1. Pointers and References
Timothy Budd
9/18/2018
C++ for Java Programmers

2. C++ for Java Programmers
Pointers
Powerful Objected-Oriented mechanisms are possible due to the indirection provided through the use of pointer values.
Often it is said that Java has no pointers, actually, everything is represented internally by pointer values.
In C++, the use of pointers is explicit and must be directly manipulated in code.
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9/18/2018
C++ for Java Programmers

3. C++ for Java Programmers
Java Pointers
class box { // Java box public int value; }
box a = new box();
a.value = 7; // set variable a
box b;
b = a;
Because internally reference the same value, changes to either a or b will be reflected in the other variable.
9/18/2018
C++ for Java Programmers

4. C++ for Java Programmers
new box() a
new box() b
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C++ for Java Programmers

5. C++ for Java Programmers
Pointers on Pointers
Pointers is simply a variable that maintain as a value the address of another location in memory.
The reasons for using pointer values
A single pointer variable must reference a variety of different values over the course of execution.
A pointer will reference only a single value, but the particular value it will reference cannot be known at run time.
The amount of memory necessary to hold a value cannot be determined at compile time, and must be allocated a run-time.
9/18/2018
C++ for Java Programmers

6. C++ for Java Programmers
Pointers on Pointers p *p
9/18/2018
C++ for Java Programmers

7. C++ for Java Programmers
Null Pointer
A null pointer is a value that does not reference any other memory location.
A null pointer is analogous to an uninitialized variable in Java.
A pointer can be tested for equality to the value zero to determine whether it is a null pointer.
9/18/2018
C++ for Java Programmers

8. C++ for Java Programmers
4 Principal Mechanisms
Can be explicitly dereferenced using the unary * operator. If p is a variable holding a pointer to a value, then *p is the value addressed by the pointer.
A pointer to a structure, or class, can combine pointer dereferencing and member field extraction using the pointer operator. p  x is the same as (*p).x
Can be subscripted. Useful only if the pointer addresses an array of objects. The index is used to determine the element accessed by the expression.
An integer value can be added to or subtracted from a pointer in order to yield a new pointer. Assumed that the pointer references an array of values.
9/18/2018
C++ for Java Programmers

9. The Address-of Operator
The address-of operator converts a name into a pointer.
int i; // location for final value int *p; // pointer variable p = & i; // set p to point to i scanf("%d", p); // scan number into i
Address-of operator can be applied directly in arguments.
int i; // location for final value
scanf("%d", &i); // scan number into i
9/18/2018
C++ for Java Programmers

10. Pointers to Simple Values
Two major operations when a pointer is referencing a primitives
Pointer vale to another pointer
To dereference the pointer value
int i = 7; int j = 11; int *p = & i; // set p to point to i *p = *p + 3; // i now has the value 10
Pointers should be compared only for equality.
9/18/2018
C++ for Java Programmers

11. Pointers on Simple Values
Difference between modifying a pointer value and modifying the value that a pointer refers to.
p = & j; // change p to point to j
9/18/2018
C++ for Java Programmers

12. Referencing a deleted value
Nothing prevents a pointer from referencing a deleted value.
int * p; // global pointer variable void Set () {
int i; // local variable i = 7; // give i a value p = & i; // set p to point to it } void Use () {
double d; d = 3.0; d += *p; // use the value p points to }
9/18/2018
C++ for Java Programmers

13. C++ for Java Programmers
Pointers to Pointer
A pointer to a value that is itself a pointer is declared using multiple levels of * symbols.
int main (int argc, char ** argv) { cout << "name of program " << **argv << '\n'; return 0; }
9/18/2018
C++ for Java Programmers

14. C++ for Java Programmers
Pointers and const
Modifier const indicates whether it is the pointer itself or the value it points to that is constant.
int i = 7; const int * p = &i; // pointer to a constant int * const q = &i; // constant pointer *p = 8; // not allowed, p points to a const *q = 8; // allowed, q is pointing to non const p = q; // allowed, p itself is not constant q = p; // not allowed q is constant
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C++ for Java Programmers

15. C++ for Java Programmers
void * Pointers
A void pointer can reference any type of value.
double d; double * dp = & d; void * p = dp;
A void * parameter must always be cast before it can be used.
double * dp2; dp2 = (double *) p; // convert p back into pointer to double
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C++ for Java Programmers

16. C++ for Java Programmers
Pointers to Functions
A function pointer can be invoked without the deference operator.
double fdiv (int i, int j) { return i / (double) j; } double (*fptr) (int, int); // declare variable fptr fptr = & fdiv; // assign value
double x = fptr(7, 14); // call ftpr directly double x = (*fptr) (7, 14); // dereference ftpr and call
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C++ for Java Programmers

17. C++ for Java Programmers
Pointers to Functions
double values[100];
int comp (void * a, void * b)
{ double * d1 = (double *) a; double * d2 = (double *) b; return (*d1) < (*d2); }
qsort (values, 100, sizeof(double), &comp);
Avoid qsort in code; use the STL routines instead.
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C++ for Java Programmers

18. C++ for Java Programmers
Pointers to Structure
The arrow operator is a combination of dereferencing and field access.
struct link { int value; link * next; // pointer to next link in chain }; link finalElement; // declare a single default element link * firstLink = & finalElement;
// set pointer to initially refer to this (*firstLink).value = 7; // these two statements firstLink->value = 7; // have the same effect
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C++ for Java Programmers

19. C++ for Java Programmers
Pointers to Structure
for (link * p = aList; p != &finalElement; p = p->next)
cout << *p << " ";
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C++ for Java Programmers

20. C++ for Java Programmers
Pointers to Arrays
Pointers can be subscripted just like arrays.
int values[100]; int * p = values;
// legal, as values is converted into a pointer
p[4] = 7; // references same value as values[4]
Neither pointer not array index values are checked to ensure they are in range.
p[310] = 7; // index value too large p[-4] = 12; // index value too small
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C++ for Java Programmers

21. C++ for Java Programmers
Pointer Arithmetic
It is legal to perform arithmetic on pointers.
char * text = " ... some text "; // p++ advances pointer to next location for (char * p = text; *p != '\0'; p++) if (isVowel(*p)) cout << "vowel value is " << *p << "\n";
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C++ for Java Programmers

22. C++ for Java Programmers
Reference
A reference is an alias, an alternative way to name an existing object.
Difference between reference and pointers
A reference can never be null; it must always refer to a legitimate object.
Once established, a reference can never be changed to make it point to a different object.
A reference does not require any explicit mechanism to dereference the memory address and access the actual data value.
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C++ for Java Programmers

23. C++ for Java Programmers
References
A reference is declared by using the ampersand.
int i = 7; int & j = i; // j is an alias for i j++; // i is now 8 i += 3; // i is now 11, as is j
A reference can be target of an assignment. Some functions will return a reference as a result for precisely this reason.
int values[100]; int & index(int i) { return values[i + 2]; } index(27) = 12; // changes values[29];
9/18/2018
C++ for Java Programmers

24. Pass by Reference Parameters
The most common use of reference is in parameter passing. A reference parameter is an alias for the corresponding actual argument value.
void passTest (int & i) { i++; i = 7; } int main ( ) { { int j = 5; passTest(j); cout << j << '\n'; return 0; }
9/18/2018
C++ for Java Programmers

25. Pass by Reference Parameters
None of the parameter passing options in C++ matches the Java semantics.
static void passTest (box i) { i.value++; i = new box(7); } public static void main (String [ ] args) { box j = new box(5); passTest(j); System.out.println("J is " + j.value); }
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C++ for Java Programmers

26. C++ for Java Programmers
References as Results
References can also be used as a result type for a function.
2 reasons for doing so:
A reference can be used as the target of an assignment. Therefore, a function call that returns a reference can be used on the left side of an assignment.
Returning a reference is more efficient than returning a value. Therefore, large values can be returned by reference.
9/18/2018
C++ for Java Programmers

27. Example of Reference as Result
class string { …..
char & operator [ ] (unsigned int index) { return buffer[index]; } ……
private: char * buffer; };
string text = "name:"; text[0] = 'f'; // change name to fame
double & min (double data[ ], int n) { double minVal = data[0]; for (int i = 1; i < n; i++) if (data[i] < minVal) minVal = data; return minVal; // error, reference to local }
9/18/2018
C++ for Java Programmers