目標C錯誤實例化一個類的對象

Question

可能有人請告訴我什麼是錯與以下程序

#import <Foundation/Foundation.h> 

@interface Fraction:NSObject 

-(void)getNumerator: (int) numerator; 
-(void)getDenominator: (int) denominator; 
-(int) calculate; 

@end 


@implementation Fraction 
{ 
    int num; 
    int den; 
    int result; 
} 

-(void) getDenominator:(int)denominator{ 
    den=denominator; 

} 

-(void) getNumerator:(int)numerator{ 
    num=numerator; 
} 

-(void) calculate{ 
    result = num/den; 
    NSLog(@"%i",result); 
    //return result; 
} 

@end 

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

    @autoreleasepool { 

     Fraction *myFrac; 
     // insert code here... 
     myFrac = [Fraction alloc]; 
     myFrac = [Fraction init]; 
     [myFrac getNumerator:4]; 
     [myFrac getDenominator:6]; 
[myFrac calculate]; 
//  NSLog(@"the result is %i",result); 
     NSLog(@"Hello, World!"); 


    } 
    return 0; 
} 

我不斷收到此錯誤：

roject3[523:303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[Fraction<0x100001200> init]: cannot init a class object.' 
*** First throw call stack: 
(
    0 CoreFoundation      0x00007fff8a01641c __exceptionPreprocess + 172 
    1 libobjc.A.dylib      0x00007fff824d5e75 objc_exception_throw + 43 
    2 CoreFoundation      0x00007fff8a019650 +[NSObject(NSObject) dealloc] + 0 
    3 project3       0x0000000100000e0c main + 108 
    4 libdyld.dylib      0x00007fff826b35fd start + 1 
    5 ???         0x0000000000000001 0x0 + 1 
) 
libc++abi.dylib: terminating with uncaught exception of type NSException 

爲什麼我不能實例化一個類的對象

Answer 1

更改此：

myFrac = [Fraction alloc]; 
myFrac = [Fraction init]; 

要這樣：

myFrac = [[Fraction alloc] init]; 

如果你不想窩，我認爲你需要做這種方式：

myFrac = [Fraction alloc]; 
myFrac = [myFrac init]; 

但它是更好地做它嵌套。雖然，只要Fraction是自定義類，您應該創建自己的factory method。 ;）

此外，您的get方法應該被稱爲set方法實際上（或甚至更好，使用@properties）。

Answer 2

myFrac = [Fraction alloc]; 
myFrac = [Fraction init]; 

是沒有意義的，應該是

myFrac = [[Fraction alloc] init];