Chapter 7 Pointers:
Java does not have pointers.
Used for dynamic memory allocation.
Example:
Employee* boss // does not create the object!
boss = new Employee or
boss = new Employee(parameters);
boss is a pointer to the object.
*boss is the object.
(*boss).method() or bossmethod() is the
proper way to access a method.
Note the syntax and common errors
on p. 309-10.
In a method, this is a pointer to the
implicit parameter (object specified
when the method was called). Key
word this is not necessary but some
like it.
Function new
boss = New Employee()
allocates memory from the heap for the
object itself.
Variable boss is allocated on the stack.
HEAP ALLOCATED MEMORY MUST
BE DISPOSED OF WHEN IT IS NO
LONGER USED.
Use delete boss at some point.
Java has automatic garbage
collection C++ does NOT.
If not used, the result can be a
memory leak (unusable memory).
dangling pointers:
A pointer that has not been initialized or
a pointer after the associated heap
memory is deleted.
COMMON ERROR is to try to access a
dangling pointer.
Do example from the snippets file.
Address operator:
If x is a pointer and y is an object then
we can write x=&y.
In this case, *x and y are the same thing.
x
y
Do example from the snippets file
under Pointers. Try a few things:
Set x to y.
Reference *x after deleting it.
Set x to y and delete y. Set x to y and
delete both x and y.
Try deleting p.
Examine all in the watch window.
Display q[0] toward the end of the main.
Set up tempdemo for department.cpp.
Code on p. 317.
Shows why pointers are useful
What happens if you write delete tina
prior to qc.print() in the main
program?
Note the comparison between a
reference parameter and specifying a
pointer as a parameter. p. 319.
Important!
Discuss common errors on p. 329.
Demo04: listing of accounts for a
customer is done by going through
ALL account objects.
This is NOT a good design.
Demo06 uses an array of pointers
inside the customer class to locate a
list of account objects.
Arrays and pointers:
int a[10] similar to int* a and a = new
int[10]
Could also use a = new int[n], where n is
input by the user.
a[i] same as *(a+i)
This is a dynamic array in contrast to a
static array discussed previously.
Example
int a=99;
int * x;
int b=99;
x = new int[5];
for (int i=0; i<5; i++)
*(x+i)=i*i;
for (int i=0; i<5; i++)
cout << x[i] << " ";
cout << endl;
cout << "a is " << a << " b is " << b << endl;
cout << endl;
See advance topic on p. 323
See Quality Tip on p. 324
COMP SCI 370 may cover this in more
detail.
See common error 7.6 on p. 325 and demo
the code as written in the notes.
See advanced topic on p. 326.
Do, also, 2D dynamic arrays. In
particular look at code from the
snippet file (optional)
While running this program take a
look at the debugger. In particular put
m, m[0], m[1], etc in a watch window.
Also, while running, take a look at the
memory window to see the actual
contents of memory.
Pointers to character strings:
Explain the difference between char c =
‘A’ and char* c = “A”. This is a COMMON
problem in C.
Explain problem with comparing char* p
and char * q with p == q using this
example
#include <iostream>
using namespace std;
int main( )
{
char* x = "Harry";
char * y;
y=new char[6];
strcpy(y, "Harry");
cout << "x is " << x << " y is " << y << endl;
if (x == y)
cout << "strings are equal";
else
cout << "string are NOT equal";
return 0;
}