C++ for Java Programmers

C++ for Java Programmers

Chapter 2. Fundamental Daty Types

Timothy Budd

Timothy BuddTimothy Budd 22

Integers

Java Integer Internal Representation• long - 32 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


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 negaitve value to an unsigned variable is confusing

Integer division involving negative numbers is platform dependent, but following equality must be preserved: a == (a / b) * b + a % b


Integers

Never use the remainder operator with negative values.

unsigned long a; // can hold largest integer value

signed short int b;

C++ does not recognize the Byte data type in Java. Instead signed char is often used to represent byte-sized quantities.


Characters

8 bit quatity - Legal to perform arithmatic on characters Character can be signed or unsigned. w_char - recent addition wide character

alias for another interger type such as short.


Booleans

Recent addtion - bool Historical boolean representation

nonzero - true zero - false

Integer and pointer types can be used as boolean values.

Cannot be signed or unsigned.


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


Booleans

Even pointer value can be used as booleans. 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.


Bit Fields

Seldome 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

};


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 rountines will not throw an exception

on error


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


Enumerated Values

Nothing in commonwith Enumeration calss in Java

enum declaration in C++

enum color {red, orange, yellow};

enum fruit {apple, pear, orange};

// error: orange redefined


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


Enumeration Values

Cast operation can be written by type(value) Cast operation can be written by type(value) or older (type)value syntax.or older (type)value syntax.

Not legal to change a pointer type.Not legal to change a pointer type.

int * i;char * c;

c = char *(i); // error: not legal syntax

static_cast would be even better.


The void type

In Java, used to represent a method or function that does not yield a result.

In C++, type can also be uses as a pointer type to describe a “universal” pointer that can hold a pointer to any type of value.


Arrays

An array need not be allocated by using new directive as in Java.

The number of element determined at compile time.

int data[100]; // create an array of 100 elements

The number of element can be omitted.char text[ ] = "an array of characters";

int limits[ ] = {10, 12, 14, 17, 0};


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 limit 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; }


Structure

The major differences in C++ between a strunct and a class is that the access is by default public rather than private as in classes.

// holds an int, a double, AND a pointer

struct myStruct {

int i;

double d;

anObject * p;

};


Unions

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

Java uses reference semantics for assignmentclass 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);


Object Values

C++ uses copy semantics.class box {class box { // C++ box// C++ boxpublic:public:

int value;int value;};};

box a; // note, explicit allocation not requiredbox a; // note, explicit allocation not requiredbox 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 << endl;cout << "b value " << b.value << endl;


Object Values

The concept of reference variable in C++, which is a variable declared as a direct alias.

box a = new box(); // java reference assignmentbox a = new box(); // java reference assignment

box b = a;box b = a;

b = new box();b = new box(); // reassignment of reference// reassignment of reference

box a; box a; // C++ example// C++ example

box & b = a;box & b = a; // reference assignment// reference assignment

box c;box c;

b = c;b = c; // error: not permitted to reassign reference// error: not permitted to reassign reference


Functions

C++ permits the definition of function that are not member 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);


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);


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.undefined and implement dependent.


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.


Altenative main Entry points

Individual libraries may provide threir 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++ for Java Programmers

Documents

Transcript of C++ for Java Programmers