【iOS】alloc、init、new

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+ (id)alloc {
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    return _objc_rootAlloc(self);
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}
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id _objc_rootAlloc(Class cls)
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{
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    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
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}
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static ALWAYS_INLINE id
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callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
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{
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    if (slowpath(checkNil && !cls)) return nil;
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#if __OBJC2__
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    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
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        if (fastpath(cls->canAllocFast())) {
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            // No ctors, raw isa, etc. Go straight to the metal.
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            bool dtor = cls->hasCxxDtor();
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            id obj = (id)calloc(1, cls->bits.fastInstanceSize());
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            if (slowpath(!obj)) return callBadAllocHandler(cls);
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            obj->initInstanceIsa(cls, dtor);
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            return obj;
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        }
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        else {
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            // Has ctor or raw isa or something. Use the slower path.
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            id obj = class_createInstance(cls, 0);
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            if (slowpath(!obj)) return callBadAllocHandler(cls);
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            return obj;
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        }
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    }
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#endif
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    // No shortcuts available.
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    if (allocWithZone) return [cls allocWithZone:nil];
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    return [cls alloc];
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}

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- (id)init {
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    return _objc_rootInit(self);
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}
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id _objc_rootInit(id obj)
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{
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    // In practice, it will be hard to rely on this function.
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    // Many classes do not properly chain -init calls.
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    return obj;
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}

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+ (id)new {
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    return [callAlloc(self, false/*checkNil*/) init];
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}