定义了一个Vehical类和一个Bus类，Bus类中包含一个Vehical的对象_vehical。  为什么不能在Bus的SetVehicalWithWheels andSpeed方法中设置_vehical的wheels和speed?代码如下：  #import <Foundation/Foundation.h>

@interface Vehical : NSObject

{

    int _wheels;

    double _speed;

}

- (void)SetWheels:(int)a;

- (void)SetSpeed:(double)b;

- (int)Wheels;

- (int)Speed;

@end

//@interface Bus : Vehical

@interface Bus : NSObject

{

    int _seat;

    Vehical *_vehical;

}

- (void)SetSeat:(int)a;

- (int)Seat;

- (void)SetVehicalWithWheels:(int)c andSpeed:(double)d;

- (void)WheelsAndSpeed;

@end

int main ()

{

    Bus *b = [Bus new];

    [b SetVehicalWithWheels:12 andSpeed:50];

    [b WheelsAndSpeed];

   

    Vehical *v = [Vehical new];

    [v SetWheels:100];

    NSLog(@"%d",[v Wheels]);

   

    return 0;

}

@implementation Vehical

- (void)SetWheels:(int)a

{

    _wheels = a;

    int b = 0;

    b++;

    NSLog(@"b = %d",b);

}

- (void)SetSpeed:(double)b

{

    _speed = b;

}

- (int)Wheels

{

    return _wheels;

}

- (int)Speed

{

    return _speed;

}

@end

@implementation Bus

- (void)SetSeat:(int)a

{

    _seat = a;

}

- (int)Seat

{

    return _seat;

}

- (void)SetVehicalWithWheels:(int)c andSpeed:(double)d

{

    [_vehical SetWheels:c];

    [_vehical SetSpeed:d];

}

- (void)WheelsAndSpeed

{

    int a = [_vehical Wheels];

    int b = [_vehical Speed];

    NSLog(@"Wheels:%d,Speed:%d",a,b);

}

@end

:( 是：  （

输出结果是：
Wheels : 0,Speed : 0
b = 1
100

因为内存中没有 Vehical , Bus 类 也没有接收 Vehical 对象的方法
解决办法:
1. 在 Bus 加上  - (void) setVehical:(Vehical *)vehical;
2. 在main函数中需要 创建一个 Vehical对象 , 然后set 给 Bus的vehical 成员变量, 这样就能有值了.
Vehical * vehical = [[Vehical alloc] init];
    [b setVehical:vehical];
3. 写法太奇怪 :L

Vehical.h

1. #import <Foundation/Foundation.h>
   
2. 
   
3. @interface Vehical : NSObject
   
4. {
   
5.     int _wheels;
   
6.     double _speed;
   
7. }
   
8. - (void)SetWheels:(int)a;
   
9. - (void)SetSpeed:(double)b;
   
10. - (int)Wheels;
    
11. - (int)Speed;
    
12. @end

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Vehical.m

1. #import "Vehical.h"
   
2. 
   
3. @implementation Vehical
   
4. 
   
5. - (void)SetWheels:(int)a
   
6. {
   
7.     _wheels = a;
   
8.     int b = 0;
   
9.     b++;
   
10.     NSLog(@"b = %d",b);
    
11. }
    
12. - (void)SetSpeed:(double)b
    
13. {
    
14.     _speed = b;
    
15. }
    
16. - (int)Wheels
    
17. {
    
18.     return _wheels;
    
19. }
    
20. - (int)Speed
    
21. {
    
22.     return _speed;
    
23. }
    
24. 
    
25. @end

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Bus.h

1. #import <Foundation/Foundation.h>
   
2. @class Vehical;
   
3. @interface Bus : NSObject
   
4. {
   
5.     int _seat;
   
6.     Vehical *_vehical;
   
7. }
   
8. 
   
9. - (void)SetSeat:(int)a;
   
10. - (int)Seat;
    
11. - (void)SetVehicalWithWheels:(int)c andSpeed:(double)d;
    
12. - (void)WheelsAndSpeed;
    
13. 
    
14. //=== 修改 ====================================
    
15. - (void) setVehical:(Vehical *)vehical;
    
16. 
    
17. @end

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Bus.m

1. #import "Bus.h"
   
2. #import "Vehical.h"
   
3. @implementation Bus
   
4. 
   
5. - (void)SetSeat:(int)a
   
6. {
   
7.         _seat = a;
   
8. }
   
9. - (int)Seat
   
10. {
    
11.         return _seat;
    
12. }
    
13. - (void)SetVehicalWithWheels:(int)c andSpeed:(double)d
    
14. {
    
15.         [_vehical SetWheels:c];
    
16.         [_vehical SetSpeed:d];
    
17. }
    
18. - (void)WheelsAndSpeed
    
19. {
    
20.         int a = [_vehical Wheels];
    
21.         int b = [_vehical Speed];
    
22.         NSLog(@"Wheels:%d,Speed:%d", a, b);
    
23. }
    
24. 
    
25. //=== 修改 =====================
    
26. - (void) setVehical:(Vehical *)vehical {
    
27.     _vehical = vehical;
    
28. }
    
29. @end

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main.m

1. #import <Foundation/Foundation.h>
   
2. #import "Vehical.h"
   
3. #import "Bus.h"
   
4. 
   
5. int main()
   
6. {
   
7.         Bus * b = [Bus new];
   
8.     //=== 修改 =====================
   
9.     Vehical * vehical = [[Vehical alloc] init];
   
10.     [b setVehical:vehical];
    
11.     //===============================
    
12. 
    
13.         [b SetVehicalWithWheels:12 andSpeed:50];
    
14.         [b WheelsAndSpeed];
    
15. 
    
16.         Vehical * v = [Vehical new];
    
17.         [v SetWheels:100];
    
18.         NSLog(@"%d", [v Wheels]);
    
19. 
    
20.         return 0;
    
21. }

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谢谢！确实是内存中没有。  我在 - (void)SetVehicalWithWheels:(int)c andSpeed:(double)d 中添加了    Vehical *b = [Vehical new];
    _vehical = b;   内存有了，程序就通啦。btw你头像很漂亮了。