C for Java Programmers Chapter 2 Fundamental Data

C++ for Java Programmers Chapter 2. Fundamental Data Types Timothy Budd

Comments Java // Comment thru end of line /* Multi line comments extend until the final */ C++ can use both styles of comments C uses only the second style 2

Integers n Java Integer Internal Representation • short - 16 bit • integer -32 bit • long - 64 bit short int x; // declare x as a small integer long y; // declare y as long integer n C++ Integer Internal Representation • long and/or short may have the same size as integer • int is usually the size of native target machine 3

C++ Integer n An unsigned integer can only hold nonnegative values int i = -3; unsigned int j = i; cout << j << endl; // will print very large positive integer n n Assigning a negative value to an unsigned variable is confusing (but legal) Integer division involving negative numbers is platform dependent, but following equality must be preserved: a == (a / b) * b + a % b 4

Integers n Never use the remainder operator with negative values. n unsigned long a; // for largest integer values signed short int b; // for smallest integers INT_MAX, INT_MIN, SHRT_MAX, etc. are constants which define the limits n C++ does not recognize the Byte data type in Java. Instead signed char is often used to represent byte 5 sized quantities.

Characters 8 bit quatity n Legal to perform arithmetic on characters n Character can be signed or unsigned. n w_char - recent addition wide character alias for another interger type such as short. (UNICODE > 1 byte) n 6

Booleans u Recent addition - bool u. Historical boolean representation unonzero – true (usually 1 or -1) uzero - false u Integer and pointer types can be used as boolean values. u Cannot be signed or unsigned. 7

Examples of Booleans 8

Booleans n Even pointer values can be used as boolean False if it is null, true otherwise. a. Class * a. Ptr; // declare a pointer variable. . . if (a. Ptr) // will be true if a. Ptr is not null n Legacy code can contain different boolean abstractions. 9

Bit Fields Seldom used feature n Programmer can specify explicitly the number of bits to be used. n struct info. Byte { int on: 1; // one-bit value, 0 or 1 int : 4; // four bit padding, not named int type: 3; // three bit value, 0 to 7 }; 10

Bit Fields: Practical Example n n n Frequently device controllers and the OS need to communicate at a low level. Example: Disk Controller Register We could define this register easily with bit fields: struct DISK_REGISTER { unsigned ready: 1; unsigned error_occured: 1; unsigned disk_spinning: 1; unsigned write_protect: 1; unsigned head_loaded: 1; unsigned error_code: 8; unsigned track: 9; unsigned sector: 5; unsigned command: 5; }; 11

Floating Point Values n float, double, long double int i; double d = 3. 14; i = d; // may generate a warning Never use float; use double instead. n math routines generally will not throw an exception on error n 12

Floating Point Values n Always check errno double d = sqrt(-1); // should generate error if (errno == EDOM). . . // but only caught if checked n Java: Nan, NEGATIVE INFINITY, POSITIVE INFINITY 13

Enumerated Values Nothing in commonwith Enumeration class in Java n enum declaration in C++ n enum animal {dog, cat, horse=7, cow}; enum color {red, orange, yellow}; enum fruit {apple, pear, orange}; // error: orange redefined 14

Enumeration Values u Can be converted into integers and can even have their own internal integer values explicitly specified. enum shape {circle=12, square=3, triangle}; n Can be assigned to an integer and incremented, but the resulting value must then be cast back into the enumrated data type before fruit a. Fruit = pear; int i = a. Fruit; // legal conversion i++; // legal increment a. Fruit = fruit(i); // fruit is probably now orange i++; a. Fruit = fruit(i); // fruit value is now undefined 15

Type Casting Cast operation can be written by type(value) or older (type)value syntax. n Not legal to change a pointer type. n int* i; char* c; // same as int *i; c = char* (i); // error: not legal syntax n static_cast would be even better. double result = static_cast<double>(4)/5; 16

The void type In Java, used to represent a method or function that does not yield a result. n In C++, type can also be used as a pointer type to describe a “universal” pointer that can hold a pointer to any type of value. n Similar to Object in Java n 17

Arrays An array need not be allocated by using new directive as in Java. n The number of elements determined at compile time. n int data[100]; // create an array of 100 elements n The number of elements can be omitted. char text[ ] = "an array of characters"; int limits[ ] = {10, 12, 14, 17, 0}; 18

Arrays n Not legal to place the square brackets after type as in Java double[ ] limits = {10, 12, 14, 17, 0}; // legal Java, not C++ n The size can be omitted when arrays are passed as arguments to a function. // compute average of an array of data values double average (int n, double data[ ] ) { double sum = 0; for (int i = 0; i < n; i++) { sum += data[i]; } return sum / n; } 19

Structures & Classes Struct Class Members public by default Members private by default In C, data members only In C, no classes struct my. Struct // holds an int, a double, AND a pointer { int i; double d; an. Object * p; }; 20

Unions n Similar to a structure, but the different data fields all share the same location in memory. // can hold an int, a double, OR a pointer union my. Union { int i; double d; an. Object * p; }; n Object-oriented languages made unions unnecessary by introducing polymorphic variables 21

Object Values Java uses reference semantics C++ uses copy semantics. class box { // Java box public int value; } class box { // C++ box public: int value; }; box a = new box(); box b; box a; // note, no explicit allocation box b; a. value = 7; // set variable a b = a; // assign b from a a. value = 12; // change variable a System. out. println("a value “+ a. value); System. out. println("b value “+ b. value); a. value = 7; b = a; a. value = 12; cout << "a value " << a. value << endl; cout << "b value " << b. value << endl; // a & b refer to the same object // a & b are different objects 22

Reference Variables (alias) JAVA box a = new box(); box c = new box(); // java reference assignment box b = a; // reassignment of reference b = new box(); C++ box a; box c; // C++ reference assignment box & b = a; // error: not permitted to reassign reference b = c; 23

Functions n C++ permits the definition of functions (and variables) that are not members of any class. // define a function for the maximum of two integer values int max (int i, int j) { if (i < j) return j; return i; } int x =. . . ; int y =. . . ; int z = max(x, y); 24

Functions n Prototypes are necessary in C++ as every function name with its associated parameter types must be known to the compiler. // declare function max defined elsewhere int max(int, int); 25

Order of Argument Evaluation n In Java, argument is evaluated from left to right. String s = "going, "; print. Test (s, s, s = "gone "); void print. Test (String a, String b, String c) { System. out. println(a + b + c); } n In C++, order of argument evaluation is undefined and implement dependent (usually right to left) 26

The function main n n In C++, main is a function outside any class. Always return zero on successful completion of the main program. int main (int argc, char *argv[ ]) { cout << "executing program " << argv[0] << 'n'; return 0; // execution successful } n n The first command line argument in C++ is always the application name. A lot of old legacy code uses: void main() 27

Altenative main Entry points Individual libraries may provide their own version of main and then require a different entry point. n Many Windows graphical systems come with their own main routine already written, which will perform certain initializations before invoking a different function such as Win. Main. n 28

C/C++ compilers Visual Studio n Borland C++ Builder n Linux cc, c++, gcc, g++ n • gcc = cc C programs only • g++ = c++ C or C++ programs • Actually all 4 compilers are the same programs making different assumptions based on input file name or contents n Dev-C++ a good g++ compiler for PC’s 29

Linux G++ Command Syntax g++ filename Input file should have extension. c, . cxx, . cpp, . c++ Usually C-programs - . c C++ programs -. cpp Although g++ is pretty smart at figuring it out regardless of the extension Most other compilers are less tolerant 30

Linux G++ (cont) Output file by default is a. out n Output filename can be specified with –o g++ -o outfilename. cpp n 31

Simple Programs JAVA public class Hello. World { public static void main(String[] args) { System. out. println("Hello World"); } } C #include <stdio. h> void main (int argc, char argv[]) { printf(“Hello Worldn); } 32

Simple Programs C C++ #include <stdio. h> #include <iostream> using namespace std; void main () { printf(“Hello Worldn”); } void main () { cout << “Hello World” << endl; } 33

Actually some would consider a void main function bad form, so … C C++ #include <stdio. h> #include <iostream> using namespace std; int main () { printf(“Hello Worldn”); return 0; } int main () { cout << “Hello World” << endl; return 0; } 34