C++ for Java Programmers

Chapter 2. Fundamental Data Types

Timothy Budd

CommentsComments

JavaJava

// Comment thru end of line// Comment thru end of line

/* Multi line comments /* Multi line comments extend until the final */extend until the final */

C++C++

C++ can use both C++ can use both styles of commentsstyles of comments

C uses only the C uses only the second stylesecond style

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Integers

Java Integer Internal Representation• short - 16 bit

• integer -32 bit

• long - 64 bitshort 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

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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

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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.

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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)

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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.

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Examples of Booleans

int i = 10;while (i) { // will loop until i is zero

...i--;

}

while (*p++ = *q++) ;

bool test = true;int i = 2 + test; // i is now 3test = test - 1; // test is now 0, or false

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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.

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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

};

Bit Fields: Practical Example Bit Fields: Practical Example Frequently device controllers and Frequently device controllers and

the OS need to communicate at a the OS need to communicate at a low level. low level.

Example: Example: Disk Controller Register Disk Controller Register We could define this register easily We could define this register easily with bit fields: with bit fields:

struct DISK_REGISTER struct DISK_REGISTER { { unsigned ready:1; unsigned ready:1;

unsigned error_occured:1; unsigned error_occured:1; unsigned disk_spinning:1; unsigned disk_spinning:1; unsigned write_protect:1; unsigned write_protect:1; unsigned head_loaded:1; unsigned head_loaded:1; unsigned error_code:8; unsigned error_code:8;

unsigned track:9; unsigned track:9; unsigned sector:5; unsigned sector:5; unsigned command:5; }; unsigned command:5; };

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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

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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

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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

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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

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Type Casting Cast operation can be written by Cast operation can be written by

type(value) or older (type)value syntax.type(value) or older (type)value syntax. Not legal to change a pointer type.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;

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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

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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};

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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 (int i = 0; i < n; i++) {

sum += data[i];}

return sum / n; }

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Structures & Classes

struct myStruct // holds an int, a double, AND a pointer {

int i;

double d;

anObject * p;

};

Struct Class

Members public by default Members private by default

In C, data members only In C, no classes

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Unions

Similar to a structure, but the different data fields Similar to a structure, but the different data fields all share the same location in memory.all share the same location in memory.

// can hold an int, a double, OR a pointer

union myUnion {

int i;

double d;

anObject * p;

};

Object-orienteObject-oriented languages made d languages made unions unions unnecessary by introducing unnecessary by introducing polymorphic polymorphic variablesvariables

Object Values

C++ uses copy semantics.

class box {class box { // C++ box// C++ boxpublic: int value;public: int value;

};};

box a; // box a; // note, no explicit allocationnote, no explicit allocationbox b;box b;

a.value = 7;a.value = 7;b = a;b = a;a.value = 12;a.value = 12;cout << "a value " << a.value << endl;cout << "a value " << a.value << endl;

cout << "b value " << b.value << endlcout << "b value " << b.value << endl;;// a & b are different objects // a & b are different objects

Java uses reference semantics

class box {class box { // Java box // Java box public int value;public int value;

}}

box a = new box();box a = new box();box b;box b;

a.value = 7; a.value = 7; // set variable a// set variable ab = a; b = a; // assign b from a// assign b from aa.value = 12; a.value = 12; // change variable a// change variable aSystem.out.println("a value “+ a.value);System.out.println("a value “+ a.value);System.out.println("b value “+ b.value);System.out.println("b value “+ b.value);

// a & b refer to the same object// a & b refer to the same object

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Reference Variables (alias)

JAVAJAVA

box a = new box(); box a = new box();

box c = new box();box c = new box();

// java reference assignment// java reference assignment

box b = a;box b = a;

// reassignment of reference// reassignment of reference

b = new box();b = new box();

C++C++

box a;box a;

box c;box c;

// C++ reference assignment // C++ reference assignment

box & b = a;box & b = a;

// error: not permitted to // error: not permitted to reassign referencereassign reference

b = c;b = c;2323

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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 valuesint max (int i, int j) {

if (i < j) return j;return i;

}int x = ...;int y = ...;int z = max(x, y);

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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);

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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 In C++, order of argument evaluation is undefined and implement dependent (usually undefined and implement dependent (usually right to left)right to left)

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The function main

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

}

The first command line argument in C++ is always the application name.

A lot of old legacy code uses: void main()

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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.

C/C++ compilersC/C++ compilers

Visual StudioVisual Studio Borland C++ BuilderBorland C++ Builder Linux cc, c++, gcc, g++Linux cc, c++, gcc, g++

• gcc = ccgcc = cc C programs onlyC programs only• g++ = c++g++ = c++ C or C++ programsC or C++ programs• Actually all 4 compilers are the same programs Actually all 4 compilers are the same programs

making different assumptions based on input file making different assumptions based on input file name or contentsname or contents

Dev-C++ a good g++ compiler for PC’sDev-C++ a good g++ compiler for PC’s

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Linux G++Linux G++

Command SyntaxCommand Syntax

g++ filenameg++ filename

Input file should have Input file should have extension .c, .cc, .cxx, .cpp, .c++extension .c, .cc, .cxx, .cpp, .c++Usually C-programs - .cUsually C-programs - .c C++ programs - .cpp C++ programs - .cppAlthough g++ is pretty smart at figuring it out Although g++ is pretty smart at figuring it out regardless of the extensionregardless of the extension

Most other compilers are less tolerantMost other compilers are less tolerant

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Linux G++ (cont)Linux G++ (cont)

Output file by default is a.outOutput file by default is a.out Output filename can be specified with –oOutput filename can be specified with –o

g++ -o outfilename filename.cpp g++ -o outfilename filename.cpp

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Simple ProgramsSimple Programs

JAVAJAVA

public class HelloWorldpublic class HelloWorld

{{

public static void main(String[] args)public static void main(String[] args)

{ {     System.out.println("Hello World");    System.out.println("Hello World");

}}

} }

CC

#include <stdio.h>#include <stdio.h>

void main (int argc, char argv[])void main (int argc, char argv[])

{{

printf(“Hello World\n);printf(“Hello World\n);

}}

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Simple ProgramsSimple Programs

CC

#include <stdio.h>#include <stdio.h>

void main ()void main ()

{{

printf(“Hello World\n”);printf(“Hello World\n”);

}}

C++C++

#include <iostream>#include <iostream>

using namespace std;using namespace std;

void main ()void main ()

{{

cout << “Hello World” << cout << “Hello World” << endl; endl;

}}

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Actually some would consider a Actually some would consider a void main function bad form, so …void main function bad form, so …

CC

#include <stdio.h>#include <stdio.h>

int main ()int main ()

{{

printf(“Hello World\n”);printf(“Hello World\n”);

return 0;return 0;

}}

C++C++

#include <iostream>#include <iostream>

using namespace std;using namespace std;

int main ()int main ()

{{

cout << “Hello World” << endl;cout << “Hello World” << endl;

return 0;return 0;

}}

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