* 获取一个NSObject实例对象的成员变量所占用的字节大小,可以用`runtime`的api, `class_getInstanceSize`来获取,得到`8`

    /** 

 * Returns the size of instances of a class.
 * 
 * @param cls A class object.
 * 
 * @return The size in bytes of instances of the class e cls, or c 0 if e cls is c Nil.
 */
OBJC_EXPORT size_t
class_getInstanceSize(Class _Nullable cls) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

* 也可以通过`malloc`库里的api,`malloc_size`来获取,得到`16`

    extern size_t malloc_size(const void *ptr);

/* Returns size of given ptr */

#import <Foundation/Foundation.h>

#import <objc/runtime.h>

#import <malloc/malloc.h>

// NSObject Implementation

struct NSObject_IMPL {

    Class isa; // 8个字节

};

// 指针

// typedef struct objc_class *Class;

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        NSObject *obj = [[NSObject alloc] init];

        // 16个字节

        // 获得NSObject实例对象的成员变量所占用的大小 >> 8

        NSLog(@"%zd", class_getInstanceSize([NSObject class]));

        // 获得obj指针所指向内存的大小 >> 16

        NSLog(@"%zd", malloc_size((__bridge const void *)obj));

        // 什么平台的代码

        // 不同平台支持的代码肯定是不一样

        // Windows、mac、iOS

        // 模拟器(i386)、32bit(armv7)、64bit（arm64）

        // 可以通过 命令行工具,生成C++文件

        // xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp

        // 然后把编译成功的cpp文件,拖入到工程中,注意 Copy items if needed 不勾选

        // 然后Build Phases中删除main-arm64.cpp编译选项(选中文件点减号或按delete键)

        // 这样文件不参与编译就不会报错了

    }

    return 0;

}

// NSObject Implementation

struct NSObject_IMPL {

    Class isa; // 8个字节

};

* 地址`https://opensource.apple.com/tarballs/objc4/`
* 选择版本号最新的下载查看查看源码发现,一旦发现

inline size_t instanceSize(size_t extraBytes) const {

        if (fastpath(cache.hasFastInstanceSize(extraBytes))) {

            return cache.fastInstanceSize(extraBytes);

        }

        size_t size = alignedInstanceSize() + extraBytes;

        // CF requires all objects be at least 16 bytes.

        if (size < 16) size = 16;

        return size;

    }

struct Student_IMPL {

    Class isa;

    int _no;

    int _age;

};

@interface Student : NSObject

{

    @public

    int _no;

    int _age;

}

@end

@implementation Student

@end

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        Student *stu = [[Student alloc] init];

        stu->_no = 4;

        stu->_age = 5;

        // 16

        NSLog(@"%zd", class_getInstanceSize([Student class]));

        // 16

        NSLog(@"%zd", malloc_size((__bridge const void *)stu));

        struct Student_IMPL *stuImpl = (__bridge struct Student_IMPL *)stu;

        // no is 4, age is 5

        NSLog(@"no is %d, age is %d", stuImpl->_no, stuImpl->_age);

    }

    return 0;

}

@interface Person: NSObject

{

    int _age;

}

@end

@implementation Person

@end

@interface Student : Person

{

    int _no;

}

@end

@implementation Student

@end

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        Person *person = [[Person alloc] init];

        // 16

        NSLog(@"person --- %zd", class_getInstanceSize([Student class]));

        // 16

        NSLog(@"person --- %zd", malloc_size((__bridge const void *)person));

        Student *stu = [[Student alloc] init];

        // 16

        NSLog(@"stu --- %zd", class_getInstanceSize([Student class]));

        // 16

        NSLog(@"stu --- %zd", malloc_size((__bridge const void *)stu));

    }

    return 0;

}