Pointers and References

Pointers and References Timothy Budd 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. 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. C++ for Java Programmers

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

Pointers on Pointers p *p 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. 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. C++ for Java Programmers

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 C++ for Java Programmers

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. C++ for Java Programmers

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 C++ for Java Programmers

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 } 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; } 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 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 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 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. 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 elementlink * firstLink = & finalElement; // set pointer to initially refer to this(*firstLink).value = 7; // these two statementsfirstLink->value = 7; // have the same effect C++ for Java Programmers

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

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; } C++ for Java Programmers

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); } 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. C++ for Java Programmers

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 } C++ for Java Programmers

Pointers and References