实例对象( instance)、类对象(class)、元类对象(meta-class)的内部结构分析

实例对象( instance)、类对象(class)、元类对象(meta-class)的内部结构分析

本文使用的objc源码版本为objc4-756.2。

Class本质上为一个结构体类型：

typedef struct objc_class *Class;

实例对象的定义

struct objc_object {
private:
    isa_t isa;    
    //以下省略是其他继承添加成员变量
}

类对象/元类对象的定义

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // 方法缓存
    class_data_bits_t bits;    // 类的具体信息
    class_rw_t *data() { 
        return bits.data();
}
    //以下省略
}

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint32_t version;

    const class_ro_t *ro;

    method_array_t methods;    //方法列表
    property_array_t properties;    //属性信息
    protocol_array_t protocols;    //协议列表
}

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;    //instance对象占用的内存大小
#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;
}

Xcode 里面的runtime里面的 API是已经废弃了如下

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

废弃如下  OBJC2_UNAVAILABLE; 

struct objc_class {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class _Nullable super_class                              OBJC2_UNAVAILABLE;
    const char * _Nonnull name                               OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list * _Nullable ivars                  OBJC2_UNAVAILABLE;
    struct objc_method_list * _Nullable * _Nullable methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache * _Nonnull cache                       OBJC2_UNAVAILABLE;
    struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;