Re: More on pointers to pointers.

Skarmander wrote:
Dave Vandervies wrote:
In article <1143773007.173184.158940@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Chad <cdalten@xxxxxxxxx> wrote:
<snip>
I guess even more to the point, why is this legal?
arg = &(*ptr) ;

ptr gets converted to a pointer to its first element, and then you follow
that pointer, so the expression inside the parens identifies an object
of type pointer to char.
Then the & operator gives you a pointer to that object, with type
pointer to pointer to char. Since this is the same type of the object
you're storing it into, this works with no problems or complaints from
the compiler.

(This one is actually a special case of a more general rule: since & (get
pointer) and * (follow pointer) are inverse operations, saying "&(*p)"
will always give you the same value as just saying "p" for any pointer p,
and "*(&x)" will always give you the same object as just saying "x".)

To be precise in a way that doesn't really matter but is still notable: this is a direct consequence of how & is defined. In particular, the standard guarantees that "&*p" is equivalent to "p" for *any* pointer p, even if p is invalid, because the subexpression "*p" is not evaluated.

Some compilers actually get this wrong if optimization is not turned on, and will insert evaluation of the pointer.

Evaluation of the indirection, obviously.

S.
.

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