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Use the Foundation as your base and not the MulleFoundation, as Foundation also include objc-compat.

Use envelope headers

Rewrite code that imports specific headers to use the envelope header.

Example: Rewrite

#import <Foundation/NSObject.h>
#import <Foundation/NSString.h>


#import "import.h" // or <Foundation/Foundation.h> without the modern workflow

The exception being <Foundation/NSDebug.h> or any other header not exposed by <Foundation/Foundation.h>.

Find and correct uses of class_getInstanceSize

Two typical uses for class_getInstanceSize are

  1. retrieve the amount of memory required to create an instance
  2. locate the “extra” bytes allocated by NSAllocateObject at the end of an instance

Instance creation

The number of bytes returned by class_getInstanceSize is the amount of bytes required to hold an instance (including mulle-objc runtime overhead). But the actual object will be at an offset. Pass the allocated memory to objc_constructInstance and use the return value as the instance pointer.

id        obj;
size_t    size;
void      *allocation;

size       = class_getInstanceSize( cls);
allocation = calloc( 1, size);
obj        = objc_constructInstance( cls, allocation);
#ifdef __MULLE_OBJC__
allocation = object_getAlloc( obj);
allocation = obj;
free( allocation);

Access extra bytes

Remember that your class may be subclassed. An offset from the last known instance variable in your class implementation may not be correct.

The proper and portable way to get a pointer to the extra bytes is:

size_t    size;
void      *allocation;
void      *extra;

size       = class_getInstanceSize( object_getClass( self));
#ifdef __MULLE_OBJC__
allocation = object_getAlloc( obj);
allocation = obj;
extra      = &((char *) allocation)[ size];

If you use objc-compat, you can use object_getExtraBytes, which does exactly the above.

Register composed selectors before using messaging

If you compose a selector from a C string, you must then use sel_registerName to acquire a SEL from the string. Then use this SEL for messaging. (You can also use NSSelectorFromString)

Find and correct uses of objc_msgSend and objc_msgSend_stret

Use the mulle-objc MetaABI convention to pass parameters and inspect return values.

Find and correct uses of Protocol

Protocol * does not exist. Replace it with PROTOCOL. You can not treat PROTOCOL as an object and message it.

Find +initialize and +load methods and add dependency information

The proper dependencies must be declared, the only known dependency to exist is the runtime during loading and initialization. Everything else must be declared.


Move class extension code to the @interface


Fix mismatching property and ivar names (rename ivars)



  • the runtime only knows about protocols that are adopted by a class
  • PROTOCOL in mulle-objc is a hash value (like a selector) a different type
  • you can not message protocols

There is no enveloping NSAutoreleasePool around +load in mulle-objc

If you create ephemeral instances in your +load method, you should wrap the code yourself inside an NSAutoreleasePool.

Tags: runtime