pointers Getting started with pointers

Remarks

Getting Started with Pointers

Pointers are variables that store the address of another variable.As language feature they are available in several programming languages like, but not limited to :

• Go
• C/C++
• Ada
• Pascal
• C# (available under certain constraints)
• COBOL
• FORTRAN

To get started with C/C++ pointers , follow these steps

1. Install compiler like Minimalistic GNU for Windows, http://www.mingw.org/wiki/Getting_Started

2. Go to the installation folder of g++ binary via commandline for example:

   C:\MinGW\bin>
    

3.Create a text file and write this C++ program

#include <iostream>

int main () {

   int  pointed=0;
   int* ptr = & pointed;

   std::cout<<"Address of pointed variable is: "<<ptr<<std::endl;

   return 0;
}
 

4. Save as pointer.cpp 5.On the command prompt run the command

g++ -o pointer.exe -c pointer.cpp

6. In the working directory you will get a executable as pointer.exe , this exe upon running will give some output like

Address of pointed variable is: 0x7e892dac0a0c

If you receive the above output, you have written your first pointer program

What is a Pointer?

It is basically address of a variable in memory. It allows us to indirectly access a variable. So using pointers we can talk about a variable's address(as well as its value by dereferencing the pointer). They are useful when we want to deal with address of a memory location rather than its value.

Consider the following simple swap function in C:

void Swap(int firstVal, int secondVal)
{
 int tempVal = firstVal;
 firstVal = secondVal;
 secondVal = tempVal;
}
 

now in main if we have the following code:

.
.
int a = 9,b = 100;
swap(a,b);
//print a and b
.
.
 

The values of a and b would remain unchanged as would be clear by printing their values in the main function. To implement the swap function correctly, instead of passing the values of variables a and b , we pass the address of variables a and b as:

swap(&a,&b);
 

The operator & , returns the address of the variable. Its used like this:

int *address_of_a = &a;
 

the int *address_of_a , states that the variable address_of_a points to(stores the address of) an integer variable.

Now our correct swap function would be:

void Swap(int *firstaddress, int *secondaddress)
{
 int tempVal = *firstaddress;
 *firsaddress = *secondaddress;
 *secondaddress = tempVal;
}
 

Now the interchanged values would be reflected in main function:

int a = 9,b = 100;
swap(&a,&b);
//print
 

You can always dereference the pointer using * , if you dont have the original variable. Suppose if in a function you do not have the original variable but have its address in a pointer variable int *x . We can simply access the value of the memory address as value = *x ;

If we did not have pointers we could never emulate pass by reference in C , because C is pass by value. But remember we can only emulate, because even when we use pointers, the int *firstaddress, int *secondaddress are only local pointer variables created, that have the address of variables a and b .