C++ for Java Programmers

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

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

3. Integers • 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 • C++ Integer Internal Representation • long and/or short may have the same size as integer • int is usually the size of native target machine

4. C++ Integer • An unsigned integer can only hold nonnegative values int i = -3; unsigned int j = i; cout << j << endl; // will print very large positive integer • 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

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

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

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

8. Examples of Booleans

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

10. Bit Fields • Seldom used feature • Programmer can specify explicitly the number of bits to be used. struct infoByte { 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 };

11. Bit Fields: Practical Example • 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; };

12. Floating Point Values • float, double, long double int i; double d = 3.14; i = d; // may generate a warning • Never use float; use double instead. • math routines generally will not throw an exception on error

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

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

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

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

17. The void type • In Java, used to represent a method or function that does not yield a result. • 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. • Similar to Object in Java

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

19. Arrays • Not legal to place the square brackets after type as in Java double[ ] limits = {10, 12, 14, 17, 0}; // legal Java, not C++ • 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 (inti = 0; i < n; i++) { sum += data[i];} return sum / n; }

20. Structures & Classes struct myStruct // holds an int, a double, AND a pointer { int i; double d; anObject * p; };

21. Unions • 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 myUnion { inti; double d; anObject * p; }; • Object-oriented languages made unions unnecessary by introducing polymorphic variables

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

23. 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;

24. Functions • 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 (inti, int j) { if (i < j) return j; return i; } int x = ...; int y = ...; int z = max(x, y);

25. Functions • 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);

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

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

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

29. C/C++ compilers • Visual Studio • Borland C++ Builder • Linux cc, c++, gcc, g++ • 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 • Dev-C++ a good g++ compiler for PC’s

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

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

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

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

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