C / C++ / MFC

12-Dec-07 0:03

Hello everyone,

I can not understand how the following code works and assign 100 to counter variable?

void Foo (int* input)
{
	*input = 100;

	return;
}

#define GETFOO  (Foo( &counter ), counter)

int counter;

int main()
{
	GETFOO;
	return 0;
}

thanks in advance,
George

CPallini12-Dec-07 0:35

12-Dec-07 0:35

the macro expands to

(Foo( &counter ), counter);

on running:

Foo(&counter) is first evaluated and it sets counter=100, then counter is evaluated giving the result 100 to the expression inside the outer braces, finally the result itself it is discarded.

Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

George_George12-Dec-07 0:45

12-Dec-07 0:45

Thanks CPallini,

I do not understand how this statement works,

(Foo( &counter ), counter);

I have tested that the simple statement (without comment) also works. What is the benefit and function to add , and counter in macro definition?

// #define GETFOO Foo((&counter), counter)
#define GETFOO Foo(&counter)

regards,
George

CPallini12-Dec-07 1:56

12-Dec-07 1:56

George_George wrote:
I do not understand how this statement works,

(Foo( &counter ), counter);

Both I and toxcct gave an answer to the above.

George_George wrote:
I have tested that the simple statement (without comment) also works.

Of course.

George_George wrote:
What is the benefit and function to add , and counter in macro definition?

The difference stands in the returned value of the expression, for instance

void Foo (int* input)
{
	*input = 100;

	return;
}

#define GETFOO  (Foo( &counter ), counter)
void main()
{
int i = GETFOO;
}

will assign 100 to i.

Anyway that way of coding is simply junk.

Big Grin | :-D

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

George_George12-Dec-07 2:03

12-Dec-07 2:03

Cool CPallini,

Thanks!

regards,
George

toxcct12-Dec-07 2:12

12-Dec-07 2:12

I gave you 5 everywhere in this thread, so did I to Iian.
answering George_george deserve courage and patience !!! Poke tongue | ;-P

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CPallini12-Dec-07 2:23

12-Dec-07 2:23

toxcct wrote:
I gave you 5 everywhere in this thread, so did I to Iian

Only 5?
Only for this thread?
The same given to Ian?
OMG, definitely you aren't my friend!
Big Grin | :-D

toxcct wrote:
answering George_george deserve courage and patience !!!

I cannot understand the overall plan behind all his questions. When I asked to him he replied that was for technical fun.
Anyway, I have to admit Blush | :O

that some questions are quite interesting.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

toxcct12-Dec-07 2:30

12-Dec-07 2:30

CPallini wrote:
OMG, definitely you aren't my friend!

please tell me this is a joke hey ! Poke tongue | ;-P

CPallini wrote:
Only 5?

i am negociating with the site administrators to leverage the votes on G_G thread up to at least 10 ! lol

BTW, please do the same to me Blush | :O

let's get stronger again that kid.

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CPallini12-Dec-07 2:44

12-Dec-07 2:44

toxcct wrote:
BTW, please do the same to me

Done buddy!

But you understand that it cannot be our voting guideline unless we want to be hounded out CP for violating the Competition Law.
Laugh | :laugh:

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

toxcct12-Dec-07 2:48

Re: how the macro works [modified]

12-Dec-07 2:48

hey, we're not enforcing anything. we're just reacting to G_G voting bad habit.
we're all good guyz here, so what's bad in this ?! lol.

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toxcct12-Dec-07 0:50

12-Dec-07 0:50

the , operator returns the last of its statements.
here, the macro calls the Foo function, to which the counter variable is passes by pointer (so, the counter variable is modified from within the Foo() function, but its modifications are still visible from the caller).
then, the counter variable is return (and as there is no = operator in the instruction GETFOO;, the returned value is not used, but still the counter variable is assigned with 100.

writing GETFOO; here is as if you were doing Foo(&counter);

BTW, this is very awful coding, and you should never use such macros !

and to finish, a question: how old are you ?

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modified on Wednesday, December 12, 2007 8:01:42 AM

George_George12-Dec-07 0:59

12-Dec-07 0:59

Thanks toxcct,

I do not agree with your answer. What do you mean class Foo? I am not using any object related concepts and techniques. It is pure C code. Smile | :)

regards,
George

toxcct12-Dec-07 2:01

12-Dec-07 2:01

George_George wrote:
I do not agree with your answer

don't say you don't agree, just say you don't understand.

now,
1) my answer was plainly explaining what the macro was doing, i just confused class with function, but the bahavior is the same,
3) my post deserves more than what you gave it,
4) you didn't answer my F*CKING QUESTION,
5) ... Mad | :mad:

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Cedric Moonen12-Dec-07 2:10

Cedric Moonen

12-Dec-07 2:10

He didn't answer your question so you should vote him a 2 or 1 Poke tongue | ;-P

Cédric Moonen
Software developer

Charting control [v1.2]

toxcct12-Dec-07 2:13

12-Dec-07 2:13

totally right. all fixed Sir.

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George_George12-Dec-07 2:35

12-Dec-07 2:35

Thanks toxcct,

Cool!

regards,
George

toxcct12-Dec-07 2:38

12-Dec-07 2:38

George_George wrote:
Thanks toxcct,

about what ?

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Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer12-Dec-07 1:42

12-Dec-07 1:42

This is an odd macro, but I presume it makes more sense in the context you're copying it from. (after all, if you wrote it yourself, you'd know what it does.

It may make more sense if main was like this

main ()
{
   int q = GETFOO;
   return 0;
}

instead.

That is expanded to...

int q = Foo(&counter), counter;

So, the function Foo is called, does it's magic, and places a result into counter.
Then the second half of the right hand expression is called, which is simply counter, so counter is then placed into q.

It's equivalent to:

int q;
Foo (&counter);
q = counter;

The comma operator is a bit odd and will take some reading.

The main place I see it is in for loops:

for (a = 0, b = 7; a < 9; a++, b++)
{
   ...
}

while allows you to combine a bunch of expressions between the ;'s.

I hope that makes some sense.

While you're asking us all the horrible questions in C, I can't recommend strongly enough The C Programming Language[^] by Kernighan & Ritchie.

Iain.

CPallini12-Dec-07 1:59

12-Dec-07 1:59

Iain Clarke wrote:
The main place I see it is in for loops:

for (a = 0, b = 7; a < 9; a++, b++)
{
...
}

I saw it also in the following kind of stuff

int i,j,k;

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

George_George12-Dec-07 2:00

12-Dec-07 2:00

Thanks CPallini,

My question is,

I think using in my sample,

#define GETFOO (Foo( &counter ), counter)

is the same as

#define GETFOO Foo( &counter )

So, no benefits of using , operator, right?

regards,
George

toxcct12-Dec-07 2:09

12-Dec-07 2:09

George_George wrote:
So, no benefits of using , operator, right?

yes there is !!! and i explained you why in my first answer.

if you were writing this:

#define GETFOO (Foo(&counter), counter)
 
void Foo(int* i) {
    &i = 100;
}
 
void main() {
    int counter;
    int i2 = GETFOO;
}

with the macro not containing the sequence operator ( , ), it would not even compile, because the Foo() function here is ot type void.
here is another example of how it could be used:

void main() {
    int counter;
    int i2 = (GETFOO * 5);
}

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George_George12-Dec-07 2:18

12-Dec-07 2:18

Cool toxcct!

My question is answered.

regards,
George

toxcct12-Dec-07 2:21

12-Dec-07 2:21

George_George wrote:
My question is answered.

so answer mine[^] now

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