iOS从源码看类:alloc

[NSObject alloc] 开源的实现：

+ (id)alloc {
    return _objc_rootAlloc(self);
}

id
_objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
    if (slowpath(checkNil && !cls)) return nil;
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }
#endif

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}

从objc开源代码运行
先是走

// Calls [cls alloc].
id
objc_alloc(Class cls)
{
    return callAlloc(cls, true/*checkNil*/, false/*allocWithZone*/);
}

然后

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
    if (slowpath(checkNil && !cls)) return nil;
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }
#endif

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}

_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}

static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)
{
    ASSERT(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;

    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        obj = (id)calloc(1, size);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (fastpath(!hasCxxCtor)) {
        return obj;
    }

    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
    return object_cxxConstructFromClass(obj, cls, construct_flags);
}

obj = (id)calloc(1, size)这里会开辟内存空间

其中 size的大小 size = cls->instanceSize(extraBytes);获取
8字节对齐 但是最少16字节（nsobject实例对象）

sizeof(nsobj) = 8字节
class_getInstanceSize(nsobj) 获取的是8字节对齐后的大小
malloc_size(nsobj) = 16字节 是按照16字节对齐
为啥16字节对齐？看malloc_size源码

segregated_size_to_fit(nanozone_t *nanozone, size_t size, size_t *pKey)
{
    size_t k, slot_bytes;
    if (0 == size) {
        size = NANO_REGIME_QUANTA_SIZE; // 1<<4 = 16
    }
    k = (size + NANO_REGIME_QUANTA_SIZE - 1) >> SHIFT_NANO_QUANTUM; //#define SHIFT_NANO_QUANTUM        4
    slot_bytes = k << SHIFT_NANO_QUANTUM;                           
    *pKey = k - 1;                                              
    return slot_bytes;
}

k = (size + 16 - 1) >> 4
slot_bytes = k << 4;