Question

Hi I came across this bit of C++ code and am trying to learn how pointers operate.

   int main ()
{
  int firstvalue, secondvalue;
  int * mypointer;

  mypointer = &firstvalue;
  *mypointer = 10;
  mypointer = &secondvalue;
  *mypointer = 20;
  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  return 0;
}

My questions are below:

1. what exactly happens in memory when int *mypointer; is declared?
2. what does mypointer represent?
3. what does *mypointer represent?
4. when *mypointer = 10; what happens in memory?

Answer 1

1. what exactly happens in memory when int *mypointer; is declared?

Sufficient memory is allocated from the stack to store a memory address. This will be 32 or 64 bits, depending on your OS.

1. what does mypointer represent?

mypointer is a variable on the stack that contains a memory address.

1. what does *mypointer represent?

*mypointer is the actual memory location pointed to by mypointer.

1. when *mypointer = 10; what happens in memory?

The value 10 is stored in the memory location pointed to by mypointer. If mypointer contains the memory address 0x00004000, for example, then the value 10 is stored at that location in memory.

Your example with comments:

int main ()
{
  int firstvalue, secondvalue;   // declares two integer variables on the stack
  int * mypointer;               // declares a pointer-to-int variable on the stack

  mypointer = &firstvalue;       // sets mypointer to the address of firstvalue
  *mypointer = 10;               // sets the location pointed to by mypointer to 10.
                                    In this case same as firstvalue = 10; because
                                    mypointer contains the address of firstvalue
  mypointer = &secondvalue;      // sets mypointer to the address of secondvalue
  *mypointer = 20;               // sets the location pointed to by mypointer to 10. 
                                    In this case same as secondvalue = 20; because 
                                    mypointer contains the address of secondvalue
  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  return 0;
}

Try this code and see if that helps:

int main ()
{
  int firstvalue, secondvalue;   
  int * mypointer;               

  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  cout << "mypointer is pointing to " << mypointer << endl;

  mypointer = &firstvalue;       

  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  cout << "mypointer is pointing to " << mypointer << endl;

  *mypointer = 10;               

  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  cout << "mypointer is pointing to " << mypointer << endl;

  mypointer = &secondvalue;      

  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  cout << "mypointer is pointing to " << mypointer << endl;

  *mypointer = 20;               

  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  cout << "mypointer is pointing to " << mypointer << endl;

  return 0;
}