Thread: Pointers.

Could somebody explain pointers to me? I've read the section on pointers twice now, but I still don't have a clue what they're for and what they do. I feel like I'm bugging you all with this n00b problem but I'm pretty determined to having adequate knowledge of C++.

- Wall

google is your best friend.

A good analogy is your home address. Your address is where you live. You are the contents stored at that address. A pointer holds the address of a variable,structure,object, function, array etc . A pointer doesn't hold the actual data just the address of the data. This is extremely useful when you want to pass large blocks of data back and forth. Instead of copying the data you can just get a pointer to it.

Originally Posted by manofsteel972

A good analogy is your home address. Your address is where you live. You are the contents stored at that address. A pointer holds the address of a variable,structure,object, function, array etc . A pointer doesn't hold the actual data just the address of the data. This is extremely useful when you want to pass large blocks of data back and forth. Instead of copying the data you can just get a pointer to it.

I was actually going to break it down for the op but for most ppl, pointers are a bit more indepth and they tend to confuse it easily. I would do a search on the board OP, and i'd also use google.

I think I kind of understand it now. Could somebody give me an example of some code that uses pointers? So I understand the point of pointers. (heh)

Originally Posted by Wall

I think I kind of understand it now. Could somebody give me an example of some code that uses pointers? So I understand the point of pointers. (heh)

dude your not serious??. I just gave you a link to a whole bunch of tutorials and code. did you even click the link?? and no you don't understand it already, even adepts get stumped with pointers still. MEH. rtfPOST!

here you have some samples:

Code:

#include <iostream>
using namespace std;

//to test pointer
void testPtr(int *ptrLocal)
{
    *ptrLocal = 300;
}

//to test without pointer
void testByVal(int intLocal)
{
    intLocal = 300;
}

int main()
{
    //integer containign value 100
    int myInt = 100;  
  	
  	//pointer to myInt(anything you do with ptrInt affects myInt too
  	//because you can see it like if its an allias of myInt
  	int *ptrInt = &myInt;
  	
  	//displays myInt original value
    cout << "Original value of myInt: " << myInt << endl;
  	
    //value(200) to ptrInt and affects myInt too
  	*ptrInt = 200;
  	cout << "Value of myInt after \"*ptrInt = 200\": " << myInt << endl;
    
    //passing myInt to test() by value(copy of myInt)
    //will not modify myInt        
    testByVal(myInt);
    cout << "Value of myInt after testByVal(myInt): " << myInt << endl;
    
    //passing direction(pointer) of myInt to test()
    //will modify myInt
    testPtr(&myInt);
    cout << "Value of myInt after testPtr(&myInt): " << myInt << endl;
    
    return 0;
}

you can use pointers for a lot of things, but 1 thing you must keep in mind is that you can perform some tasks without the use of pointers but working that way you give up performance, why?
ie: when u pass data to a function(like "testPtr" or "testByValue") the compiler creates a copy of that data to work with it, know imagine you are passing a big value like a big array of doubles, big objects etc, the compiler will create a copy of that and all this means less memory, but if you send a pointer to a funciton the compiler will not create a copy because you are sending the memory direction of the data in other words you are working directly to the data.

another use could be to navigate data like an array, you can use a pointer to that array to navigate the array like finding a special char modify a value(element) of that array, etc you can do that because you can point that pointer to any place(see pointer aritmethic )

hope it helps and plase excuse my poor english

if you dont understand me just forget it

dude your not serious??. I just gave you a link to a whole bunch of tutorials and code. did you even click the link?? and no you don't understand it already, even adepts get stumped with pointers still. MEH. rtfPOST!

o.o;;;;

I just wanted to see some blocks of code where pointers would come in handy

This one is pretty good for the regular kinds of pointers
http://pweb.netcom.com/~tjensen/ptr/pointers.htm

This one for the scary stuff of when you want to point to functions
http://www.function-pointer.org/
Actually, they're pretty easy once you get past the syntax (and then hide it behind typedefs)

I just wanted to see some blocks of code where pointers would come in handy

well here is an option then.

PHP Code:

/*finds yearly totals, yearly average, and monthly average for several years of rainfall data using ptr notation */
#include <stdio.h>
#define MONTHS 12    
#define YEARS   5    
int main(void)
{
 // initializing rainfall data 
 const float rain[YEARS][MONTHS] = {
 {10.1, 8.1, 6.8, 4.2, 2.1, 1.8, 0.2, 0.3, 1.1, 2.3, 6.1, 7.4},
 {9.2, 9.8, 4.4, 3.3, 2.2, 0.8, 0.4, 0.0, 0.6, 1.7, 4.3, 5.2},
 {6.6, 5.5, 3.8, 2.8, 1.8, 0.2, 0.0, 0.0, 0.0, 1.3, 2.6, 4.2},
 {4.3, 4.3, 4.3, 3.0, 2.0, 1.2, 0.2, 0.2, 0.4, 2.4, 3.5, 6.6},
 {8.5, 8.2, 1.2, 1.6, 2.4, 0.0, 5.2, 0.9, 0.3, 0.9, 1.4, 7.3}
 };
 int year, month;
 float subtot, total;
 const float (*data)[MONTHS];
 data = rain;

 printf(" YEAR    RAINFALL  (inches)\n");
 for (year = 0, total = 0; year < YEARS; year++)
 {             // for each year, sum rainfall for each month
    for (month = 0, subtot = 0; month < MONTHS; month++)
       subtot +=  *(*(rain + year) + month); 
       /* or data[year][month] */
    printf("%5d %15.1f\n", 1997 + year, subtot);
    total += subtot;                  // total for all years 
 }
 printf("\nThe yearly average is %.1f inches.\n\n", total/YEARS);
 printf("MONTHLY AVERAGES:\n\n");
 printf(" Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct ");
 printf(" Nov  Dec\n");

 for (month = 0; month < MONTHS; month++)
 {             // for each month, sum rainfall over years
    for (year = 0, subtot =0; year < YEARS; year++)
       subtot +=  *(*(rain + year) + month);
    printf("%4.1f ", subtot/YEARS);
 }
 printf("\n");
 getchar();
 return 0;
} 


note: this could easily have been written in array notation as well. As you'll see sometimes it is not mandatory to use pointers. "the dual nature of them tends to confuse ppl"

salem's two links are pretty good as well.