Frequently Asked Questions - comp.lang.objective-c

compiled by David Stes (

November 9, 2000


1. About this FAQ

1.1 Where can I find the latest version of the FAQ ?

It's posted once a month to comp.lang.objective-c, comp.answers and news.answers. It is archived at

2. Objective-C Compiler Commands

2.1 What's the file suffix for Objective-C source ?

It's .m for implementation files, and .h for header files. Objective-C compilers usually also accept .c as a suffix, but compile those files in plain C mode.

2.2 How do I compile .m files with the Stepstone compiler ?

objcc -c class.m
objcc -o class class.o

See for more information.

2.3 How do I compile .m files with the Apple compiler ?

cc -c class.m
cc -o class class.o

See for more information.

2.4 How do I compile .m files with the GNU C compiler ?

gcc -c class.m
gcc -o class class.o -lobjc -lpthread

See for more information.

2.5 How do I compile .m files with the POC ?

objc -c class.m
objc -o class class.o

See for more information.

3. Objective-C preprocessor issues

3.1 What's the syntax for comments ?

The Objective-C preprocessor usually supports two styles of comments :

// this is a BCPL-style comment (extends to end of line)


/* this is a C-style comment */

3.2 How do I include the root class ?

On Stepstone and the POC, the header file to include is :


On GNU cc and Apple cc, it's :


The root class is located in a directory called runtime for the Stepstone compiler, and in a directory called objcrt for the POC, but because of implicit -I options passed on to the preprocessor, these locations are automatically searched.

3.3 What is #import ?

It's a C preprocessor construct to avoid multiple inclusions of the same file.

#import <Object.h>

is an alternative to

#include <Object.h>

where the .h file is protected itself against multiple inclusions :

#ifndef _OBJECT_H_
#define _OBJECT_H_

3.4 Why am I lectured about using #import ?

The GNU Objective-C compiler emits a warning when you use #import because some people find using #import poor style. You can turn off the warning by using the -Wno-import option, you could modify the compiler source code and set the variable warn_import (in the file cccp.c) or you could convert your code to use pairs of #ifndef and #endif, as shown above, which makes your code work with all compilers.

4. Object datatype (id)

4.1 What is id ?

It's a generic C type that Objective-C uses for an arbitrary object. For example, a static function that takes one object as argument and returns an object, could be declared as :

static id myfunction(id argument) { ... }

4.2 What is the difference between self and super ?

self is a variable that refers to the object that received a message in a method implementation. super refers to the same variable, but directs the compiler to use a method implementation from the superclass.

Using pseudo-code, where copy (from super) is the syntax for the copy implementation of the superclass, the following are equivalent :

myObject = [super copy];


myObject = [self copy (from super)]; // pseudo-code

4.3 What is @defs() ?

It's a compiler directive to get access to the internal memory layout of instances of a particular class.

typedef struct { @defs(MyClass) } *TMyClass;

defines a C-type TMyClass with a memory layout that is the same as that of MyClass instances.

5. Message selectors (SEL)

5.1 What is a SEL ?

It's the C type of a message selector; it's often defined as a (uniqued) string of characters (the name of the method, including colons), but not all compilers define the type as such.

5.2 What is perform: doing ?

perform: is a message to send a message, identified by its message selector (SEL), to an object.

5.3 How do I know the SEL of a given method ?

If the name of the method is known at compile time, use @selector :

[myObject perform:@selector(close)];

At runtime, you can lookup the selector by a runtime function that takes the name of the message as argument, as in :

SEL mySel = selUid(name); // for Stepstone
SEL mySel = sel_getUid(name); // for Apple
SEL mySel = sel_get_any_uid(name); // for GNU Objective C
SEL mySel = selUid(name); // for POC

6. Implementation pointers (IMP)

6.1 What is an IMP ?

It's the C type of a method implementation pointer, a function pointer to the function that implements an Objective-C method. It is defined to return id and takes two hidden arguments, self and _cmd :

typedef id (*IMP)(id self,SEL _cmd,...);

6.2 How do I get an IMP given a SEL ?

This can be done by sending a methodFor: message :

IMP myImp = [myObject methodFor:mySel];

6.3 How do I send a message given an IMP ?

By dereferencing the function pointer. The following are all equivalent :

[myObject myMessage];


IMP myImp = [myObject methodFor:@selector(myMessage)];


[myObject perform:@selector(myMessage)];

6.4 How can I use IMP for methods returning double ?

For methods that return a C type such as double instead of id, the IMP function pointer is casted from pointer to a function returning id to pointer to a function returning double :

double aDouble = ((double (*) (id,SEL))myImp)(self,_cmd);

6.5 Can I use perform: for a message returning double ?

No. The method perform: is for sending messages returning id without any other argument. Use perform:with: if the message returns id and takes one argument. Use methodFor: for the general case of any number of arguments and any return type.

7. Copying objects

7.1 What's the difference between copy and deepCopy ?

copy is intented to make a bytecopy of the object, sharing pointers with the original, and can be overridden to copy additional memory. deepCopy is intented to make a copy that doesn't share pointers with the original. A deep copy of an object contains copies of its instance variables, while a plain copy is normally just a copy at the first level.

8. Objective-C and C++

8.1 How can I link a C++ library into an Objective-C program ?

You have two options : either use the Apple compiler or use the POC. The former accepts a mix of C++ and Objective-C syntax (called Objective-C++), the latter compiles Objective-C into C and then compiles the intermediate code with a C++ compiler. See the compiler specific questions for more information.

9. Messages

9.1 How do I make a static method ?

Methods are always implemented in Objective-C as static functions. The only way to obtain the IMP (implementation pointer) of a method is through the runtime (via methodFor: and friends), because the function itself is static to the file that implements the method.

9.2 How do I prevent an object from sending a given message ?

You can't. If your object responds to a message, any other class can send this message. You could add an extra argument sender and check, as in :

- mymethod:sender
  if ([sender isKindOf:..]) ...

But this still requires cooperation of the sender, to use a correct argument :

  [anObject mymethod:self];

9.3 Do I have to recompile everything if I change the implementation of a method ?

No, you only have to recompile the implementation of the method itself. Files that only send that particular messages do not have to be recompiled because Objective-C has dynamic binding.

10. Instance and Class Variables

10.1 Do I have to recompile everything if I change instance variables of a class ?

You have to recompile that class, all of its subclasses, and those files that use @defs() or use direct access to the instance variables of that class. In short, using @defs() to access instance variables, or accessing instance variables through subclassing, breaks the encapsulation that the Objective-C runtime normally provides for all other files (the files that you do not have to recompile).

11. Objective-C and X-Windows

11.1 How do I include X Intrinsics headers into an Objective-C file ?

To avoid a conflict between Objective-C's Object and the X11/Object, do the following :

#include <Object.h>
#define Object XtObject
#include <X11/Intrinsic.h>
#include <X11/IntrinsicP.h>
#undef Object

12. Stepstone Specific Questions

12.1 How do I allocate an object on the stack ?

To allocate an instance of 'MyClass' on the stack :

MyClass aClass = [MyClass new];

13. GNU Objective-C Specific Questions

13.1 Why do I get a 'floating point exception' ?

This used to happen on some platforms and is described at A solution was to add -lieee to the command line, so that an invalid floating point operation in the runtime did not send a signal. DJGPP users can consult AIX users may want to consult In some cases, you can fix the problem by upgrading to a more recent version of the GNU Objective-C runtime and/or compiler.

14. Apple Objective-C Specific Questions

14.1 What's the class of a constant string ?

It's an NXConstantString.

NXConstantString *myString = @"my string";

14.2 How can I link a C++ library into an Objective-C program ?

c++ -c file.m
c++ file.o -lcpluslib -o myprogram

15. Portable Object Compiler Objective-C Specific Questions

15.1 What's the syntax for class variables ?

List the class variables after the instance variables, and group them together in the same way as instance variables, as follows :

@implementation MyClass : Object { id ivar1; int ivar2; } : { id cvar1; }

15.2 How do I forward messages ?

You have to implement doesNotUnderstand: to send a sentTo: message.

- doesNotUnderstand:aMsg
  return [aMsg sentTo:aProxy];

15.3 How can I link a C++ library into an Objective-C program ?

objc -c -cplus file.m
objc -cplus file.o -lcpluslib -o myprogram

16. Books and further reading

16.1 Object-Oriented Programming : An Evolutionary Approach, 2nd Ed.

Brad Cox & Andy Novobilski, ISBN 0201548348.

16.2 An Introduction To Object-Oriented Programming, 2nd Ed.

Timothy Budd, ISBN 0201824191

16.3 Objective-C : Object-Oriented Programming Techniques

Pinson, Lewis J. / Wiener, Richard S., ISBN 0201508281

16.4 Applications of Object-Oriented Programming; C++ SmallTalk Actor Objective-C Object PASCAL

Pinson, Lewis J. / Wiener, Richard S., ISBN 0201503697