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iOS Objective-C Basics (Retired) Advanced Objective-C Dynamic Typing

Posted by J.R. Sedivy
J.R. Sedivy
2,545 Points

Dynamic Typing Exercise

I am attempting to follow along with the instructor with the implementation of the dynamic typing exercise but my compiler output doesn't match. The program compiles but varies from the example. The instructor mentioned that he created a Shape class (which creates the Shape.h file and Shape.m files) and I had done this as well but suspect that I should have code in this file which I don't have.

The example compiler output is:

23 tango I am Shape. My parent is NSObject. area -1.000000

My compiler output is:

23 tango <Shape: 0x10010bd70>

Here is the code: main.m:

#import <Cocoa/Cocoa.h>
#import "Shape.h"

int main()
{
    Shape *shape = [[Shape alloc] init];

    NSArray *mixed = @[@23, @"tango", shape];
    for (id thing in mixed) {
        NSLog(@"%@", thing);
    }

    return 0;
}

Shape.h:

#import <Foundation/Foundation.h>

@interface Shape : NSObject

@end

Shape.m: 

#import "Shape.h"

@implementation Shape

@end

3 Answers

Taylor Weeks
Taylor Weeks
4,009 Points
Taylor Weeks
Taylor Weeks
4,009 Points

Try entering this as your Shape.h file. All the header file does is declare the properties and methods that the class will use, and based on what is in his implementation file, these are the properties and methods that need to be declared.

#import <Foundation/Foundation.h>

@interface Shape : NSObject

@property(nonatomic,strong) NSString* color;

-(double)area;

-(BOOL)hitTestAtLocation:(CGPoint)location;

-(NSString *)description;

@end
J.R. Sedivy
J.R. Sedivy
2,545 Points

That did the trick. The compiler output is now:

2014-03-25 11:52:51.737 MyFirstObjectiveCProgram[5765:303] 23
2014-03-25 11:52:51.740 MyFirstObjectiveCProgram[5765:303] tango
2014-03-25 11:52:51.741 MyFirstObjectiveCProgram[5765:303] I am Shape. My parent is NSObject. area -1.000000.

Thanks for your help!

Taylor Weeks
Taylor Weeks
4,009 Points
Taylor Weeks
Taylor Weeks
4,009 Points

The reason your console output is different from the instructor's is because his output was the result of 

NSLog(@"%@", [thing description]);

where you have 

NSLog(@"%@", thing);
J.R. Sedivy
J.R. Sedivy
2,545 Points

I updated as you had suggested but unfortunately this didn't correct the issue. Here's the updated "main.m" file (the other files remained unchanged:

#import <Cocoa/Cocoa.h>
#import "Shape.h"

int main()
{
    Shape *shape = [[Shape alloc] init];

    NSArray *mixed = @[@23, @"tango", shape];
    for (id thing in mixed) {
        NSLog(@"%@", [thing description]);
    }

    return 0;
}

Here is the XCode output:

2014-03-24 16:17:05.108 MyFirstObjectiveCProgram[416:303] 23 2014-03-24 16:17:05.111 MyFirstObjectiveCProgram[416:303] tango 2014-03-24 16:17:05.112 MyFirstObjectiveCProgram[416:303] <Shape: 0x10010bd80>

Taylor Weeks
Taylor Weeks
4,009 Points
Taylor Weeks
Taylor Weeks
4,009 Points

Okay, I misunderstood the question. Go to 4:44 in the video and you will see that you are missing code in your shape.m file. Here, he has defined an instance method "description" that returns their values in a string.

J.R. Sedivy
J.R. Sedivy
2,545 Points

No problem, I appreciate you taking the time to talk through this. After continuing on with the Dynamic Typing video I also noticed the updated "Shape.m" file. I had copied it and updated the original Shape.m to the following:

#import "Shape.h"
@implementation Shape

@synthesize color = _color;

-(double)area {
    return -1;
}

-(BOOL)hitTestAtLocation:(CGPoint)location {
    return NO;
}

-(NSString *)description {
    return [NSString stringWithFormat:@"I am %@. My parent is %@. area %f.",
            [self class], [self superclass], [self area]];
}

@end

I ran this with the suggested edits from "main.m" in our previous discussion but I now get a compiler error for Shape.m specific to the "@synthesize line of code: "Property implementation must have its declaration in interface "Shape"." I suppose there may have been an update to "Shape.h" which wasn't shown in the video.