Discussion
Which of the following does not initialize ptr to null (assuming variable declaration of a as int a=0)?
Answer: A
Correct answer: int *ptr = &a;
That initialiser gives ptr the address of a, which is a real object and therefore not a null pointer.
The other two do yield null. &a - &a is zero, and a - a is zero because a is 0 - and a constant expression evaluating to 0 is a null pointer constant, so it converts. Both are legal but poor style: NULL states the intention, and a subtraction that happens to give zero conceals it.
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