Index: src/OFNull.h
==================================================================
--- src/OFNull.h
+++ src/OFNull.h
@@ -24,10 +24,11 @@
 /*!
  * @class OFNull OFNull.h ObjFW/OFNull.h
  *
  * @brief A class for representing null values in collections.
  */
+OF_SUBCLASSING_RESTRICTED
 @interface OFNull: OFObject <OFCopying, OFSerialization, OFJSONRepresentation,
     OFMessagePackRepresentation>
 /*!
  * @brief Returns an OFNull singleton.
  *

Index: src/OFStdIOStream.h
==================================================================
--- src/OFStdIOStream.h
+++ src/OFStdIOStream.h
@@ -24,10 +24,11 @@
  * @brief A class for providing standard input, output and error as OFStream.
  *
  * The global variables @ref of_stdin, @ref of_stdout and @ref of_stderr are
  * instances of this class and need no initialization.
  */
+OF_SUBCLASSING_RESTRICTED
 @interface OFStdIOStream: OFStream
 {
 	int  _fd;
 	bool _atEndOfStream;
 }

Index: src/macros.h
==================================================================
--- src/macros.h
+++ src/macros.h
@@ -219,10 +219,17 @@
 #if __has_attribute(__objc_root_class__)
 # define OF_ROOT_CLASS __attribute__((__objc_root_class__))
 #else
 # define OF_ROOT_CLASS
 #endif
+
+#if __has_attribute(__objc_subclassing_restricted__)
+# define OF_SUBCLASSING_RESTRICTED \
+	__attribute__((__objc_subclassing_restricted__))
+#else
+# define OF_SUBCLASSING_RESTRICTED
+#endif
 
 #ifdef __GNUC__
 # if defined(__x86_64__) || defined(__amd64__)
 #  define OF_X86_64_ASM
 # elif defined(__i386__)