What`s the Address? Pointers - Passing a Single Variable Using Pointer Notation

(Page 6 of 9 )

Passing an array name by address is relatively simple because the value of the array name is an address. However, you may often want to pass a single variable by address. By single variable I don’t mean a variable that is unmarried, but instead, for example, an int as opposed to an int array.

With a single variable, subscript notation is not an option. Subscripts make sense only with an array. Rather, you need to use pointer notation to pass a single variable by address.

Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function—only the syntax is different. For comparison, let’s start with the following program from Chapter 9 that passes the variable to be doubled by reference:

#include <iostream>
using namespace std;
void doubleIt(int&);
int main ()
{
   int num;
   cout << "Enter number: ";
   cin >> num;
   doubleIt(num);
   cout << "The number doubled in main is " << num << endl;
   return 0;
}
void doubleIt (int& x)
{
   cout << "The number to be doubled is " << x << endl;
   x *= 2;
   cout << "The number doubled in doubleIt is " << x << endl;
}

Here is some sample input and output:

Enter number: 3
The number to be doubled is 3
The number doubled in doubleIt is 6
The number doubled in main is 6

Let’s now modify this program so it passes the variable to be doubled by address instead of by reference:

#include <iostream>
using namespace std;
void doubleIt(int*);
int main ()
{
   int num;
   cout << "Enter number: ";
   cin >> num;
   doubleIt(&num);
   cout << "The number doubled in main is " << num << endl;
   return 0;
}
void doubleIt (int* x)
{
   cout << "The number to be doubled is " << *x << endl;
   *x *= 2;
   cout << "The number doubled in doubleIt is " << *x << endl;
}

There are four syntax differences between these two programs.

1. In the function prototype, you use an ampersand (&) for passing by reference but an asterisk (*) for passing by address:
   
   void doubleIt(int&);  // by reference
   void doubleIt(int*);  // by address
   
2. Similarly, in the function header, you use an ampersand (&) for passing by reference but an asterisk (*) for passing by address:
   
   void doubleIt (int& x)   // by reference
   void doubleIt (int* x)   // by address
   
3. When you call the function, you don’t need the address operator (&) for passing by reference, but you do need one for passing by address since you are supposed to be passing by the address of x.:
   
   doubleIt(num);   // by reference
   doubleIt(&num);  // by address
   
4. In the body of the called function, you don’t need to dereference the argument when you pass it by reference, but you do need to when you pass by address since x, being passed by address, is not a value but is instead a pointer:

// by reference -no dereference
{
   cout << "The number to be doubled is " << x << endl;
   x *= 2;
   cout << "The number doubled in doubleIt is " << x << endl;
}
// by address -need to dereference
{
   cout << "The number to be doubled is " << *x << endl;
   *x *= 2;
   cout << "The number doubled in doubleIt is " << *x << endl;
}

You may legitimately be wondering why, with a single variable argument, I would want to pass it by address when the syntax for passing it by reference seems easier. The pat answer I give my students is that there are certain sadistic computer science teachers (I’m not mentioning any names here) who insist their students pass by address to make them suffer. All kidding aside though, there are actually certain library functions that do use pass by address. Additionally, when using dynamic memory allocation and returning pointers from functions (to be covered in the following sections), passing by address may be the only option.

Next: Dynamic Memory Allocation >>
 

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This article is excerpted from chapter 11 of the book C++ DeMYSTifieD, written by Jeff Kent (McGraw-Hill, 2004; ISBN: 0072253703). Check it out at your favorite bookstore. Buy this book now.