Expert: Joseph Moore - 11/13/2009

Question
Is there any difference between *ptr and &ptr in following codes
code 1
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   int x = 10;
   const int *ptr;
   ptr = &x;
Code 2
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   int &ptr = x;

In code 1 ptr is an another variable than x pointing x.
Is ptr in code 2 an another variable than x, or, is another name to  same location x?

Answer
Hi, Ajay.

First, looking at your code, it appears to be mis-categorized.  Your second piece of code is creating a reference -- a C++ type.

References and pointers are remarkably similar.  They are both methods to access the same memory with different variables.  The differences are rather subtle.

First, a reference cannot be NULL.  This means that you are guaranteed that a references accesses some (supposedly) valid memory address.  This means that you do not have to validate a reference before using it, saving a branch statement (you do always check that your pointers aren't NULL, right? :)  With poor coding, it is possible to have a reference be bad, but it requires going from pointer to reference, which usually is not a good idea anyway.

Second, a reference need not be dereferenced before using it.  When you use a pointer, I'm sure you are used to putting a * in front of the pointer to dereference it or using the -> operator to access into pointers to structs.  With a reference, this is not required.  You can simply use the referenced variable as if it were the original variable, no dereferencing required.

A reference also cannot be made to reference a different variable once it has been created.  With pointers, you can reassign them to point to other variables at any time, but a reference, once created, always points to its original referenced variable.

Wikipedia has a pretty solid article on references for you to read:

http://en.wikipedia.org/wiki/Reference_%28C%2B%2B%29

If you are left with any questions, please don't hesitate to ask.  I'm here to help. :)