pointers

I can not understand the following program:

Code:

#include <iostream.h>

int main ()
{
  int value1 = 5, value2 = 15;
  int *p1, *p2;

  p1 = &value1;     // p1 = address of value1
  p2 = &value2;     // p2 = address of value2
  *p1 = 10;         // value pointed by p1 = 10
  *p2 = *p1;        // value pointed by p2 = value pointed by p1
  p1 = p2;           // p1 = p2 (value of pointer copied)
  *p1 = 20;         // value pointed by p1 = 20
  
  cout << "value1==" << value1 << "/ value2==" << value2;
  return 0;
}

why the out put 10(value1) and 20(value2) here?
I do not understand the statement p1=p2 specially.
Thank you.

 

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v1 v2
10 10
10 20

p1 = p2 assigns p1 the value of pointer p2. So because p2 == &value2, after the assignment, p1 == &value2. This is why *p2 changes value2 and not value1.

 

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What about the last line *p1=20?
Doesn't it mean value1=20?

 

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No. Your local variables are stored off the stack. For example,
value1 might be stored in location 0x8000 and value2 in location 0x8004.

When you assign p2 = &value2, you have p2 == 0x8004. Then when you assign p1 = p2, you get p1 = 0x8004. By dereferancing p1 you change value2.

 

Internet Tutorials

Okinrus,I am struggling to understand what dereference operator is.Isn't it *?

Then you see the online tutorial:

http://www.cplusplus.com/doc/tutorial/tut3-3.html

They mean it the other way round!!!!!!

It really fooled me really...

 

Yeah that tutorials a bit <a href="http://www-numi.fnal.gov/offline_software/srt_public_context/WebDocs/Companion/cxx_crib/dereference.html">inaccurate</a> They confused referance operator with dereferance operator.

 

Thank you,okinrus.