一、OC对象的本质

struct NSObject_IMPL {
    Class isa;   // 指针占8个字节
};

 NSObject *obj = [[NSObject alloc] init];
 NSLog(@"instanceSize---%ld",class_getInstanceSize([NSObject class]));
 // NSObject类的实例对象成员变量所占用的内存大小
 NSLog(@"mallocSize---%ld",malloc_size((__bridge const void *)(obj)));  
// obj指针所指向的内存大小

1、class_getInstanceSize调用栈

class_getInstanceSize调用栈01 class_getInstanceSize调用栈02

2、malloc_size调用栈

malloc_size调用栈1 malloc_size调用栈2 malloc_size调用栈3
malloc_size调用栈4

3、Student的内存大小

@interface Student : NSObject
{
    @public
    int _no;   //4
    int _age;  //4
    double _weight; //8
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
 
        Student *student = [[Student alloc] init];
        student->_age = 4;
        student->_weight = 5;
        student->_no = 8;
        NSLog(@"%ld",class_getInstanceSize([Student class]));
        NSLog(@"%ld",malloc_size((__bridge const void *)(student)));
        //由于有内存对齐的概念，分配内存大小为16的倍数，结构体的内存也有对齐的概念，是8的倍数
    }
    return 0;
} 

内存对齐