Read up on the difference between class and instance methods
http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/chapter_3_section_3.html#/
/apple_ref/doc/uid/TP30001163-CH12-TPXREF122
And the how memory allocation/initiation works
http://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/Tasks/AllocInitObjects.html
+alloc is a class method that is sent to the meta class instance
(Fraction)
Your -initWith:over: is an instance method that can only be sent to a
normal instance of your class. +alloc will return this instance, so
you will almost always be using a [[MyClass alloc]init....] pattern.
So, the Fraction class does not respond to the selector -
initWith:over:, but the object returned by [Fraction alloc] will.
Hope that makes sense! Those little +'s and -'s can be confusing.
Adam Leonard
On Mar 12, 2008, at 8:25 PM, K. Darcy Otto wrote:
I'm trying to create an initializer that allows me to combine
initialization and some setting of variables, but I get an error
that an "unrecognized selector" has been sent to the object. Here is
what I have so far:
Fraction.h:
#import <Cocoa/Cocoa.h>
@interface Fraction : NSObject {
NSInteger numerator;
NSInteger denominator;
}
@property(readwrite) NSInteger numerator;
@property(readwrite) NSInteger denominator;
-(id)initWith:(NSInteger)aNumerator over:(NSInteger)aDenominator;
@end
Fraction.m:
#import "Fraction.h"
@implementation Fraction
@synthesize numerator,denominator;
-(id)init
{
return [self initWith:1 over:1];
}
-(id)initWith:(NSInteger)aNumerator over:(NSInteger)aDenominator
{
if (self = [super init]){
self.numerator = aNumerator;
self.denominator = aDenominator;
}
return self;
}
@end
Now, I can do the following with no ill effects:
Fraction *frac = [[Fraction alloc] init];
frac.numerator = 2;
frac.denominator = 2;
But if I do this, I get the unrecognized selector error:
Fraction *frac1 = [Fraction initWith:2 over:2];
Can anyone explain the error or suggest a solution? Thanks.
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