_cmd value inside C functions

Question

What's the value of _cmd variable when I access it from C-style function's body?

Is it defined inside Objective-C methods only?

P.S. This question may originate from my non-understanding of what _cmd is. I would greatly appreciate if someone provided me with a good explanation source.

score 25 · Answer 1 · answered May 05 '14 at 09:36

The major use of the _cmd function is to get the method name in which it is called.

The use of the _cmd with some other functions has been written below.

NSLog(@"<%@:%@:%d>", NSStringFromClass([self class]), NSStringFromSelector(_cmd), __LINE__);

Instead of above line you can also use PrettyFunction

NSLog(@"%s", __PRETTY_FUNCTION__);

score 21 · Accepted Answer · edited Jul 24 '16 at 02:39

21

It's for Objective-C methods only, so you can't access it. The first two parameters passed to all Objective-C methods are self and _cmd, then whatever other arguments the actual method takes. Since neither self nor _cmd are passed to regular C functions, you can't access them.

There's nothing particularly magic about either variable.

edited Jul 24 '16 at 02:39

cjohnson318

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answered Dec 11 '10 at 18:31

Jason Coco

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For reference: [Objective-C methods](http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html). Note that any C function that takes an `id` and `SEL` as its first two arguments can be used as a method implementation. Similarly, any method implementation is a C function that takes an `id` and `SEL` as its first two arguments. – outis Dec 18 '10 at 20:08
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To be extra clear, `_cmd` is an `SEL` -- the method selector for the Objective-C method. "A method selector is a C string that has been registered (or 'mapped') with the Objective-C runtime." – Ben Flynn Apr 23 '13 at 20:29

score 0 · Answer 3 · answered Feb 10 '17 at 06:27

Please look here for further explanations.

func class_addMethod(_ cls: AnyClass!, 
                   _ name: Selector!, 
                   _ imp: IMP!, 
                   _ types: UnsafePointer<Int8>!) -> Bool

imp:

A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.

_cmd value inside C functions

3 Answers3