Question

我对以下代码有疑问。输出错误：frac1应打印出来＆＃34; 2/3＆＃34;，而是将其覆盖到＆＃34; 3/7＆＃34;。我似乎无法弄清楚为什么会被覆盖。

#import <Foundation/Foundation.h>

// ---- INTERFACE -----
@interface Fraction : NSObject
-(void) print;
-(void) setNumerator: (int) n;
-(void) setDenominator: (int) d;
@end


//------implementation section ------
@implementation Fraction
int numerator;
int denominator;


-(void) print{

    NSLog(@"%i/%i", numerator,denominator);
}


-(void) setNumerator:(int)n{

    numerator = n;
}

-(void) setDenominator:(int)d{

    denominator = d;
}

@end


// ---------- EXECUTION--------
int main (int argc, char * argv[])
{
@autoreleasepool {
    Fraction * frac1 = [[Fraction alloc]init];
    Fraction * frac2 = [[Fraction alloc]init];

    [frac1 setNumerator:2];
    [frac1 setDenominator:3];

    // set second fraction
   [frac2 setNumerator:3];
   [frac2 setDenominator:7];


    NSLog(@"this is the value of frac1:");
    [frac1 print];


    NSLog(@"this is the value of frac2:");
    [frac2 print];


}
    return 0;
}

Answer 1

问题是numerator和denominator被设置为全局变量，而不是实例变量。为了使实例变量将它们作为@implementation定义的一部分包含在{}中。

修正：

@implementation Fraction {
int numerator;
int denominator;
}

输出：

这是frac1的价值：
  2/3
  这是frac2的价值：
  3/7

Answer 2

使用此代码：

@interface Fraction : NSObject
@property (assign, nonatomic) int numerator;
@property (assign, nonatomic) int denominator;
- (void)print;
@end

@implementation Fraction

- (void)setNumerator:(int)numerator {
   if (_numerator != numerator) {
      _numerator = numerator;
   }
}

- (void)setDenominator:(int)denominator {
   if (_denominator != denominator) {
      _denominator = denominator;
   }
}

-(void) print{
   NSLog(@"%i/%i", _numerator, _denominator);
}

@end

// OUTPUT

2015-01-18 18:12:58.844 fraction[29486:2733137] this is the value of frac1:
2015-01-18 18:12:58.845 fraction[29486:2733137] 2/3
2015-01-18 18:12:58.846 fraction[29486:2733137] this is the value of frac2:
2015-01-18 18:12:58.846 fraction[29486:2733137] 3/7

在你的情况下，int numerator;分母;是全局变量，而不是对象属性。

设置一个实例的成员变量会覆盖另一个实例

2 个答案: