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Dangling, Void, Null and Wild Pointers in C++
Dangling pointer
Dangling pointer is a pointer pointing to a memory location that has been freed (or deleted). There are different ways where Pointer acts as dangling pointer
Function Call
The pointer pointing to local variable becomes dangling when local variable is not static.
int *show(void) {
int n = 76; /* ... */ return &n;
}Output
Output of this program will be garbage address.
De-allocation of memory
int main() {
float *p = (float *)malloc(sizeof(float));
//dynamic memory allocation.
free(p);
//after calling free()
p becomes a dangling pointer p = NULL;
//now p no more a dangling pointer.
}Variable goes out of scope
int main() {
int *p //some code// {
int c; p=&c;
}
//some code//
//p is dangling pointer here.
}Void pointer
Void pointer is a pointer which is not associate with any data types. It points to some data location in storage means points to the address of variables. It is also called general purpose pointer.
It has some limitations
- Pointer arithmetic is not possible of void pointer due to its concrete size.
- It can’t be used as dereferenced.
Example
#include<stdlib.h>
#include<iostream>
using namespace std;
int main() {
int a = 7;
float b = 7.6;
void *p;
p = &a;
cout<<*( (int*) p)<<endl ;
p = &b;
cout<< *( (float*) p) ;
return 0;
}Output
7 7.600000
Algorithm
Begin Initialize a variable a with integer value and variable b with float value. Declare a void pointer p. (int*)p = type casting of void. p = &b mean void pointer p is now float. (float*)p = type casting of void. Print the value. End.
Null Pointer
Null pointer is a pointer which points nothing.
Some uses of null pointer are
To initialize a pointer variable when that pointer variable isn’t assigned any valid memory address yet.
To pass a null pointer to a function argument if we don’t want to pass any valid memory address.
To check for null pointer before accessing any pointer variable. So that, we can perform error handling in pointer related code e.g. dereference pointer variable only if it’s not NULL.
Example
#include <iostream>
using namespace std;
int main() {
int *p= NULL; //initialize the pointer as null.
cout<<"The value of pointer is ";
cout<<p;
return 0;
}Output
The value of pointer is 0.
Wild Pointer
Wild pointers are pointers those are point to some arbitrary memory location. (not even NULL)
int main() {
int *ptr; //wild pointer
*ptr = 5;
}
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