Question

I’m working on one simple objective-c program which contain categories. My class .h:

#import <Foundation/Foundation.h>

@interface Fraction : NSObject

@property int numerator, denominator;

-(void)setNumerator:(int)n andDenominator:(int)d;

@end

In .m file I synthesized my numerator and denominator. In main.m created category of my Fraction class:

#import "Fraction.h"    
@interface Fraction (MathOps)
-(Fraction *) add: (Fraction *) f;
@end

@implementation Fraction (MathOps)
-(Fraction *) add: (Fraction *) f
{
    // To add two fractions:
    // a/b + c/d = ((a*d) + (b*c)) / (b * d)
    Fraction *result = [[Fraction alloc] init];
    result.numerator = (numerator * f.denominator) +
    (denominator * f.numerator);
    result.denominator = denominator * f.denominator;
    [result reduce];
    return result;
}
@end

But my program does not see numerator and denominator in category’s implementation section. Error “Use of undeclared identifier ‘numerator'(the same for denominator). What am I doing wrong?

Answer 1

Use the properties instead of using the ivars directly:

result.numerator = (self.numerator * f.denominator) + (self.denominator * f.numerator);
result.denominator = self.denominator * f.denominator;

The instance variables aren’t visible to your category because they’re not declared in Fraction’s interface — they’re only created when you @synthesize them in your implementation. That leads to the other possible solution, which is to declare the instance variables in Fraction’s interface:

@interface Fraction : NSObject {
    int numerator;
    int denominator;
}

@property int numerator, denominator;

-(void)setNumerator:(int)n andDenominator:(int)d;

@end