1.对象 类对象，元类对象，根对象，根元类对象关系

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直接上代码

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2.构建对象内部分析

(1)先介绍几个结构体

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TestObject * obj = [[TestObject alloc]init];

+ (id)alloc {

    return _objc_rootAlloc(self);

}

id _objc_rootAlloc(Classcls)

{

    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);

}

static ALWAYS_INLINE id

callAlloc(Classcls,boolcheckNil,boolallocWithZone=false)

{

    if(slowpath(checkNil && !cls))returnnil;

#if __OBJC2__

    if(fastpath(!cls->ISA()->hasCustomAWZ())) {

        // No alloc/allocWithZone implementation. Go straight to the allocator.

        // fixme store hasCustomAWZ in the non-meta class and 

        // add it to canAllocFast's summary

        if(fastpath(cls->canAllocFast())) {

            // No ctors, raw isa, etc. Go straight to the metal.

            booldtor = cls->hasCxxDtor();

            idobj = (id)calloc(1, cls->bits.fastInstanceSize());

            if(slowpath(!obj))returncallBadAllocHandler(cls);

            obj->initInstanceIsa(cls, dtor);

            returnobj;

        }

        else{

            // Has ctor or raw isa or something. Use the slower path.

//它会走这个方法

            idobj =class_createInstance(cls,0);

            if(slowpath(!obj))returncallBadAllocHandler(cls);

            returnobj;

        }

    }

#endif

    // No shortcuts available.

    if(allocWithZone)return[clsallocWithZone:nil];

    return[clsalloc];

}

id 

class_createInstance(Classcls,size_textraBytes)

{

    return _class_createInstanceFromZone(cls, extraBytes, nil);

}

static __attribute__((always_inline)) 

id

_class_createInstanceFromZone(Classcls,size_textraBytes,void*zone, 

                              boolcxxConstruct =true, 

                              size_t*outAllocatedSize =nil)

{

    if(!cls)returnnil;

    assert(cls->isRealized());

    // Read class's info bits all at once for performance

    boolhasCxxCtor = cls->hasCxxCtor();

    boolhasCxxDtor = cls->hasCxxDtor();

    boolfast = cls->canAllocNonpointer();

    size_tsize = cls->instanceSize(extraBytes);

    if(outAllocatedSize) *outAllocatedSize = size;

    idobj;

    if(!zone  &&  fast) {

        obj = (id)calloc(1, size);

        if(!obj)returnnil;

        obj->initInstanceIsa(cls, hasCxxDtor);

    } 

    else{

        if(zone) {

            obj = (id)malloc_zone_calloc((malloc_zone_t*)zone,1, size);

        }else{

            obj = (id)calloc(1, size);

        }

        if(!obj)returnnil;

        // Use raw pointer isa on the assumption that they might be 

        // doing something weird with the zone or RR.

        obj->initIsa(cls);

    }

    if(cxxConstruct && hasCxxCtor) {

        obj =_objc_constructOrFree(obj, cls);

    }

    returnobj;

}

//获取size

size_tinstanceSize(size_textraBytes) {

        size_tsize =alignedInstanceSize() + extraBytes;

        // CF框架需要所有对象大小最少是16字节

        // CF requires all objects be at least 16 bytes.

        if(size <16) size =16;

        returnsize;

    }

uint32_talignedInstanceSize() {

        return word_align(unalignedInstanceSize());

    }

 uint32_tunalignedInstanceSize() {

        assert(isRealized());

        return data()->ro->instanceSize;

    }