Question

Please help with understanding how #import works without using inheritance.

So I have created a Class named Person and another Class named Body (both classes are inherited from NSObject).

The Body class has 3 ivars (set with properties)

  int hands;
  int feet;
  int legs;

I then imported and created an instance of the Body class ( *body) into the Person class. My understanding is that I have now made a ‘type’ from the Body class. This ivar was then set with properties.

In main, I then created an instance from the Person class named person.
Main recognizes person.body.feet but I cannot get and set its value directly. The only way it can be done is passing its value into a new int variable and then print out the new variable.

Obviously I’m a struggling newbe but I really want to understand this problem –

  #import <Foundation/Foundation.h>
    #import "Person.h"

    int main (int argc, const char * argv[])
    {

        NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

        Person *person = [[Person alloc]init];

        // this works and NSLog prints out the value is 2
        int feet = person.body.feet = 2;
        NSLog(@"person.body = %i",feet);


        // this does not work - NSLog prints out the value is 0; 
        [person.body setFeet:2];
        NSLog(@"person.body = %i", person.body.feet );



        [person release];
        [pool drain];
        return 0;
    }

    #import <Foundation/Foundation.h>
#import "Body.h"

@interface Person : NSObject {

    Body *body;

}

   @property (assign) Body *body;

@end


#import "Person.h"

@implementation Person

  @synthesize  body;

@end


#import <Foundation/Foundation.h>

@interface Body : NSObject {

    int hands;
    int feet;
    int legs;

}

@property int hands;
@property int feet;
@property int legs;

@end



#import "Body.h"

@implementation Body

@synthesize hands;
@synthesize feet;
@synthesize legs;


@end

Answer 1

My guess is that you haven’t assigned a Body object to person.body anywhere.

All objects in Objective-C are allocated on the heap. All those asterisks denote that the variables are pointers, which are mere numbers pointing to memory locations where the objects live. When you declare a property that holds an object, all you are really doing is creating a place to store a pointer to the object. You aren’t creating the object itself.

So somewhere (most likely in Person‘s designated initialiser), you’ll want to actually create a Body object and assign it to the body property.

If you don’t do this, then the memory location will remain at 0, which is equivalent to nil. When you chain the assignment here:

int feet = person.body.feet = 2;

…you’re assigning 2 to both feet and person.body.feet, but since person.body is nil, the setter message to it is ignored. When later you try to log person.body.feet, again, it will be nil and return nil/0.

Edit: Having looked at your update, yes, this is your problem.

This line does not create a Body object:

Body *body;

It creates an instance variable for a pointer to a Body object.

This line does not create a Body object:

@property (assign) Body *body;

It declares a property, which is syntactic sugar for getter and setter method declarations.

This line does not create a Body object:

@synthesize  body;

It synthesises your previously declared property by creating the getter and setter methods.

You need to actually create and initialise a Body object and then assign it to your Person object’s body property. For instance:

- (id)init {
    if (self = [super init]) {
        self.body = [[Body alloc] init];
    }
    return self;
}

Also, as you aren’t using ARC, you should almost certainly declare that property to be retain not assign, and release it in your Person‘s dealloc method.