How to assign Objective-C properties using dot notation?

Question

I'm new to Objective-C. I'm currently testing properties with the following code. Note this is on windows using GNUstep:

#import <Foundation/Foundation.h>

@interface Car : NSObject
    @property NSString *color;
@end

@implementation Car
@end

int main (int argc, const char * argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    Car *honda = [[Car alloc] init];
    honda.color = @"Red";
    NSLog(@"%s", honda.color);   

    [pool drain]; 
    return 0;
}

But getting the following:

C:\Users\Bab\Desktop\main.m:5:2: warning: object property 'color' has no 'assign', 'retain' or 'copy' attribute; assuming 'assign' [enabled by default]
C:\Users\Bab\Desktop\main.m:5:2: note: 'assign' can be unsafe for Objective-C objects; please state explicitly if you need it
C:\Users\Bab\Desktop\main.m:9:1: warning: incomplete implementation of class 'Car' [enabled by default]
C:\Users\Bab\Desktop\main.m:9:1: warning: method definition for '-setColor:' not found [enabled by default]
C:\Users\Bab\Desktop\main.m:9:1: warning: method definition for '-color' not found [enabled by default]
: Uncaught exception NSInvalidArgumentException, reason: Car(instance) does not recognize setColor:
[Finished in 0.4s with exit code 1]

What _language_ is this? Okay, I know what language it is. But gosh, nowadays no one does manual memory management in Objective-C. What aren't you using ARC? — matt, Dec 08 '14 at 16:11
There's not much wrong with your code. You should say `@property (nonatomic, retain) NSString *color;` and `NSLog(@"%@", honda.color);` and your code will compile and run just fine. The fact that it doesn't suggests you aren't even using Xcode or something. — matt, Dec 08 '14 at 16:14
"I think it might be because I'm using GNUstep on windows." But that wouldn't be worth mentioning in your question. Okay, yes, people might see the `C:\`. But what you're doing is not exactly "normal". — matt, Dec 08 '14 at 16:15
@matt - that's not really fair. The OP tagged (and titled) the question with `objective-c`, not `XCode`, not `iOS`, `mobile` or anything of the like. — Adam Jenkins, Dec 08 '14 at 16:17
@Adam But not with `GNUStep` or `Windows`. Nor were those words mentioned in the original post until a lot of kerfuffle in comments. I think the "not really fair" belongs elsewhere. — matt, Dec 08 '14 at 16:19
@matt Actually, I edited the post after the first comment, but ok. — Babiker, Dec 08 '14 at 16:20

matt · Answer 1 · 2014-12-08T22:38:12.347

2

I don't know or care a hoot about GNUStep on Windows. But my guess is that if GNUStep on Windows lacks ARC it lacks autosynthesis of properties. So you would have to say @synthesize explicitly or maybe even write your own accessor methods.

Some further discussion here: http://wiki.gnustep.org/index.php/ObjC2_FAQ#Which_Bits_of_Objective-C_2_Work.3F

Basically it appears that large sections of the "modern runtime" may not be there.

edited Dec 08 '14 at 22:38

answered Dec 08 '14 at 16:17

matt

515,959
87
875
1,141

ARC isn't GNUstep's feature but Objective-C's. – Fred Frith-MacDonald Dec 10 '14 at 13:52

score 0 · Answer 2 · answered Dec 08 '14 at 16:22

If you use gcc to compile those codes under mac terminal, you might got these:

SinriMac:overstack Sinri$ gcc 03.m -framework Foundation
03.m:4:5: warning: no 'assign', 'retain', or 'copy' attribute is specified -
  'assign' is assumed [-Wobjc-property-no-attribute]
@property NSString *color;
^
03.m:4:5: warning: default property attribute 'assign' not appropriate for
  non-GC object [-Wobjc-property-no-attribute]
03.m:16:18: warning: format specifies type 'char *' but the argument has type
  'NSString *' [-Wformat]
NSLog(@"%s", honda.color);   
        ~~   ^~~~~~~~~~~
        %@
3 warnings generated.

SinriMac:overstack Sinri$ ./a.out
2014-12-09 00:16:55.804 a.out[86907:5686808] `踺u…
SinriMac:overstack Sinri$

So it should be the issue of GNUStep. Use @synthesize is the method to solve this.

PS Anyway, the %s is strange here, why not use %@?

score 0 · Answer 3 · edited Mar 20 '21 at 06:51

The problem is not the notation. The problem is here:

: Uncaught exception NSInvalidArgumentException, reason: Car(instance) does not recognize setColor:

The runtime is looking for the expected "setColor" accessor for the property "color". To generate accessors automatically, use @synthesize as matt said, or create them explicitly in the implementation.

Explanation of synthesize: What exactly does @synthesize do?

However, @synthesize is an objective-c 2.0 keyword which is not supported by the gcc in GNUStep. You need clang to enable this: How can i use Objective-C's Property feature in GNUstep?

I installed clang with this guide: https://solarianprogrammer.com/2012/03/21/clang-objective-c-windows/

FYI this is not an ARC issue - ARC is part of objective-c, not GNUStep as Fred Frith-MacDonald said. ARC can also be used under GNUStep via clang/llvm.

How to assign Objective-C properties using dot notation?

3 Answers3