6/10/2015C++ for Java Programmers1 Pointers and References Timothy Budd

6/10/2015C++ for Java Programmers 2 Java Primitive & Reference Types Primitive types: Numeric – byte, short, long, float, double Boolean Char Reference types: String Array Class

6/10/2015C++ for Java Programmers 3 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. box box b; b 27 box data

6/10/2015C++ for Java Programmers 4 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.

6/10/2015C++ for Java Programmers 5 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.

6/10/2015C++ for Java Programmers 6 a a b new box()

6/10/2015C++ for Java Programmers 7 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)

6/10/2015C++ for Java Programmers 8 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.

6/10/2015C++ for Java Programmers 9 Pointers on Pointers p*p

6/10/2015C++ for Java Programmers 10 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.

6/10/2015C++ for Java Programmers 11 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.

6/10/2015C++ for Java Programmers 12 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

6/10/2015C++ for Java Programmers 13 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 i int *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) …

6/10/2015C++ for Java Programmers 14 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

6/10/2015C++ for Java Programmers 15 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 }

Value vs. Reference Parameters Pass by Value // statement in main() swap(x, y); void swap (int a, int b) {int temp; temp = a; a = b; b = temp; } Pass by Reference // statement in main() swap(x, y); void swap (int &a, int &b) {int temp; temp = a; a = b; b = temp; } 6/10/2015C++ for Java Programmers 16

Value vs. Reference Parameters Pass by Value Copy of actual parameter is passed to function Variable, constants, expressions can be passed Modifying parameter affects only local copy Safe–no external side effects Inefficient for large structures (must make local copy) Pass by Reference Address of actual parameter is passed to function Only variables (things with addresses can be passed Modifying parameter affects the original variable External side effects Efficient for large structures (only address is passed) 6/10/2015C++ for Java Programmers 17

C has only value parameters (but you can cheat) Pass by Reference // statement in main() swap(x, y); void swap (int &a, int &b) {int temp; temp = a; a = b; b = temp; } 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; } 6/10/2015C++ for Java Programmers 18

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 … but when passing a reference type you are actually passing the reference (address) by value so it works just like “call by reference” 6/10/2015C++ for Java Programmers 19

6/10/2015C++ for Java Programmers 20 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; }

6/10/2015C++ for Java Programmers 21 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; }

6/10/2015C++ for Java Programmers 22 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

6/10/2015C++ for Java Programmers 23 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

6/10/2015C++ for Java Programmers 24 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

6/10/2015C++ for Java Programmers 25 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.

6/10/2015C++ for Java Programmers 26 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

6/10/2015C++ for Java Programmers 27 Pointers to Structure for (link *p = aList; p != &finalElement; p = p->next) cout value << " ";

6/10/2015C++ for Java Programmers 28 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

6/10/2015C++ for Java Programmers 29 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";

6/10/2015C++ for Java Programmers 30 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

6/10/2015C++ for Java Programmers 31 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];

6/10/2015C++ for Java Programmers 32 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; }

6/10/2015C++ for Java Programmers 33 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); }

6/10/2015C++ for Java Programmers 34 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.

6/10/2015C++ for Java Programmers 35 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 }