CODEWITHSUNDEEP
objective-c
Mr_Vlasov
Mr_Vlasov

Reputation: 537

Did author make a logical mistake in his explanation?

I'm learning objective-c from "Programming in objective-c" author Kochan. 3rd edition. In Chapter 8 "Inheritance" Mr. Kochan gives following explanation to the method: 

-(void) setOrigin: (XYPoint *) pt
{
if (! origin)
origin = [[XYPoint alloc] init];
origin.x = pt.x;
origin.y = pt.y;
}

"The method first tests to see if the instance variable origin is nonzero (make sure you understand that test and the use of the logical negation operator ! that’s used). Recall that all instance variables are initially set to zero. So when a new Rectangle object is allocated, its instance variables, which includes origin, will be set to zero.

If the origin is zero, the setOrigin: method will allocate and initialize a new XYPoint object and store the reference to it in the origin."

Is there a logical mistake? Doesn't the "setOrigin" method allocates a new XYPoint object only if the origin is non zero?

Upvotes: 0

Views: 128

Answers (3)

Mawg
Mawg

Reputation: 40159

The author is, indeed, correct.

This is why I don't like incomplete comparisons in C.

If he codes if (origin) then that boolean expression is true for any value of origin which is non-zero.

So, when he negates it, the boolean expression if (! origin) is true only when origin is zero.

IMO, this is horrendous and should be coded if (origin == NULL) which makes it much clearer.

It seems to me that he is going out of his way to teach bad coding habits. This sort of construct always gives off a Code Smell.

Upvotes: 0

Dithermaster
Dithermaster

Reputation: 6343

No. The "!" is a logical NOT operation in this case. When "origin" is zero, !origin is TRUE; when "origin" is non-zero, !origin is FALSE. Therefore, the next line (which allocates and initializes an XYPoint object) is only executed when origin is zero. Just as the author states.

Upvotes: 1

Greg Hewgill
Greg Hewgill

Reputation: 993333

Your quote says:

If the origin is zero, ...

That is, when origin is zero (nil), then ! origin will be true, and a new XYPoint will be allocated.

I don't see a logical inconsistency here.

Upvotes: 5

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