The bottom of iOS explores the nature of classes

ISA walk a

Define a class ABPerson that inherits NSObject to see the reference to ISA through the LLDB command.

The ISA of the instance object points to the class object

• p/x pPrint instance objectpThe address of the
• x/4gx 0x0000000100538ae0Print the memory of this address, output 4 segments, the first segment is storedISAThe relevant information
• p/x 0x011d800100008185 & 0x00007ffffffffff8ULLBy aboutISA_MASKX00007ffffffffff8ull (0)&Operation to get the class objectABPerson
• po 0x0000000100008180Print class object

Conclusion: The ISA of the instance object points to the class object

The ISA of a class object points to its metaclass

According to the figure above,0x0000000100008180Is the address of the class objectISATo:You can see that you’ve got an address0x0000000100008158So is printingABPersonThis is the metaclass, which is generated by the system. Only one class object can be proved as follows:

Class objectABPersonThe addresses are:0x100008190.0x0000000100008158Is a metaclass address.

Conclusion: ISA of a class object points to its metaclass

The ISA of the metaclass points to the root metaclass

According to the figure above,0x0000000100008158Is the metaclass address of the classISATo:That can see metaclassesISAIs pointing toNSObject, which is the root metaclass. Conclusion: metaclassISAIt points to the root metaclass

The ISA of the root metaclass points to itself

According to the figure above,0x00007fff88967fe0Is a metaclassNSObjectContinue to look at the root metaclassISATo:The addresses in the red box are all the same, they’re all themselves. Conclusion: The ISA of the root metaclass points to itself

ISA bitmap

Inheritance chain

Define a subclassABTeacherinheritanceABPerson

• NSObjectThe root metaclass, root metaclass, root metaclass are all0x7fff88967fe0Is a thing.
• ABPersonisABTeacherIs the parent class,ABTeacherThe superclass address of the metaclass is0x100008210.ABPersonAs is the metaclass address of0x100008210, soABTeacherThe parent of a metaclass isABPersonThe metaclass.
• The root classNSObjectThe parent classnil
• The parent address of the root metaclass is0x7fff88968008isNSObject

Finally, attached is the official picture of Apple

The structure of the class

Objc source code for Class definition:

typedef struct objc_class *Class;

Objc_class struct definition:

struct objc_class : objc_object {
  objc_class(const objc_class&) = delete;
  objc_class(objc_class&&) = delete;
  void operator=(const objc_class&) = delete;
  void operator=(objc_class&&) = delete;
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flagsOmit some codeCopy the code

The structure layout of the class looks like this:

• ISAandsuperclassBoth take up the size of a structure pointer8byte
• cacheAccount for16Bytes,cache_tStructure:

struct cache_t {
private:
    explicit_atomic<uintptr_t> _bucketsAndMaybeMask; / / 8
    union {
    // Combinations are mutually exclusive, so they account for 8
        struct {
            explicit_atomic<mask_t>    _maybeMask;  //uint32_t 4
#if __LP64__
            uint16_t                   _flags;  //uint16_t 2
#endif
            uint16_t                   _occupied; //uint16_t 2
        };
        explicit_atomic<preopt_cache_t *> _originalPreoptCache; / / 8
    };
Copy the code

_bucketsAndMaybeMask is 8, the current Union is 8, so cache_t is 16

To access bits, memory shift 8+8+16=32 bytes.

Gets the attributes of the class

LLDB debugging:

Debug project download

@interface LGPerson : NSObject
// isa
@property (nonatomic.copy) NSString *name;
@property (nonatomic) int age;

- (void)saySomething;

@end
Copy the code

• x/4gx LGPerson.classGets the first address of the class
• p/x 0x1000083a0 + 0x20, the first address is offset32One byte, get itbits
• p (class_data_bits_t *)0x00000001000083c0To forcibly convert the address toclass_data_bits_ttype
• p $2->data(), the callclass_data_bits_tIn thedata()function

structPublic: class_rw_t* data()const {
        return(class_rw_t *)(bits & FAST_DATA_MASK); } omit some code}Copy the code

• p $3->properties()Gets the attributes of the class

 const property_array_t properties() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>(&ro_or_rw_ext)->properties;
        } else {
            return property_array_t{v.get<constclass_ro_t *>(&ro_or_rw_ext)->baseProperties}; }}Copy the code

class property_array_t : 
    public list_array_tt<property_t, property_list_t, RawPtr>
{
    typedef list_array_tt<property_t, property_list_t, RawPtr> Super;

 public:
    property_array_t() : Super() { }
    property_array_t(property_list_t *l) : Super(l) { }
};
Copy the code

class list_array_tt {
    struct array_t {
        uint32_t count;
        Ptr<List> lists[0];

        static size_t byteSize(uint32_t count) {
            return sizeof(array_t) + count*sizeof(lists[0]);
        }
        size_t byteSize() {
            returnbyteSize(count); }}; Omit some code}Copy the code

Property_array_t inherits from list_array_tt. List_array_tt has a structure array_t, which contains variables lists

• p $4.listgetproperty_list_tThe first address
• p $5.ptrprintproperty_list_tThe first address
• p *$6Get address getproperty_list_t
• p $7.get(0)readproperty_list_tMember variables in

Gets the instance method of the class

• The same steps are omitted here
• p $3->methods()Method to get the class

 const method_array_t methods() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>(&ro_or_rw_ext)->methods;
        } else {
            return method_array_t{v.get<constclass_ro_t *>(&ro_or_rw_ext)->baseMethods()}; }}Copy the code

class method_array_t : 
    public list_array_tt<method_t, method_list_t, method_list_t_authed_ptr>
{
    typedef list_array_tt<method_t, method_list_t, method_list_t_authed_ptr> Super;

 public:
    method_array_t() : Super() { }
    method_array_t(method_list_t *l) : Super(l) { }

    const method_list_t_authed_ptr<method_list_t> *beginCategoryMethodLists() const {
        return beginLists();
    }
    
    const method_list_t_authed_ptr<method_list_t> *endCategoryMethodLists(Class cls) const;
};
Copy the code

class list_array_tt {
    struct array_t {
        uint32_t count;
        Ptr<List> lists[0];

        static size_t byteSize(uint32_t count) {
            return sizeof(array_t) + count*sizeof(lists[0]);
        }
        size_t byteSize() {
            returnbyteSize(count); }}; Omit some code}Copy the code

Method_array_t inherits from list_array_tt, which has a structure array_t and variable lists in it

• p $4.listgetmethod_list_tThe first address
• p $5.ptrprintmethod_list_tThe first address
• p *$6Get address getmethod_list_t
• p $7.get(0).big()readmethod_list_tInstance methods in

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

Copy the code

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;
        const char*types; MethodListIMP imp; }; Omit some code}Copy the code

$7.get(0) gets the property from the property_t structure. Method_t also has a big structure inside big, so it calls big().

Gets a member variable of the class

Modify the LGPerson code to add a member variable and a class method

@interface LGPerson : NSObject
{
    // Member variables
    NSString *subject;
}
@property (nonatomic.copy) NSString *name;
@property (nonatomic) int age;

- (void)saySomething;
/ / class methods
+ (void)doSomething;
@end
Copy the code

@implementation LGPerson
- (void)saySomething{
    NSLog(@"%s",__func__);
}
+ (void)doSomething{
    NSLog(@"%s",__func__);
}
@end

Copy the code

• The same steps are omitted here

– $3->ro() to get the class_ro_t address

 const class_ro_t *ro() const {
        auto v = get_ro_or_rwe();
        if (slowpath(v.is<class_rw_ext_t *>())) {
            return v.get<class_rw_ext_t *>(&ro_or_rw_ext)->ro;
        }
        return v.get<const class_ro_t *>(&ro_or_rw_ext);
    }
Copy the code

• p *$4Get address, getclass_ro_t
• p $5.ivarsTo obtainclass_ro_tIs a list of member variablesivar_list_tThe first address

structClass_ro_t {omit some codeconstivar_list_t * ivars; Omit some code}Copy the code

• p *$6Get the address to get the list of member variablesivar_list_t
• p $7.get(0)Print member variables

Get class methods

• x/4gx LGPerson.classTo printLGPersonMemory, get the first 8 bytes0x00000001000083c0
• p/x 0x00000001000083c0 & 0x00007ffffffffff8ULLTo get the metaclass address
• The following steps are the same as obtaining the instance method (omitted)

conclusion

• ISABit: of the instance objectISAReferring to a class objectISAReferring to its metaclass, metaclassISAPointing to the root metaclass, root metaclassISAPoint to itself
• Inheritance chain:
• • A subclass inherits from a parent class, which inherits from a root class, which inherits from a root classnil
• • The child metaclass inherits from the parent metaclass, which inherits from the root metaclass, which inherits from the root metaclass
• Class structure:ISA,superclass,cache,bits
• Get class attributes: Get the first address of the class -> offset32Who getclass_data_bits_t->data()->properties()->property_list_t->get()
• Method for getting an instance of a class: Get the first address of the class -> offsetThe 32 bits get class_datA_bits_t->data()->methods()->method_list_t->get().big()
• Get a class member variable: Get the first address of the class -> offset32Who getclass_data_bits_t->data()->ro()->ivar_list_t->get()
• Get class method: Get metaclass address -> offsetThe 32 bits get class_datA_bits_t->data()->methods()->method_list_t->get().big()