Dangling, Void , Null and Wild Pointers

Dangling pointer

A pointer pointing to a memory location that has been deleted (or freed) is called dangling pointer. There are three different ways where Pointer acts as dangling pointer

1. De-allocation of memory

C++

// Deallocating a memory pointed by ptr causes
// dangling pointer

#include <cstdlib>
#include <iostream>

int main()
{
    int* ptr = (int *)malloc(sizeof(int));

    // After below free call, ptr becomes a
    // dangling pointer
    free(ptr);

    // No more  dangling pointer
    ptr = NULL;
}

// This code is contributed by sarajadhav12052009

C

// Deallocating a memory pointed by ptr causes
// dangling pointer
#include <stdlib.h>
#include <stdio.h>

int main()
{
    int *ptr = (int *)malloc(sizeof(int));

    // After below free call, ptr becomes a
    // dangling pointer
    free(ptr);

    // No more dangling pointer
    ptr = NULL;
}

2. Function Call

C++

// The pointer pointing to local variable becomes
// dangling when local variable is not static.

#include <iostream>

int* fun()
{
    // x is local variable and goes out of
    // scope after an execution of fun() is
    // over.
    int x = 5;

    return &x;
}

// Driver Code
int main()
{
    int *p = fun();
    fflush(stdin);

    // p points to something which is not
    // valid anymore
    std::cout << *p;

    return 0;
}

// This code is contributed by sarajadhav12052009

C

// The pointer pointing to local variable becomes
// dangling when local variable is not static.
#include<stdio.h>

int *fun()
{
    // x is local variable and goes out of
    // scope after an execution of fun() is
    // over.
    int x = 5;

    return &x;
}

// Driver Code
int main()
{
    int *p = fun();
    fflush(stdin);

    // p points to something which is not
    // valid anymore
    printf("%d", *p);
    return 0;
}

Output

0

The above problem doesn’t appear (or p doesn’t become dangling) if x is a static variable. 

C++

// The pointer pointing to local variable doesn't
// become dangling when local variable is static.

#include <iostream>
using namespace std;

int *fun()
{
    // x now has scope throughout the program
    static int x = 5;

    return &x;
}

int main()
{
    int *p = fun();
    fflush(stdin);

    // Not a dangling pointer as it points
    // to static variable.
    cout << *p;

    return 0;
}

// This code is contributed by sarajadhav12052009

C

// The pointer pointing to local variable doesn't
// become dangling when local variable is static.
#include <stdio.h>

int *fun()
{
    // x now has scope throughout the program
    static int x = 5;

    return &x;
}

int main()
{
    int *p = fun();
    fflush(stdin);

    // Not a dangling pointer as it points
    // to static variable.
    printf("%d",*p);
}

Output

5

3. Variable goes out of scope 

void main()
{
   int *ptr;
   .....
   .....
   {
       int ch;
       ptr = &ch;
   } 
   .....   
   // Here ptr is dangling pointer
}

Void pointer

Void pointer is a specific pointer type – void * – a pointer that points to some data location in storage, which doesn’t have any specific type. Void refers to the type. Basically the type of data that it points to is can be any. If we assign address of char data type to void pointer it will become char Pointer, if int data type then int pointer and so on. Any pointer type is convertible to a void pointer hence it can point to any value. 

Important Points

1. void pointers cannot be dereferenced. It can however be done using typecasting the void pointer
2. Pointer arithmetic is not possible on pointers of void due to lack of concrete value and thus size.

Example:

C

#include <stdlib.h>

int main()
{
    int x = 4;
    float y = 5.5;

    // A void pointer
    void* ptr;
    ptr = &x;

    // (int*)ptr - does type casting of void
    // *((int*)ptr) dereferences the typecasted
    // void pointer variable.
    printf("Integer variable is = %d", *((int*)ptr));

    // void pointer is now float
    ptr = &y;
    printf("\nFloat variable is = %f", *((float*)ptr));

    return 0;
}

C++

#include <iostream>
using namespace std;
int main()
{
    int x = 4;
    float y = 5.5;
    // A void pointer
    void* ptr;
    ptr = &x;
    // (int*)ptr - does type casting of void
    // *((int*)ptr) dereferences the type casted
    // void pointer variable.
    cout << "Integer variable is = " << *((int*)ptr)
         << endl;
    ptr = &y;
    cout << "Float variable is = " << *((float*)ptr) << endl;
    return 0;
}

// This code is contributed by Sachin Pal

Output

Integer variable is = 4
Float variable is = 5.500000

Refer void pointer article for details.  

NULL Pointer

NULL Pointer is a pointer which is pointing to nothing. In case, if we don’t have address to be assigned to a pointer, then we can simply use NULL. 

C++

#include <iostream>
using namespace std;

int main()
{
    // Null Pointer
    int *ptr = NULL;

    cout << "The value of ptr is " << ptr;
    return 0;
}

// This code is contributed by sarajadhav12052009

C

#include <stdio.h>
int main()
{
    // Null Pointer
    int *ptr = NULL;

    printf("The value of ptr is %p", ptr);
    return 0;
}

Output

The value of ptr is 0

Important Points

1. NULL vs Uninitialized pointer – An uninitialized pointer stores an undefined value. A null pointer stores a defined value, but one that is defined by the environment to not be a valid address for any member or object.
2. NULL vs Void Pointer – Null pointer is a value, while void pointer is a type

Wild pointer

A pointer that has not been initialized to anything (not even NULL) is known as wild pointer. The pointer may be initialized to a non-NULL garbage value that may not be a valid address. 

CPP

int main()
{
    int *p;  /* wild pointer */

    int x = 10;

    // p is not a wild pointer now
    p = &x;

    return 0;
}

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