pass by address between functions

Xiangbuilder · 10-10-2003, 01:47 AM

Code:

void print (int* ip) {
   cout<<*ip<<endl;
} 
int main ()
{
   int i;
   print (&i);
}

I think the compiler will think int* ip=&i in this case.

Code:

void print (int* i) {
   cout<<*i<<endl;
} 
int main ()
{
   int i;
   print (&i);
}

I think the compiler will think int* i=&i in this case.
Then my trouble comes up:
We know in c++, the expression int* i=&i is evil. Howeve, The code still make sense, why?
thank you.

mr_segfault · 10-10-2003, 04:47 AM

The reason is that they are different i's and not the same.

They are in different scope and therefore do not do as you said.

int *i=&i; // would set the value of i to the address of the pointer i, probably not much use, ie its sets the pointer to the address of its self since they are the same variable!

But since the two i's your talking about are in different scope it does not do this.

Look at it this way..

int i; // lets say this gets addres 0x000300

print(&i); // passes 0x000300 to print

void print (int *i) // another variable called *i which gets address say 0x000302

now when you call print(&i) from main, the value 0x000302 is written to memory location 0x000300.

Then when you call cout << *i <<endl; it will print out the value which is stored at memory location 0x000300.

but when you say:

int *i = &i;

lets say *i gets the address 0x000700, then you it says, set the pointer to point to 0x000700, so now it points to its self, now your data type for i is a pointer to an int, but you have assigned it to point to a pointer (which is a pointer to an int, and not an int), so the types are probably not compatible.

Have I made any sense?

One more example to help clarify what I am saying.

int *i = &i; // This is similar to this:

int *k;
int *m;

k=&m; // bad...

k=m; // makes more sense because now they are both the same type...

Have I made sense yet? (I dont think i'm very good at explaining things : )

Cheers

Xiangbuilder · 10-10-2003, 06:54 AM

"I dont think i'm very good at explaining things"
My English and c++ skill are so poor, still can understand your words.
Thank you.
For your detailed clear information.