Theory Question, Reading Text Book

[marak]

G'day guys, ive been reading Brian Overlands C++ Without Fear for a few weeks now on and off when i get the time, and im getting a little confused with the following.

pg 146:
Pointers(he's discussing why pointers are used and his reasoning is:
"The problem here is that when an argument is passed to a funciton, the function gets a copy of the argument, but upon return that copy is thrown away" - his example is a void double_it(int n) with a variable reference from above inside it.

**
Think i figured it out while i was typing this out, but i want to ask a question based on his code so ill type it out here
**

his original pointer code:

#include <iostream>
using namespace std;

void double_it(int *p);

int main(){
int a =  5, b = 6;
cout<<"Value of a before doubling is: " << a << endl;
cout<<"Value of b before doubling is: " << b << endl;
double_it(&a); // pass address of a.
double_it(&b); // pass address of b.
cout<<"value of a after doubling is: " << a << endl;
cout<<"Value of b after doubling is: " << b << endl;
return 0;
}

void double_it(int *p){
*p = *p * 2;
}

Ok thats the base code, now if instead of using a pointer to passt the values to Double_it, could i instead use a return value on double_it? Wouldnt that negate the need for pointers in this case?

I understand how pointers work(going to be playing with them today to get more familiar), infact this language has been quite easy so far, just a few questions the book doesnt answer :)

- Marak

[LB]

http://www.cplusplus.com/doc/tutorial/pointers/

^That will help. ;)





Also, another way to do that double_it function could be like this: (I think)

void DoubleIt(int& D)
{
	D *= 2;
}

Then to call it just use DoubleIt(MyIntVariable);

I am not sure of which is the better method, though.

[marak]

Thanks for the reply. I understand the points, what im trying to figure out is why he's saying that we should use pointers as passing values wouldnt work(and yet im pretty sure it would work).

[ne555]

What if you want more than one result
bool func(int &foo, int &bar);
(You could also return a struct tough)

Or if your variable is too big that make a copy will cost you.

[marak]

Oh i couldnt return two values? (ive never tried it, it was kind of a mistake to assume it was possible lol).

Now im starting to understand where its usefull to use pointers, instead of passing variables. Thanks guys :)

[LB]

Yes, you can only return one thing at a time. As soon as you use the return function, no matter what, the function will exit right then and there and return the given value. No further code will get processed in the function. Example: This returns true if the parameters are equal and false if they are not.

bool IsEqual(int param1, int param2)
{
	if(param1 == param2)
	{
		return(true); //This immediately exits the function, no questions asked
	}
	return(false); //If the above return(true); did not fire because the integers did not equal, then this returns false
}

[marak]

Thanks for that :) makes things a little easier to understand.