[移动开发]OC面试题-一个类占多少字节

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import <malloc/malloc.h>
@interface Person:NSObject
{
    @public
        int _age;
}
@end

@implementation Person
@end

@interface Student: Person
{
    @public
        int _no;
}

@end

@implementation Student
-(instancetype)init
{
    if(self = [super init])
    {
        NSLog(@"custom");
    }
    return self;
}
@end
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
//        NSLog(@"Hello, World!");
        Person *person = [[Person alloc] init];
        person->_age = 15;
        Student *stu = [[Student alloc] init];
        stu->_age = 16;
        stu->_no = 10001;
        NSLog(@"student sizeof%ld",sizeof(stu));
//       NSObject 实例对象的实际大小
        NSLog(@"student class_getInstanceSize%zd",class_getInstanceSize([Student class]));
//      NSObject 分配的内存大小
        NSLog(@"student malloc_size%zd",malloc_size((__bridge const void *)stu));
        NSLog(@"person sizeof%ld",sizeof(person));
        //       NSObject 实例对象的实际大小
        NSLog(@"person class_getInstanceSize%zd",class_getInstanceSize([Person class]));
        //      NSObject 分配的内存大小
        NSLog(@"person malloc_size%zd",malloc_size((__bridge const void *)person));
    }
    return 0;
}

这个面试题有三个知识点,1.所有类都只有8个字节,根本原因每个类都有一个指针,指向类信息的真正存储地址,这个指针就是isa指针 2.类的实际内存,这就根据实际的内容来了? 3.类的分配内存,这里涉及到内存对齐,就是类分配内存会是最大成员所内存的倍数,比如成员所占内存是24,但最大内存是16,那么会分配至少 32个字节