}
+
+	/**
+	 * Restore the variable upon object destruction.
+	 */
+	~AutoRestoreBackup()
+	{
+		this->original = this->original_value;
+	}
+
+private:
+	T &original;
+	T original_value;
+
+	/* Prevent copy, assignment and allocation on stack. */
+	AutoRestoreBackup(const AutoRestoreBackup&) = delete;
+	AutoRestoreBackup& operator=(AutoRestoreBackup&) = delete;
+	static void *operator new(std::size_t) = delete;
+	static void *operator new[](std::size_t) = delete;
+};
+
 #endif /* BACKUP_TYPE_HPP */