Help with pointers (confused by sample)

[ragecoder]

So recently I have been learning in combination of this websites tutorials and a Sams C++ book I have found. Anyway onto the subject at hand. When I run the fallowing code it works just fine but I'm confused why firstValue equals 10 when it was assigned the value of 20 where as secondvalue is 20 when I think it should be ten. Specifically what is going on in line 14. Thanks to any one who could explain what's going on here.
Cheers.

// more pointers
#include <iostream>

int main()
{
int firstValue = 5, secondValue = 15;
int* p1, * p2;

p1 = &firstValue; // p1 = adress of firstValue
p2 = &secondValue; // p2 = adress of secondValue
*p1 = 10; // value pointed by p1 = 10
*p2 = *p1; // value pointed by p2 is now equal to value pointed by p1

p1 = p2; // p1 = p2 (value of pointer is copied)
*p1 = 20; // value pointed by p1 = 20

std::cout << "firstValue is " << firstValue << std::endl;
std::cout << "secondValue is " << secondValue << std::endl;
return 0;
}

[Disch]

Variables contain data... for example an int contains a number.

Pointers are the same, only they contain an address to another variable. When a pointer contains an address of another variable, it is said to "point to" that variable.

When you do *pointer you are not accessing or modifying the pointer... you are instead modifying the variable it points to.


With that in mind... let's take a look at this:

int firstValue = 5, secondValue = 15;
int* p1, * p2;

p1 = &firstValue;
p2 = &secondValue;

// at this point:
//   firstValue  = 5
//   secondValue = 15
//   p1          = &firstValue   (it "points to firstValue")
//   p2          = &secondValue

*p1 = 10;   // <- since p1 points to firstValue, this modifies firstValue

// at this point:
//   firstValue  = 10
//   secondValue = 15
//   p1          = &firstValue
//   p2          = &secondValue

*p2 = *p1;  // <- again, not modifying the pointers, but rather the variables they
            //   point to.  So here, we are saying whatever p2 points to (secondValue)
            //   = whatever p1 points to (firstValue).  So basically...
            //   secondValue = firstValue
            
// at this point:
//   firstValue  = 10
//   secondValue = 10
//   p1          = &firstValue
//   p2          = &secondValue

p1 = p2;    // <- here.. we ARE accessing the pointers... and not the data they point to
            //   this effectively makes p1 and p2 point to the same thing
            
// at this point:
//   firstValue  = 10
//   secondValue = 10
//   p1          = &secondValue
//   p2          = &secondValue

*p1 = 20;   // <- p1 now points to secondValue, so this will modify secondValue

// at this point:
//   firstValue  = 10
//   secondValue = 20
//   p1          = &secondValue
//   p2          = &secondValue

std::cout << "firstValue is " << firstValue << std::endl;   // <- prints 10
std::cout << "secondValue is " << secondValue << std::endl; // <- prints 20 

[ragecoder]

Thanks I sort of understand it now It was driving me mad at school all day couldn't figure it out.