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Re: string buffer

Iain Clarke, Warrior Programmer10-Dec-07 2:04

Iain Clarke, Warrior Programmer

10-Dec-07 2:04

Your examples aren't all that correct. But I think I get the idea of the question.

This will be brief. For a longer version, read a C book. The Kernigan & Richie one taught me loads in a small book.

char array [] = "hello world";

ends up with a constant pointer to chunk of memory holding hello world followed by a zeroed character (NULL).

Same with buf, but it's a constant pointer to a chunk of memory 256 characters long - which could hold anything.

buf is a pointer.

buf [37] is the value in the 37th character in the chunk of memory pointed to by buf.

array[buf] would be the character pointed to by array + an offset of the amount of buf. If buf = 0x12344, this is a big offset.
array(buf) will get the compiler moaning at you, because array is not a function.

Going to array...

array is a pointer to the start of memory holding "hello world".
array[4] is the 4th character in that array. ie 'o', not 'l'.
*array is the character pointed to by array, ie 'h'.
*array and array [0] are equivalent.
array+4 is the pointer to the memory of helloe world, with 4 added to it. So it points to 'o'.
&array[4] is the pointer to the 4th char in the array. same as above.

So...
&array[x] is the same as array+x. The only difference is how similar it looks to the code around it. If the memory is allocated as an array, and that's how you're working on it, &array[0] may be more readable.

If you working on pointers to structures, and your function doesn't know how they're allocated, using array may be more readable.

I hope this helps a little!

Iain.

Re: string buffer

George_George10-Dec-07 2:18

10-Dec-07 2:18

Thanks Iain,

How about array and &array, are they always the same?

(in your reply, I have not seen any discussion about &array).

regards,
George

Re: string buffer

Iain Clarke, Warrior Programmer10-Dec-07 2:38

Iain Clarke, Warrior Programmer

10-Dec-07 2:38

Erm, no, they're never the same. That's like asking "Is 'Bob' the same as '#1 The Street, Townsville'?"

The & means pointer to.

So, &array is a pointer to a pointer to chars.

This can make the brain hurt, but is very useful when you get the hang of it.

Iain.

Re: string buffer

George_George10-Dec-07 2:50

10-Dec-07 2:50

Hi Iain,

I have tested that buf and &buf are of the same value -- means same effect. Here is my program to test. Any comments?

int main()
{
char buf[] = "Hello World";

int p1 = buf;

int p2 = &amp;buf;

return 0;
}

regards,
George

Re: string buffer

Iain Clarke, Warrior Programmer10-Dec-07 5:26

Iain Clarke, Warrior Programmer

10-Dec-07 5:26

CPallini's answer was good, and correct.

All it means though is the value of array and &array are the same as C nicely hacks it that way.

BUT...

They are of different types, as no doubt C will complain if you try to do this...

char array [] = "hello world";
char *p;
char **pp

p = array;   // happy
ASSERT(*p == 'h';
p = &array;  // not happy.

pp = &array;
ASSERT(**p == 'h');

Getting there?

Iain.

Re: string buffer

CPallini10-Dec-07 6:06

10-Dec-07 6:06

Actually:

Iain Clarke wrote:
char array [] = "hello world";
char *p;
char **pp

p = array; // happy
ASSERT(*p == 'h';
p = &array; // not happy.

is OK, while

Iain Clarke wrote:

char **pp;
pp = &array;
ASSERT(**p == 'h');

is NOT so OK!

It is a very tricky topic...
Smile | :)

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.

Re: string buffer

George_George10-Dec-07 15:49

10-Dec-07 15:49

Hi Iain,

I have tested that this line,

pp = &array;

also can not compile. I am using Visual Studio 2008 and C compiler.

regards,
George

Re: string buffer

Iain Clarke, Warrior Programmer11-Dec-07 4:44

Iain Clarke, Warrior Programmer

11-Dec-07 4:44

I think Cpallini has pretty thoroughly covered the whole field of pointers to arrays.

And while my answers may have been longer, his are that little bit more accurate with respect to &array.

If it doesn't compile, then the error message you get should be worth a read.

It's probably worth understanding pointers before you go back to socket questions!

Iain.

Re: string buffer

George_George11-Dec-07 21:59

11-Dec-07 21:59

Thanks for your advice, Iain!

regards,
George

Re: string buffer

CPallini10-Dec-07 3:25

10-Dec-07 3:25

Iain Clarke wrote:
So, &array is a pointer to a pointer to chars.

Your argument is good when applied on standard pointers. On the other hand, the address-of operator has a strange behaviour whenever is applied to array identifiers.
See, for instance.
http://www.fredosaurus.com/notes-cpp/arrayptr/arraysaspointers2.html[^]

You can also make a little test.
Smile | :)

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.

Re: string buffer

George L. Jackson10-Dec-07 4:43

George L. Jackson

10-Dec-07 4:43

&array is a pointer to an array pointer:

int values[] = { 10, 20, 30, 40, 55, 60, 70, 80, 90, 100 };
int (*pval)[10] = &values;
wcout << *pval[0] << endl;
// or
wcout << **pval << endl;

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Re: string buffer

George_George10-Dec-07 15:42

10-Dec-07 15:42

Thanks George,

I do not understand why **pval is the same as *pval[0]? I think *pval should be the same as *pval[0]. But during testing, you are correct. Why?

regards,
George

Re: string buffer

George L. Jackson10-Dec-07 16:53

George L. Jackson

10-Dec-07 16:53

pval[0] is a pointer to the first value and *pval[0] is the first value. pval is a pointer-to-a-array pointer. *pval dereferences to an array pointer and **pval dereferences to the first value. So, to get the second value using both:

  wcout << *(*pval + 1) << endl;
  wcout << *(pval[0] + 1) << endl;

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Re: string buffer

George_George10-Dec-07 17:50

10-Dec-07 17:50

Cool, thanks George!

regards,
George

Re: string buffer

David Crow10-Dec-07 4:01

10-Dec-07 4:01

George_George wrote:
Sometimes, I noticed that we either use,

1. array (buf)
or use,
2. &array (&buf)
or use
3. &array[0] (&buf[0])

Where have you seen this convention? I certainly don't profess to being a C expert, but I've never used nor have I ever seen this syntax before.

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Re: string buffer

CPallini10-Dec-07 6:13

10-Dec-07 6:13

DavidCrow wrote:
1. array (buf)

Probably (I'm guessing), with the above notation he means

array

or

buf

an so on.
If it stands then the syntax (apart of case (2)) it is used quite a lot.

Smile | :)

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.

Re: string buffer

David Crow10-Dec-07 6:20

10-Dec-07 6:20

CPallini wrote:
If it stands then the syntax (apart of case (2)) it is used quite a lot.

How, since it is syntactically incorrect?

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Re: string buffer

CPallini10-Dec-07 6:32

10-Dec-07 6:32

The syntax is in fact correct.
For instance, if you have

char array[] = "hello world";

all the three following statements are syntactically (and semantically) correct:
(1)

printf("%s\n", array);

(2)

printf("%s\n", &array);

(3)

printf("%s\n", &array[0]);

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.

Re: string buffer

David Crow10-Dec-07 7:15

10-Dec-07 7:15

But none of array(buf), &array(&buf), or &array[0](&buf[0]) are correct.

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Re: string buffer

CPallini10-Dec-07 9:01

10-Dec-07 9:01

DavidCrow wrote:
But none of array(buf), &array(&buf), or &array[0](&buf[0]) are correct.

Sure.

I think you missed the meaning of my previous reply http://www.codeproject.com/script/Forums/View.aspx?fid=1647&msg=2352543[^]

I assume (or, better, I'm guessing...) that he is using

array(buf)

as a shortucut to say that the same argument appliese both to array and buffer. For example, if I write the following sentence:

a number is even (odd) if, divided by two, gives 0 (1) as reminder.

then I'm obviously meaning:
(1) a number is even if, divided by two, gives 0 as reminder.
(2) a number is odd if, divided by two, gives 1 as reminder.

Smile | :)

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.

Re: string buffer

George L. Jackson10-Dec-07 7:15

George L. Jackson

10-Dec-07 7:15

But they are not equivalent!

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Re: string buffer

CPallini10-Dec-07 9:07

10-Dec-07 9:07

Well they aren't strictly equivalent (if you'll read all current thread posting then you'll find some links to related documentation), anyway the three statements produce the same result.
Smile | :)

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.

Re: string buffer

George L. Jackson10-Dec-07 9:44

George L. Jackson

10-Dec-07 9:44

The three statements don't always produce the same result:

wchar_t me[] = L"George Jackson\n";

wprintf(me);
wprintf(&me[0]);
wprintf(&me); // Won't compile: expects const wchar_t* not wchar_t (*)[16]

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Re: string buffer

CPallini10-Dec-07 10:42

10-Dec-07 10:42

I know there are differences (I already suggested you to check out all of the current thread posts).

The statements I shown produce the same results while the compiler error in yours has nothing to do with the wide char version of printf, for instance

wchar_t you[] = L"George Jackson\n";
wprintf(L"%s\n", you);
wprintf(L"%s\n", &you[0]);
wprintf(L"%s\n", &you);

compiles fine.
Smile | :)

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.

Re: string buffer

George L. Jackson10-Dec-07 11:27

George L. Jackson

10-Dec-07 11:27

You are using a format string; I am not. The following definitely doesn't have the same result:

#include <iostream>
#include <string>
 
using namespace std;
 
int _tmain(int argc, _TCHAR* argv[])
{
	wchar_t me[] = L"George Jackson\n";
 
	wcout << me << endl;
	wcout << &me[0] << endl;
	wcout << &me << endl; // You get the address of "me";
 
	wchar_t *me_ptr = me;
	wcout << me_ptr << endl;
 	
	me_ptr = &me[0];
	wcout << me_ptr << endl;
 	
	//me_ptr = &me; // Does not compile
	wchar_t (*me2_ptr)[16] = &me;
	wcout << *me2_ptr << endl;
 
	return 0;
}

"We make a living by what we get, we make a life by what we give." --Winston Churchill

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