iOS - isa、class-rw-t、class-ro-t结构体

struct objc_object {
private:
    isa_t isa;
public:
    // ISA() assumes this is NOT a tagged pointer object
    Class ISA(bool authenticated = false);
    ···
}

union isa_t {
    isa_t() { }
    isa_t(uintptr_t value) : bits(value) { }
    uintptr_t bits;
private:
    // Accessing the class requires custom ptrauth operations, so
    // force clients to go through setClass/getClass by making this
    // private.
    Class cls;
public:
#if defined(ISA_BITFIELD)
    struct {
        ISA_BITFIELD;  // defined in isa.h
    };
};

# if __arm64__
// ARM64 simulators have a larger address space, so use the ARM64e
// scheme even when simulators build for ARM64-not-e.
#   if __has_feature(ptrauth_calls) || TARGET_OS_SIMULATOR
#     define ISA_MASK        0x007ffffffffffff8ULL
#     define ISA_MAGIC_MASK  0x0000000000000001ULL
#     define ISA_MAGIC_VALUE 0x0000000000000001ULL
#     define ISA_HAS_CXX_DTOR_BIT 0
#     define ISA_BITFIELD                                                      \
        uintptr_t nonpointer        : 1;                                       \
        uintptr_t has_assoc         : 1;                                       \
        uintptr_t weakly_referenced : 1;                                       \
        uintptr_t shiftcls_and_sig  : 52;                                      \
        uintptr_t has_sidetable_rc  : 1;                                       \
        uintptr_t extra_rc          : 8
#     define RC_ONE   (1ULL<<56)
#     define RC_HALF  (1ULL<<7)

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint16_t witness;
#if SUPPORT_INDEXED_ISA
    uint16_t index;
#endif
    // explicit_atomic 是为了安全操作
    explicit_atomic<uintptr_t> ro_or_rw_ext;
    Class firstSubclass;
    Class nextSiblingClass;
}

struct class_rw_ext_t {
    DECLARE_AUTHED_PTR_TEMPLATE(class_ro_t)
    class_ro_t_authed_ptr<const class_ro_t> ro;
    method_array_t methods;
    property_array_t properties;
    protocol_array_t protocols;
    char *demangledName;
    uint32_t version;
};

class method_array_t : 
public list_array_tt<method_t, method_list_t, method_list_t_authed_ptr>
class property_array_t : 
    public list_array_tt<property_t, property_list_t, RawPtr>
class protocol_array_t : 
    public list_array_tt<protocol_ref_t, protocol_list_t, RawPtr>

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;
#ifdef __LP64__
    uint32_t reserved;
#endif
    union {
        const uint8_t * ivarLayout;
        Class nonMetaclass;
    };
    explicit_atomic<const char *> name;
    // With ptrauth, this is signed if it points to a small list, but
    // may be unsigned if it points to a big list.
    void *baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;
    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;

using MethodListIMP = IMP;
struct method_t {
    static const uint32_t smallMethodListFlag = 0x80000000;
    method_t(const method_t &other) = delete;
    // The representation of a "big" method. This is the traditional
    // representation of three pointers storing the selector, types
    // and implementation.
    struct big {
        SEL name;  //方法、函数名
        const char *types;  //编码（参数类型、返回值类型）
        MethodListIMP imp;  //方法实现，指向方法、函数的指针
    };