functions and pointer arguments

Ok, i'm a bit confused with passing a pointer to a function e.g.

void change_val(int *var, int val)
{
*var = val;
}

int main(void)
{
int a = 5;
int *p = &a;

p = &a;

cout << "*p: " << *p << endl;

change_val(p, 10);

cout << "*p: " << *p << endl;

return 0;
}

Most text i read say if you want to change the value of a pointer passed to a function you have to declare the parameter as a pointer to a pointer (int **var) and pass the pointer memory address (&p). So how does the above program work?

Thanks for any help.

OK, well, a pointer holds a memory address. I'm sure you know that. What you've done in the main function, is give the pointer, p, the memory address of a.

So when you call the function change_val, you give the function a memory address, which is the memory address stored in p.

What the program does, is uses the operator *, the dereferencing operator, and changes the memory at that address, not the pointer.

And that's why the program works, since you're passing a memory address, and changing the data at that address. You're not changing the actual pointer.

Ok, so when would i need to use int **ptr as a function parameter instead of just int *ptr?

Read this (several times)
http://pw2.netcom.com/~tjensen/ptr/cpoint.htm

If you wanted to change the actual pointer's memory address, you would use two * operators (**), showing that you want the memory address of a pointer.

why couldn't one just use the address of operater(&)

I don't mean the pointer's memory address as in where it is stored the data is stored, but I mean the memory address it is pointing to.