isa initialization
isa structure
union isa_t {
isa_t () {}
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#if defined(ISA_BITFIELD)
struct {
ISA_BITFIELD; // defined in isa.h
};
#endif
};
Thus be seen isa a consortium, a data type, the 8 bytes occupied. It features: shared memory, or with exclusive features, meaning that the assignment of cls, not to other members of the assignment. Wherein ISA_BITFIELD is a macro definition, it is a bit field, which defines the following structure
#if SUPPORT_PACKED_ISA
// extra_rc must be the MSB-most field (so it matches carry/overflow flags)
// nonpointer must be the LSB (fixme or get rid of it)
// shiftcls must occupy the same bits that a real class pointer would
// bits + RC_ONE is equivalent to extra_rc + 1
// RC_HALF is the high bit of extra_rc (i.e. half of its range)
// future expansion:
// uintptr_t fast_rr : 1; // no r/r overrides
// uintptr_t lock : 2; // lock for atomic property, @synch
// uintptr_t extraBytes : 1; // allocated with extra bytes
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 33; /*MACH_VM_MAX_ADDRESS 0x1000000000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 19
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
# ELif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 44; /*MACH_VM_MAX_ADDRESS 0x7fffffe00000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 8
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
# else
# error unknown architecture for packed isa
# endif
// SUPPORT_PACKED_ISA
#endif
#if SUPPORT_INDEXED_ISA
# if __ARM_ARCH_7K__ >= 2 || (__arm64__ && !__LP64__)
// armv7k or arm64_32
# define ISA_INDEX_IS_NPI_BIT 0
# define ISA_INDEX_IS_NPI_MASK 0x00000001
# define ISA_INDEX_MASK 0x0001FFFC
# define ISA_INDEX_SHIFT 2
# define ISA_INDEX_BITS 15
# define ISA_INDEX_COUNT (1 << ISA_INDEX_BITS)
# define ISA_INDEX_MAGIC_MASK 0x001E0001
# define ISA_INDEX_MAGIC_VALUE 0x001C0001
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t indexcls: 15; \
uintptr_t magic : 4; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 7
# define RC_ONE (1ULL<<25)
# define RC_HALF (1ULL<<6)
# else
# error unknown architecture for indexed isa
# endif
// SUPPORT_INDEXED_ISA
#endif
isa arm64 arguments detailed:
nonpointer: indicates whether to turn on isa pointer to optimize 0: pure isa pointer, 1: not only is the class object address, isa contains the class information, reference counting objects
has_assoc: associated object flag 0 no 1 is present
has_cxx_dtor: whether the object has Objc or C ++ destructor, if destructor, destructor logic needed to be done, and if not, the object is released faster
shiftcls:
value storing pointer to a class. Open pointer optimization case, there are 33 classes to store pointers arm64 architecture.
magic: a debugger to determine the current object or objects is really no room for initialization weakly_referenced: Chi whether the object is a point to point or ARC had weak variables,
objects no weak references can be released faster. deallocating: flag whether the object is freeing memory
has_sidetable_rc: When the object reference technology is greater than 10, then the need to borrow the variable storage carry
extra_rc: when indicates reference count of the object actually reference count is decremented by 1, e.g., if the object the reference count is 10, then extra_rc 9. If the reference count is greater than 10, it is necessary to use the following has_sidetable_rc.
Isa point of analysis
- Associated with an object class
LGPerson * object = [LGPerson alloc] ;
Brief lldb command
information memory section 4 x / 4gx objc objc of printing. Extended: x / 6gx is to print information memory section 6.
p / tp / t binary print information; p / o octal print information; p / x hexadecimal print information; p / d print decimal information, where the print information memory section 4, a first object property must be ISA, because it is inherited from NSObject,
@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
Class isa OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}
Return to the topic, by object_getClass (object); find related information
Class object_getClass(id obj)
{
if (obj) return obj->getIsa();
else return Nil;
}
objc_object::getIsa()
{
if (!isTaggedPointer()) return ISA();
uintptr_t ptr = (uintptr_t)this;
if (isExtTaggedPointer()) {
uintptr_t lock =
(ptr >> _OBJC_TAG_EXT_SLOT_SHIFT) & _OBJC_TAG_EXT_SLOT_MASK;
return objc_tag_ext_classes[slot];
} else {
uintptr_t lock =
(ptr >> _OBJC_TAG_SLOT_SHIFT) & _OBJC_TAG_SLOT_MASK;
return objc_tag_classes[slot];
}
}
inline Class
objc_object::ISA()
{
assert(!isTaggedPointer());
#if SUPPORT_INDEXED_ISA
if (isa.nonpointer) {
uintptr_t slot = isa.indexcls;
return classForIndex((unsigned)slot);
}
return (Class)isa.bits;
#else
return (Class)(isa.bits & ISA_MASK);
#endif
}
Analysis of the source can be seen from the last class is returned (Class) (isa.bits & ISA_MASK) results, and then start the verification, as follows, which proves the point isa class:
Analytical object number memory exists
We know that you can create multiple objects, the class does, whether it can create more, now verify:
Class class1 = [LGPerson class];
Class class2 = [LGPerson alloc].class;
Class class3 = object_getClass([LGPerson alloc]);
Class class4 = [LGPerson alloc].class;
NSLog(@"\n%p-\n%p-\n%p-\n%p",class1,class2,class3,class4);
The output is
2020-02-1000: 27: 37.559203 + 0800001- objects isa [7942: 604270]
0x100002400-
0x100002400-
0x100002400-
0x100002400
Proof can only create a class, only one in memory inside
isa walk & inheritance
As can be seen from the object points to the class isa Through the above analysis, and then continues to walk analyzed according to FIG isa
虚线代表了isa的走位。实例对象->类->元类->根元类->根根元类(根元类本身)。
实线代表了继承关系。这里值得注意的就是根元类的父类是NSObject，NSObject父类是nil。
开始验证：
1.isa从对象指向类
XDPerson *person = [XDPerson alloc];
(lldb) x/4gx person
0x10185eac0: 0x001d8001000011a9 0x0000000000000000
0x10185ead0: 0x000000010185eba0 0x000000010185ede0
(lldb) p/x 0x001d8001000011a9 & 0x0000000ffffffff8
(long) $1 = 0x00000001000011a8
(lldb) po $1
XDPerson
2.isa从类指向元类
(lldb) x/4gx $1
0x1000011a8: 0x001d800100001181 0x00000001000011f8
0x1000011b8: 0x00000001003a1e50 0x0000000000000000
(lldb) p/x 0x001d800100001181 & 0x0000000ffffffff8
(long) $2 = 0x0000000100001180
(lldb) po $2
XDPerson
我们打印类XDPerson的内存信息，通过类的isa&ISA_MASK获取到了另外一个类的信息。即isa从类XDPerson又指向了类XDPerson（其实这个类是我们XDPerson的metaClass元类，它与第一步的XDPerson的内存地址并不同）。
3.isa从元类指向根元类
(lldb) x/4gx $2
0x100001180: 0x001d800100aff0f1 0x00000001000011d0
0x100001190: 0x0000000101e142b0 0x0000000100000007
(lldb) p/x 0x001d800100aff0f1 & 0x0000000ffffffff8
(long) $3 = 0x0000000100aff0f0
(lldb) po $3
NSObject
我们打印元类XDPerson的内存信息 通过元类的isa & ISA_MASK 获取到了另外一个元类类NSObject的信息。即isa从元类XDPerson又指向了元类NSObject。这里我们可以来打印NSObjcr.class来观察，元类NSObject的内存地址NSObjcr.class的内存地址并不同。
4.isa从根元类指向根根元类
(lldb) x/4gx $3
0x100aff0f0: 0x001d800100aff0f1 0x0000000100aff140
0x100aff100: 0x0000000101e146e0 0x0000000300000007
(lldb) p/x 0x001d800100aff0f1 & 0x0000000ffffffff8
(long) $4 = 0x0000000100aff0f0
(lldb) po $4
NSObject
这里补充一下几个概念
对象 是程序员根据类实例化出来的
类 是代码写出来的，内存中只有一份，是系统创建的
元类 是系统编译的，发现有了这么一个类 - 系统也同时创建了
方法 是编译阶段产生的