Pointers and References

Pointers and References Timothy Budd C++ for Java Programmers

Java Primitive & Reference Types Primitive types: Numeric – byte, short, long, float, double Boolean Char Reference types: String Array Class C++ for Java Programmers

Java Primitive & Reference Types Primitive type variables contain the values of the things they represent. int x; x Reference type variables contain references to (addresses of) the objects they represent. boxbox b; b 27 box data 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 both a and b 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

C++ Primitive & Reference Types Primitive types: Numeric – byte, short, long, float, double Boolean Char Class & Struct Reference types: Array String (simple strings implemented by arrays) C++ for Java Programmers

Pointers on Pointers • A pointer is simply a variable that holds as its 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 compile time or at the beginning of execution. • 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 or the constant NULL 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 ( p[5] ) 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 (p + 5) This use assumes 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 as above C++ for Java Programmers

Pointers to Simple Values • Two major operations when a pointer is referencing a primitive • To dereference the pointer value int i = 7;int j = 11;int *p = & i; // set p to point to iint *q = & j; // set q to point to j*p = *p + 3; // i now has the value 10 • Change pointer value to another pointer value p = q; // i now has the value 10 • Pointers should be compared only for equality. if (p == q) … while (p !=q) … 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 p = q; // now p points to different object 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 (Note: i is local) p = & i; // set p to point to it } void Use () { double d; d = 3.0; d += *p; // use the value p points to ?????? // i no longer exists, memory recycled } C++ for Java Programmers

Value vs. Reference Parameters Pass by Value Pass by Reference // statement in main() swap(x, y); void swap (int &a, int &b) { int temp; temp = a; a = b; b = temp; } // statement in main() swap(x, y); void swap (int a, int b) { int temp; temp = a; a = b; b = temp; } C++ for Java Programmers

Value vs. Reference Parameters Pass by Value Pass by Reference Address of actual parameter is passed to function Only variables (things with addresses can be passed Modifying parameter affects the original variableExternal side effects Efficient for large structures(only address is passed) • Copy of actual parameter is passed to function • Variable, constants, expressions can be passed • Modifying parameter affects only local copySafe–no external side effects • Inefficient for large structures (must make local copy) C++ for Java Programmers

C has only value parameters(but you can cheat) Pass by Reference Pass by Reference (sort of) // statement in main() swap(&x, &y); void swap (int *a, int *b) { int temp; temp = *a; *a = *b; *b = temp; } // statement in main() swap(x, y); void swap (int &a, int &b) { int temp; temp = a; a = b; b = temp; } C++ for Java Programmers

C has only value parameters(Ooops – except for arrays) Arrays are always passed by reference Saves time & space Array names are base addresses int a[100]; a == &a[0]; Java has only value parameters … butwhen passing a reference type you are actually passing the reference (address) by valueso it works just like “call by reference” 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

Pointers to Pointers • A pointer to a value that is itself a pointer can be 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 int * const q = &i; // constant pointer *p = 8; // not allowed, p points to a const int *q = 8; // allowed, q is pointing to non const int 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 -> value << " "; 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. • A pointer is a variable that can contain a reference 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++; } 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 structures 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; // danger, reference to local which will soon be recycled } C++ for Java Programmers

Pointers and References