iOS isa

一般都说isa指针，那么要了解isa，就得先搞清楚何为指针 https://zhidao.baidu.com/question/1372805343211755539.html

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objc-private.h中，搜索isa，即可查询到如下信息

unionisa_t {
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#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__
# defineISA_MASK0x0000000ffffffff8ULL
# defineISA_MAGIC_MASK0x000003f000000001ULL
# defineISA_MAGIC_VALUE0x000001a000000001ULLstruct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 33; // MACH_VM_MAX_ADDRESS 0x1000000000uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 19;
# defineRC_ONE (1ULL<<45)
# defineRC_HALF (1ULL<<18)
};
# elif __x86_64__
# defineISA_MASK0x00007ffffffffff8ULL
# defineISA_MAGIC_MASK0x001f800000000001ULL
# defineISA_MAGIC_VALUE0x001d800000000001ULLstruct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 44; // MACH_VM_MAX_ADDRESS 0x7fffffe00000uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 8;
# defineRC_ONE (1ULL<<56)
# defineRC_HALF (1ULL<<7)
};
# else
# error unknown architecture for packed isa
# endif// SUPPORT_PACKED_ISA
#endif
#if SUPPORT_INDEXED_ISA
# if __ARM_ARCH_7K__ >= 2
# defineISA_INDEX_IS_NPI1
# defineISA_INDEX_MASK0x0001FFFC
# defineISA_INDEX_SHIFT2
# defineISA_INDEX_BITS15
# defineISA_INDEX_COUNT (1 << ISA_INDEX_BITS)
# defineISA_INDEX_MAGIC_MASK0x001E0001
# defineISA_INDEX_MAGIC_VALUE0x001C0001struct {
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;
# defineRC_ONE (1ULL<<25)
# defineRC_HALF (1ULL<<6)
};
# else
# error unknown architecture for indexed isa
# endif// SUPPORT_INDEXED_ISA
#endif
};

isa属于C语言共用体