Chapter 6

Data Types

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Pointer Type

A pointer has a memory address as it value (r-value), and also has a special value, nil (NULL in C)

Pointer operations:

•Assignment: Sets a pointer variable’s value to some useful address(&) .

•Dereferncing: Fetches the value in the memory cell whose address is referenced

by the pointer(*) .

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Pointer Types (C)

#include<stdio.h;>int main{()

int k, n; int *p;//Pointer declaration

k = 560; p = &k;//Pointer assignment

n = *p;//Pointer dereferencing printf("\n n = %d ",n); //Prints 560

return(0);

}

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Pointer Variables - Applications

Indirect addressing ( p=&k )

Array addressing ( *(mat+3) )

Record Field addressing ( s->id )

Allocation of dynamic storage

)p = (int*) malloc(64) ;(

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Dangling Reference

•A dangling reference is a pointer that contains address of a dynamic variable that has been

deallocated .

–Pointer “p1” is set to point to a new heap-dynamic variable.

–Pointer “p2” is assigned p1’s value.–The heap dynamic variable pointed to by “p1” is

explicitly deallocated , and p1 is set to null. Pointer p2 is not changed. p2 is now a dangling reference.

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Dangling Reference

Heap

p1

p2

Heap

p1 (NULL)

p2

(Deallocated)

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Dangling Reference

ProblemsThe location being pointed to by p2 may have been reallocated to some new heap-dynamic

variable .

The new value will bear no relationship with the old pointer’s dereferenced value .

If p2 is used to change the heap dynamic variable, then the value of the new heap dynamic variable will be destroyed.

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Lost ObjectA lost heap-dynamic variable (lost object) is an allocated heap-dynamic variable that is no longer accessible to the user program, but still contains some useful data.

Pointer p1 is set to point to a heap-dynamic variable.

p1 is set to point to another heap dynamic variable. The first heap-dynamic variable is now inaccessible, or lost.

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Lost Object

Heap

p1

Heap

p1

Heap

Lost object

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Pointers and Java

•Java does not provide indirect addressing (&) and dereferencing

operators .(*)

•Java also does not allow any pointer arithmetic .

•Java does not allow explicit deallocation of storage. There is no explicit deallocation operator (such as “delete”).

Since storage cannot be explicitly deallocated, you cannot have a dangling reference in Java.

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Heap Management

•Never reassign a pointer variable which has a dynamic storage allocation. Before reassigning a pointer, make sure that the storage it points to, is deallocated.

•Immediately after deallocating a heap-dynamic storage, reset to null all the pointer variables pointing to it.

•Whenever a large storage allocation from heap is requested, check if enough memory is available.