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Hey all, I'm new to programming and going through an objective-c book to learn the language and programing fundamentals. I've looked through the code repeatedly, went back to the book's example, and attempted to understand the gcc comple errors. Here's my code:

#import <stdio.h>
#import <objc/Object.h>

@interface Point: Object
    {
        int xaxis;
        int yaxis;
    }

    -(void) print;
    -(void) setx:   (int)x;
    -(void) sety:   (int)y;

@end

@implementation Point;

    -(void) print
        {
            printf("(%i,%i)", xaxis, yaxis);
        }

    -(void) setx:   (int) x
        {
            xaxis = x;
        }

    -(void) sety:   (int) y
        {
            yaxis = y;
        }
@end

int main (int argc, char *argv[])
    {
        Point *myPoint;

        myPoint = [Point alloc];

        myPoint = [myPoint init];

        [myPoint setx: 4];
        [myPoint sety: 5];

        printf("The coordinates are: ");
            [myPoint print];
        printf("\n");

        [myPoint free];

        return 0;

    }

Then the compile errors from gcc look like this:

urban:Desktop alex$ gcc point.m -o point -l objc
point.m: In function ‘main’:
point.m:38: warning: ‘Point’ may not respond to ‘+alloc’
point.m:38: warning: (Messages without a matching method signature
point.m:38: warning: will be assumed to return ‘id’ and accept
point.m:38: warning: ‘...’ as arguments.)
point.m:40: error: ‘mypoint’ undeclared (first use in this function)
point.m:40: error: (Each undeclared identifier is reported only once
point.m:40: error: for each function it appears in.)
point.m:49: warning: ‘Point’ may not respond to ‘-free’

Where am I going wrong?

btw I'm going through "Programming in Objective-C" by Stephen Kochan if you wanted to know.

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3
  • In which platform are you compiling your program? Commented Apr 13, 2011 at 6:58
  • I'm using Mac OS X. 10.6 Commented Apr 13, 2011 at 7:38
  • 1
    In that case, it’s best to use NSObject instead of Object since the stock compilers on Mac OS X use Objective-C 2.0, which won’t build Object as a fully functional root class. Commented Apr 13, 2011 at 7:42

3 Answers 3

Reset to default
2

First the base class should be NSObject, not Object

the normal way to do the initialization is to write the alloc and init in the same statement. You would typically have an -(id)init; method in your class:

-(id)init
{
  if ( ( self = [super init] ) )
  {
    ; // additional initialization goes here
  }
  return self;
}

and

int main (int argc, char *argv[])
    {
        Point *myPoint = [[Point alloc] init];

Properties are better used, then you get the setter and getter automatically generated for you

instead of

@interface Point: Object
    {
        int xaxis;
        int yaxis;
    }

write

@interface Point : NSObject
{
}

@property int xaxis;
@property int yaxis;

then when you assign you can either write

[myPoint setXaxis:4]

or 

myPoint.xaxis = 4;

when you release the object write release, not free

[myPoint release];

hth

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1 Comment

Depending on the runtime, Object is a valid root class.
0

You have warnings and an error. The warnings seem to suggest that Object, which you are subclassing, doesn't implement alloc, init or free. Normally, on an Apple platform, you'd subclass NSObject, which does implement these, but without knowing which platform you're on, it's not possible to advise the correct option.

Secondly, you had a typo, but that now seems to be corrected. This

point.m:40: error: ‘mypoint’ undeclared (first use in this function)

suggests that you had mypoint in your code, rather than myPoint.

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3 Comments

Thanks for the response. I corrected the typo, initialized and allocated the object in the same statement (Point *myPoint = [[Point alloc] init];) changed the subclass to NSObject and switched <objc/Object.h> to <Foundation/Foundation.h>. And yes, I'm on a Mac. When I compile I get point.m:6: error: ‘Point’ redeclared as different kind of symbol /System/Library/Frameworks/CoreServices.framework/Headers/../Frameworks/CarbonCore.framework/Headers/MacTypes.h:501: error: previous declaration of ‘Point’ point.m:9: error: redefinition of ‘struct Point’ and when I attempt to run the program:
urban:Desktop alex$ ./point objc[60472]: Point: Does not recognize selector forward:: Illegal instruction urban:Desktop alex$
@imgeekboy The problem with using Apple’s frameworks is that Point is already declared as a struct in Carbon. Rename your Point class to something else, e.g. MyPoint, and your code should compile and run just fine.
0

You forgot to include the header Foundation.h:

#import <Foundation/Foundation.h>
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