Pointers in C++

Run

#include <iostream> 
using namespace std;
int main() 
{
  
int arr[5] = { 1, 2, 3, 4, 5 };
  
int *ptr;
  
cout << "Displaying address using arrays: " << endl;
  
for (int i = 0; i < 5; ++i)
    
    {
    cout << "&arr[" << i << "] = " << &arr[i] << endl;
    } 
// ptr = &arr[0]
  ptr = arr;			//assiing base address to pointer
  cout << "\nDisplaying address using pointers: " << endl;
  
for (int i = 0; i < 5; ++i)
    
    {
cout << "ptr + " << ptr + i << " = " << *(ptr + i) << endl;	//access by incerementing address
    } 
return 0;

}

Output

Displaying address and values using arrays:
&arr[0x7fff5fbff880] = 1
&arr[0x7fff5fbff884] = 2
&arr[0x7fff5fbff888] = 3
&arr[0x7fff5fbff88c] = 4
&arr[0x7fff5fbff890] = 5
Displaying address using pointers:
ptr + 0 = 0x7fff5fbff880
ptr + 1 = 0x7fff5fbff884
ptr + 2 = 0x7fff5fbff888
ptr + 3 = 0x7fff5fbff88c
ptr + 4 = 0x7fff5fbff890

Wild/Bad pointers

• A pointer which just declared but not initialized with any address is called wild pointer
• They point to some arbitrary memory location that and may be used by the system and cause a program to crash or behave badly.
• This problem can be solved using null pointers

Example

void main()
{
int *p;//declartion but no initialisation
cout<<"*p="<<*p<<"\n";//junk
cout<<"p="<<p<<"\n";//gives an arbitraryaddresss that is reserved already
}

Null Pointer

• It is always a good practice to assign the pointer NULL to a pointer variable at the declaration case you do not have an address to be assigned.
• A pointer that is assigned NULL is called a null pointer.
• The NULL pointer is a constant with a value of zero, defined in several standard libraries, including iostream.
• Many times, uninitialized variables hold some junk values and it becomes difficult to debug the program.

Example

int main () 
{
int *ptr = NULL;//inilialising with NULL
cout << "The value of ptr is " << ptr ;
return 0;
}

Output:

The value of ptr is 0

• On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system.
• But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

Void Pointer

• Generally, Pointers are type specific i.e a pointer of integer type can hold the address of an integer variable only, It some times may cause problems if by mistake if we assign a pointer variable to a different datatype
• Hence a Void pointer is a Generic Pointer that can point to any datatype object, provided it must be typecasted while accessing so as compiler should the required number of bytes for that value
• If a pointer’s type is void *, the pointer can point to any variable that is not declared with the const or volatile keyword.

Example

int main()
{
int a=10;
float b=1.5;
void *p=&b;//pointing to float variable address
//int *c=&b//invalid types c must be float pointer
cout<<"b value:"<<*(float *)p;
}

Output:

b value:1.5

Dangling pointer

• A pointer that is still pointing to a memory address that has been deleted is called dangling pointer
• To over this null pointers can be used

Example:

int main()
{
int *p=new int(100);//allocating memory
cout<<"*p="<<*p<<endl;
cout<<"p="<<p<<endl;
delete p;// adress in p is no more
cout<<"p value="<<*p<<endl;//garbage value
p=NULL;//point no where
cout<<"p value="<<*p<<endl;
}