CS 117 Spring 2002

More Classes

IEEE Student Conference

• Student Professional Awareness Conference– Paul Kostek: career options for 21st Century– Panel discussion by local companies

• April 4, 2002 • Lookout room in SUB• email to Ben Millemon ieee@boisestate.

edu• posters in MEC and ET buildings

4/1/02

• exam 3 on April 10– files– arrays– strings– classes

Random numbers

• use the rand function from stdlib.h– int rand() returns an integer from 0 to

RAND_MAX (a big number)– the sequence you get is always the same

• use % to get numbers from 1 to 6dieValue = 1 + rand() % 6;

• randomize.h

User Defined Types

• Define what data we want in the class– what data is needed to represent the class

• Create the functionality to operate on the data– class member functions provide access to data

and operations that can be done on or with it

Money Class

• Two attributes– dollars– cents

• Behavior– decimal value– addition– comparison– input/output

Money.h

class Money {public:

Money() {dollars = cents =0;}Money( double);double decValue() const;

private:int dollars, cents;

// overloaded operators

};

Money Class

• Allocate a Money object– money unitPrice( 5000.00);

• Assignments– taxAmount = salePrice * taxPercent/100.0;– itemPrice = unitPrice * quantity;– finalCost = itemPrice + taxAmount;

Objects and Functions

• class types can be used as both parameters and return types for functions– use C ob1 to pass by value

• a copy is made (copy constructor)

• not efficient for big objects

– Use C& ob1 as formal reference argument– Use const C& ob1 to specify object can’t be

modified by a function

Objects as Operands

• Assignment operator has a default definitionob2 = ob1;

• Others must be "overloaded"– >> for input– << for output– arithmetic if appropriate– comparison

Object Input and Output

• Often need to be able to – read information to initialize an object– print out information about an object

• Reading/Printing the information from your program or another class is contrary to the goal of encapsulation

• Define functions for input and output in the class

Approaches

• Write member functions for each operator needed– read(), print(), equals(), lessThan(), ...

• Overload the appropriate operators– consistency with primitive variables

Class functions

• two approaches • a.lessThan (b)

– A and b are objects that are compared by passing one object to the other objects member function lessThan

• lessThan2 (a, b)– Both objects are treated the same

– Must use a friend function to do this

Two ways to write less than

bool Money ::lessThan (const Money & m) const

{

return (decValue()<m.decValue());

}

bool lessThan2 (const Money& m1, const Money& m2)

{

return (m1.decValue() < m2. decValue());

}

Friends

• Second version of lessThan needs access to the private data of the class – but it isn't a member function so

• Make it a friend function– declared in the class declaration– allows access to private data– example: getline for strings

• classes can be friends too

Friend Function Declaration

• Form: (in class declaration)– friend result-type function-name (arg list);

– friend result-type operator opSymbol (arg list);

• Example:– friend bool equals (const Money& , const Money& );

– friend bool operator == (const Money& , const Money& );

Overloading Operators

• lets you use objects the way you do primitive types

• can be either member or friend functions

• prototypes are relatively fixed

string class

string s1, s2 = " a string", s3;

• can use << to print: cout << s2;

• can use >> to read: cin >> s1;

• can use + to concatenate: s1 + s2

• can use = to assign: s3 = "stuff"

• can use ==, <, etc. to compare

operator<

bool Money ::operator<(const Money & m) const

{

return (decValue()<m.decValue());

}

bool operator< (const Money& m1, const Money& m2)

{

return (m1.decValue() < m2.decValue());

}

Operators for Money Class

• want input and output – do it once so you don't have to write it again

every time you use the class

• addition makes sense for Money objects

• multiply by either a double or an int

Common Programming Errors

• Function argument & return value errors• Not defining a function as a member of a

class• Prefixing a class function call

– obj.function();

• Referencing a private attribute of a class• Missing header files• Missing ; on class definition