例項物件( instance)、類物件(class)、元類物件(meta-class)的內部結構分析

本文使用的objc原始碼版本為objc4-756.2。

Class本質上為一個結構體型別：

typedef struct objc_class *Class;

例項物件的定義

struct objc_object {
private:
    isa_t isa;    
    //以下省略是其他繼承新增成員變數
}

類物件/元類物件的定義

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             
 
// 方法快取
    class_data_bits_t bits;    // 類的具體資訊
    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;    //屬性資訊
    protocol_array_t protocols;    //協議列表
}

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;    //instance物件佔用的記憶體大小
#ifdef __LP64__
    uint32_t reserved;
#endif

    const uint8_t * ivarLayout;
    
    const char
 
 * name;    //類名
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;    //成員變數列表

    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;
}

Xcode 裡面的runtime裡面的 API是已經廢棄瞭如下

/// 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;
};

廢棄如下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;