Question about Pointers

Livnat · 09-06-2008, 05:14 AM

Is this function

Code:

void sum_and_product(int *pa, *int pb)
{
  int sum = *pa + *pb;
  int diff = *pa - *pb;
 
  *pa = sum;
  *pb = diff;
}

equivalent to this one? -

Code:

void sum_and_product(int *pa, *int pb)
{
  int sum = *pa + *pb;
  int diff = *pa - *pb;
 
  pa = &sum;
  pb = &diff;
}

anon · 09-06-2008, 05:23 AM

No, the second changes the addresses the pointers point to, which luckily for you won't be updated in the calling code (since they are addresses to local variable which go out of scope).

Otherwise, it looks somewhat strange that you'd want to completely lose the original data.

jason_m · 09-06-2008, 09:03 AM

Quote:

Originally Posted by anon

...which luckily for you won't be updated in the calling code...
data.

What do you mean by this? I figured the behavior would be to change the addresses, the function returns, the memory locations held by sum and diff are discarded (they are popped off the stack) and values of the variables passed as arguments to the function are now unreliable.

Jason

tabstop · 09-06-2008, 09:08 AM

pa and pb are values passed to the function. As such, any changes made to those values are discarded at the end of the function.

In other words, you can change what a pointer points to in a function, but not where a function points, if you want the changes seen in the calling code.

jason_m · 09-06-2008, 09:23 AM

Right. I need a 1 hour wait rule after getting out of bed before being able to post :-p. It occurred to me how wrong my post was when I got in the shower and I almost bolted back out to the computer to edit it before anyone could see it, but oh well.

So yes, the parameters pa and pb are two newly created pointers to ints setup when sum_and_product(...) is called. They are not the original arguments supplied to sum_and_product, but rather hold the addresses of the variables supplied as those arguments.

Changing the addresses held by pa and pb is fine, now they point to sum and diff. So, if you were to change sum later in the function, *pa would produce this new value. However, you've lost any means by which to update the arguments supplied in the calling function.

Jason

matsp · 09-06-2008, 06:28 PM

To change ANYTHING in a function, you need it's address (or in other words, it needs to be a pointer to what you want to change).

E.g

Code:

void func(int a)
{
   a = 7;
}

int main()
{
   int x = 18;

   func(x);  
   ...

x is 18, and the number 18 is passed to func in a, a is set to 7, but x is still 18 when you get back to main.

If we change it to be a pointer:

Code:

void func(int *a)
{
   *a = 7;
}

int main()
{
   int x = 18;

   func(&x);  
   ...

Now x would be 7 when we get back from func to main.

If you want to change what a pointer points to, you would have to pass a pointer to a pointer.

And finally, if you ever take an address of a local variable, and use that address after you have returned from the function that had the local variable, don't expect it to work - that variable is no longer the variable you took the address of - it is either free space on the stack or being used for something else.

--
Mats

Livnat · 09-07-2008, 12:42 AM

That really cleared things up!

09-06-2008, 09:08 AM	#4
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,809	pa and pb are values passed to the function. As such, any changes made to those values are discarded at the end of the function. In other words, you can change what a pointer points to in a function, but not where a function points, if you want the changes seen in the calling code.
tabstop is offline

09-06-2008, 09:23 AM	#5
jason_m Registered User Join Date: May 2008 Posts: 54	Right. I need a 1 hour wait rule after getting out of bed before being able to post :-p. It occurred to me how wrong my post was when I got in the shower and I almost bolted back out to the computer to edit it before anyone could see it, but oh well. So yes, the parameters pa and pb are two newly created pointers to ints setup when sum_and_product(...) is called. They are not the original arguments supplied to sum_and_product, but rather hold the addresses of the variables supplied as those arguments. Changing the addresses held by pa and pb is fine, now they point to sum and diff. So, if you were to change sum later in the function, *pa would produce this new value. However, you've lost any means by which to update the arguments supplied in the calling function. Jason
jason_m is offline

09-06-2008, 06:28 PM	#6
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	To change ANYTHING in a function, you need it's address (or in other words, it needs to be a pointer to what you want to change). E.g Code: void func(int a) { a = 7; } int main() { int x = 18; func(x); ... x is 18, and the number 18 is passed to func in a, a is set to 7, but x is still 18 when you get back to main. If we change it to be a pointer: Code: void func(int a) { a = 7; } int main() { int x = 18; func(&x); ... Now x would be 7 when we get back from func to main. If you want to change what a pointer points to, you would have to pass a pointer to a pointer. And finally, if you ever take an address of a local variable, and use that address after you have returned from the function that had the local variable, don't expect it to work - that variable is no longer the variable you took the address of - it is either free space on the stack or being used for something else. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
matsp is offline

09-07-2008, 12:42 AM	#7
Livnat Registered User Join Date: May 2008 Posts: 29	Thanks! That really cleared things up!
Livnat is offline

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