面试题如下

@interface DXPerson : NSObject
@property (copy, nonatomic) NSString *name;

- (void)print;
@end
// ----------分割线----------
@implementation DXPerson

- (void)print{
    NSLog(@"my name is %@", self->_name);
}
@end

1.编译能否通过? 2.如果能通过,打印结果是什么?

- (void)viewDidLoad {
    [super viewDidLoad];

    id cls = [DXPerson class];
    void *obj = &cls;
    [(__bridge id)obj print];

打印结果

my name is <ViewController: 0x7f8ba1f1a300>

复习以前的知识

对象的isa指针指向哪里?

Objective-C对象的本质

@interface Student : NSObject{
    int _no;
    int _age;
}
struct Student_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    int _no;
    int _age;
}

struct NSObject_IMPL {
    Class isa;
}

因此归根结底:

struct Student_IMPL {
    Class isa;
    int _no;
    int _age;
}

demo

根据这张图,我们就明白了为什么可以通过编译,并且可以调用成功,本质上都是找到类对象,然后调用对象方法.

- (void)viewDidLoad {
    [super viewDidLoad];

    NSString *test = @"123";
    id cls = [DXPerson class];
    void *obj = &cls;
    [(__bridge id)obj print];
}

发现name打印竟然是123

my name is 123

为什么如此神奇呢??

局部变量分配在栈空间

栈空间分配，是连续的,从高地址到低地址

void test(){
    // 8个字节
    long long a = 4; // 0x7ffee638bff8
    long long b = 5; // 0x7ffee638bff0
    long long c = 6; // 0x7ffee638bfe8
    long long d = 7; // 0x7ffee638bfe0
    
    NSLog(@"%p %p %p %p", &a, &b, &c, &d);
}

demo

struct DXPerson_IMPL{
    Class isa;
    NSString *_name;
};

如图cls相当于isa,他们所在的栈空间地址是连续的,查找_name,是根据DXPerson的结构体查找,从第8个字节开始查找,于是就找到了test