preincrement puzzle

[Saturday7]

You have a struct:

struct a {
char *p;
};

struct a *apple;

Which does this preincrement: apple or p?

++apple->p = 0;

 

[Karthik Kumar]

You have a struct:

struct a {
char *p;
};

struct a *apple;

Which does this preincrement: apple or p?

++apple->p = 0;

preincrement applied to apple->p .

Check out the rules of 'Operator precedence' in any standard C book.

 

[Ben Pfaff]

Which does this preincrement: apple or p?

++apple->p = 0;

Postfix operators precede prefix operators, so p.
This is the same rule that applies to an expression like *p++,
which increments p, not *p.

 

[Joe Wright]

You have a struct:

struct a {
char *p;
};

struct a *apple;

Which does this preincrement: apple or p?

++apple->p = 0;

Note you have apple as a pointer to a 'struct a' object. But apple
is uninitialized and doesn't point anywhere. Nothing works. Allow me
to presume to re-state your problem and question..

struct a *apple = malloc(sizeof *apple);

Now apple is a pointer to memory declared a 'struct a' object.

apple->p = "Hi Sailor, Buy a girl a drink?";

This assigns the address of the string literal array to apple->p, a
pointer to char and a proper lvalue. ++apple->p increments apple->p
and yields a pointer value but is not an lvalue and cannot be
assigned to. puts(apple->p) now prints ..

i Sailor, Buy a girl a drink?

Again, '++apple->p = 0;' cannot work and should, I think, result in
a diagnostic.

 

[Bradley Bungmunch]

Postfix operators precede prefix operators, so p.

Yes, but not so in this case. While it is p that gets incremented, it
does so because -> is a primary operator while ++ is a unary operator
and primary operators take precedence over unary operators.

This is the same rule that applies to an expression like *p++,
which increments p, not *p.

=====================



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[Keith Thompson]

Warning:

The previous article was cross-posted to alt.fan.karl.malden.nose.
Please do not cross-post any followups. According to the
alt.fan.karl.malden.nose FAQ,
<http://faqs.jmas.co.jp/FAQs/1999/Feb/990211.01>, they'll take this as
an invitation to start spamming comp.lang.c. I don't know how
seriously to take this idiocy, but let's not take any chances.

Apologies to those who have already wisely killfiled cross-posts to
alt.fan.karl.malden.nose.

 

[Chris Barts]

On Wed, 06 Oct 2004 23:57:09 +0000, Bradley Bungmunch wrote:

<snip all, including massive sig>

Intelligent people have sigs that are no more than four lines long.

Really intelligent people don't have sigs at all.

 

[Alan Balmer]

Warning:

The previous article was cross-posted to alt.fan.karl.malden.nose.
Please do not cross-post any followups. According to the
alt.fan.karl.malden.nose FAQ,
<http://faqs.jmas.co.jp/FAQs/1999/Feb/990211.01>, they'll take this as
an invitation to start spamming comp.lang.c. I don't know how
seriously to take this idiocy, but let's not take any chances.

Apologies to those who have already wisely killfiled cross-posts to
alt.fan.karl.malden.nose.

I was puzzled as to why I didn't see the previous post, until I
checked my filters. I've had "bungmunch" kill-filed for so long I had
completely forgotten it.