父类有一个静态变量子类中改变

父类中有个静态变量，子类会自动继承：
注意：通过对象对象方法和通过类方法修改static静态变量的结果是不一样的。
通过类(+方法)方法对static变量进行修改：父类和子类都会对彼此的静态变量进行修改。
通过对象(-方法)方法对static变量进行修改：只能修改本类中对象方法中获取的静态变量。
在对象(-方法)方法中修改了静态变量的值，通过类方法获取静态变量的值，得到的始终是父类中静态变量的值。

//父类Person
static NSString *_st = @"11";

@interface Person : NSObject

@property (nonatomic, copy) NSString *name;
@property (nonatomic, strong) NSHashTable *family;

- (void)test;

+ (void)setSt:(NSString *)st;
+ (NSString *)st;

@end

- (void)test {
    NSLog(@"Person1:%@",_st);
    _st = @"Person";
    NSLog(@"Person2:%@",_st);
}


+ (void)setSt:(NSString *)st {
    _st = [st copy];
}

+ (NSString *)st {
    return _st;
}

//子类
@interface Son : Person

@end

@implementation Son

- (void)test {
    NSLog(@"Son1:%@",_st);
    _st = @"Son";
    NSLog(@"Son2:%@",_st);
}

@end

int main()
{
        Person *peron = [[Person alloc] init];
        Son *son = [[Son alloc] init];
        [peron test];
        [son test];
        [Person setSt:@"person class"];
        NSLog(@"person class===");
        [peron test];
        [son test];
        
        NSLog(@"Person class ===");
        [Son setSt:@"Son class"];
        [peron test];
        [son test];
        NSLog(@"get class ====");
        NSLog(@"Person class:%@",[Person st]);
        NSLog(@"Son class:%@",[Son st]);
}

//打印结果
2018-05-10 14:08:36.228924+0800 TestOC[2183:485436] Person1:11
2018-05-10 14:08:36.228946+0800 TestOC[2183:485436] Person2:Person
2018-05-10 14:08:36.228956+0800 TestOC[2183:485436] Son1:11
2018-05-10 14:08:36.228964+0800 TestOC[2183:485436] Son2:Son
2018-05-10 14:08:36.228977+0800 TestOC[2183:485436] person class==========
2018-05-10 14:08:36.228985+0800 TestOC[2183:485436] Person1:person class
2018-05-10 14:08:36.228992+0800 TestOC[2183:485436] Person2:Person
2018-05-10 14:08:36.229000+0800 TestOC[2183:485436] Son1:Son
2018-05-10 14:08:36.229007+0800 TestOC[2183:485436] Son2:Son
2018-05-10 14:08:36.229014+0800 TestOC[2183:485436] Person class ======
2018-05-10 14:08:36.229022+0800 TestOC[2183:485436] Person1:Son class
2018-05-10 14:08:36.229029+0800 TestOC[2183:485436] Person2:Person
2018-05-10 14:08:36.229037+0800 TestOC[2183:485436] Son1:Son
2018-05-10 14:08:36.229044+0800 TestOC[2183:485436] Son2:Son
2018-05-10 14:08:36.229052+0800 TestOC[2183:485436] get class ====
2018-05-10 14:08:36.229061+0800 TestOC[2183:485436] Person class:Person
2018-05-10 14:08:36.229206+0800 TestOC[2183:485436] Son class:Person