OC对象的本质

我们平时编写的OC代码,底层实现都是C\C++代码
Xcode新建一个app工程.
终端指向main.m的上一级目录
终端打印
clang -rewrite-objc main.m -o main.cpp
clang 是苹果内置的编程语言的编译器前端
-rewrite-objc 重新Objc代码,写为C或者C++代码
-o输出 -o main.cpp输出main.cpp (cpp指C plus plus 即C++)

因为一份OC代码跑到iPhone上面和跑在mac上面编译成的C++和汇编语言是不一样的.因为硬件不一样.不同平台支持的代码肯定不一样.还有是针对于iOS有模拟器(架构i386),32bit(架构armv7), 64bit(架构arm64)
一般我们要指定平台
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp
这个是针对arm64的手机生成的代码
Xcode工具指定sdk为iphone 架构为arm64,输出文件为main-arm64.cpp
发现main-arm64.cpp比main.cpp小的多,因为指定了具体平台

1一个NSObject对象占用多少内存
系统分配了16个字节给NSObject对象(通过malloc_size函数获得),但是NSObject对象内部只适用了8个字节的空间(64bit环境下,可以通过class_GetInstanceSize函数获得)




一个16进制位代表4个二进制位,2个16进制位代表8个二进制位.
一个字节是8个二进制位

OC对象的分类
OC中的对象,简称OC对象,主要分为3种
instance对象(实例对象)

class对象(类对象)


meta-class对象(元类对象)

isa指针
消息机制

对象方法存储在类对象中,所以调用对象方法是在实例对象的isa指向类对象,让类对象调用对象方法.很方便的通过实例对象的isa找到类对象,也可以通过类对象的isa找到元类对象来完成方法的调用
类方法在元类对象中,类对象的isa指向它的元类对象

superclass

总结:

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//
//  MJClassInfo.h
//  TestClass
//
//  Created by MJ Lee on 2018/3/8.
//  Copyright © 2018年 MJ Lee. All rights reserved.
//

#import <Foundation/Foundation.h>

#ifndef MJClassInfo_h
#define MJClassInfo_h

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
# endif

#if __LP64__
typedef uint32_t mask_t;
#else
typedef uint16_t mask_t;
#endif
typedef uintptr_t cache_key_t;

struct bucket_t {
    cache_key_t _key;
    IMP _imp;
};

struct cache_t {
    bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
};

struct entsize_list_tt {
    uint32_t entsizeAndFlags;
    uint32_t count;
};

struct method_t {
    SEL name;
    const char *types;
    IMP imp;
};

struct method_list_t : entsize_list_tt {
    method_t first;
};

struct ivar_t {
    int32_t *offset;
    const char *name;
    const char *type;
    uint32_t alignment_raw;
    uint32_t size;
};

struct ivar_list_t : entsize_list_tt {
    ivar_t first;
};

struct property_t {
    const char *name;
    const char *attributes;
};

struct property_list_t : entsize_list_tt {
    property_t first;
};

struct chained_property_list {
    chained_property_list *next;
    uint32_t count;
    property_t list[0];
};

typedef uintptr_t protocol_ref_t;
struct protocol_list_t {
    uintptr_t count;
    protocol_ref_t list[0];
};

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;  // instance对象占用的内存空间
#ifdef __LP64__
    uint32_t reserved;
#endif
    const uint8_t * ivarLayout;
    const char * name;  // 类名
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;  // 成员变量列表
    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;
};

struct class_rw_t {
    uint32_t flags;
    uint32_t version;
    const class_ro_t *ro;
    method_list_t * methods;    // 方法列表
    property_list_t *properties;    // 属性列表
    const protocol_list_t * protocols;  // 协议列表
    Class firstSubclass;
    Class nextSiblingClass;
    char *demangledName;
};

#define FAST_DATA_MASK          0x00007ffffffffff8UL
struct class_data_bits_t {
    uintptr_t bits;
public:
    class_rw_t* data() {
        return (class_rw_t *)(bits & FAST_DATA_MASK);
    }
};

/* OC对象 */
struct mj_objc_object {
    void *isa;
};

/* 类对象 */
struct mj_objc_class : mj_objc_object {
    Class superclass;
    cache_t cache;
    class_data_bits_t bits;
public:
    class_rw_t* data() {
        return bits.data();
    }

    mj_objc_class* metaClass() {
        return (mj_objc_class *)((long long)isa & ISA_MASK);
    }
};

#endif /* MJClassInfo_h */

//
//  main.m
//  Interview04-class的结构
//
//  Created by MJ Lee on 2018/4/15.
//  Copyright © 2018年 MJ Lee. All rights reserved.
//
// objective-c++
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "MJClassInfo.h"

// MJPerson
@interface MJPerson : NSObject <NSCopying>
{
@public
    int _age;
}
@property (nonatomic, assign) int no;
- (void)personInstanceMethod;
+ (void)personClassMethod;
@end

@implementation MJPerson

- (void)test
{

}

- (void)personInstanceMethod
{

}
+ (void)personClassMethod
{

}
- (id)copyWithZone:(NSZone *)zone
{
    return nil;
}
@end

// MJStudent
@interface MJStudent : MJPerson <NSCoding>
{
@public
    int _weight;
}
@property (nonatomic, assign) int height;
- (void)studentInstanceMethod;
+ (void)studentClassMethod;
@end

@implementation MJStudent
- (void)test
{

}
- (void)studentInstanceMethod
{

}
+ (void)studentClassMethod
{

}
- (id)initWithCoder:(NSCoder *)aDecoder
{
    return nil;
}

- (void)encodeWithCoder:(NSCoder *)aCoder
{

}
@end


int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MJStudent *stu = [[MJStudent alloc] init];
        stu->_weight = 10;

        mj_objc_class *studentClass = (__bridge mj_objc_class *)([MJStudent class]);
        mj_objc_class *personClass = (__bridge mj_objc_class *)([MJPerson class]);

        class_rw_t *studentClassData = studentClass->data();
        class_rw_t *personClassData = personClass->data();

        class_rw_t *studentMetaClassData = studentClass->metaClass()->data();
        class_rw_t *personMetaClassData = personClass->metaClass()->data();

        NSLog(@"1111");
    }
    return 0;
}