视频上老师讲解组合一带而过，感觉应该不难，可能设置变量的时候就蒙了
#import <Foundation/Foundation.h>

@interface Score : NSObject
{
    int _cScore;
}
- (void)setCScore:(int)cScore;
- (int)cScore;
@end

@implementation Score
- (void)setCScore:(int)cScore
{
    _cScore = cScore;
    NSLog(@"设置C的成绩%d",_cScore);
}

- (int)cScore
{
    NSLog(@"取得C的成绩%d",_cScore);
    return _cScore;
}

@end

@interface Student : NSObject
{
    Score *_score;  //组合
    int _age;
}
- (void)setScore:(int)cScore;
- (int)score;
@end

@implementation Student
- (void)setScore:(int)cScore
{
    [_score setCScore:cScore];
}
- (int)score
{
    int cs = [_score cScore];
    return cs;
}
@end


int main()
{
    Student *s = [Student new];
    Score *score = [Score new];
    [score setCScore:100];
    [s setScore:78];
    NSLog(@"C语言成绩是：%d",[s score]);
    return 0;
}

看到论坛里一同学发过这样的帖子，说原因是因为另一个类没有实例化，是在要main函数里用Score创建对象吗？

各位师兄们不要吝啬啊，为我指点迷津啊！！