Pointers such as Dangling, Void, Null, and Wild

Dangling pointer

A dangling pointer is a pointer that points to a deleted (or freed) memory location. Pointer can function as a dangling pointer in three ways.

1. De-allocation of memory

C++ Code

// 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 a dangling pointer
	ptr = NULL;
}

2. Function Call

C++ Code

// 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;
}

C Code

// 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:

A garbage Address

If x is a static variable, the above problem does not occur (or p does not become dangling).

C++ Code

// 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;
}

Output:

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 an unspecified data location in storage. The type is referred to as void. Basically, the data that it points to can be anything. If we assign a char data type address to a void pointer, it will become a char pointer, an int data type address will become an int pointer, and so on. Because any pointer type can be converted to a void pointer, it can point to any value.

Important Considerations

It is not possible to dereference void pointers. However, it is possible to do so by typecasting the void pointer.
Due to the lack of a concrete value and thus size, pointer arithmetic is not possible on void pointers.

C++ Code

#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;
}

Output:

Integer variable is = 4
Float variable is= 5.500000

NULL pointer

C++ Code

#include <iostream>
using namespace std;

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

Output:

The value of ptr is (nil)

Important Considerations

Uninitialized pointer vs. NULL - An uninitialized pointer stores an undefined value. A null pointer stores a defined value, but one that the environment defines as not being a valid address for any member or object.
NULL vs. Void Pointer - A value is a null pointer, whereas a void pointer is a type.

Wild pointer

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

C++ Code

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

	int x = 10;

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

	return 0;
}

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