defining new data types in c++

7/29/2019 Defining New Data Types in c++

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Defining new data types in C++

Part 1: enums & structs


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Enumerated types

An enumeration is a data type in which labels for

all possible values of the type can be listed

The type declaration consists of the keywordenumfollowed by the name of the new type and a

block of code in which labels for all values of the

type are listed

Syntax:

enum NewTypeName {value1, value2, , valueN};


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Enumeration constants

The value labels listed in an enumeration arecalled enumeration constants

Enumeration constants must be valid C++

identifiers; they are not string or char literals Enumeration constants are stored in memory as

integers; by default, the first is assigned the value0, the second 1, etc.

Thus the order of the listing determines therelative magnitude of enumeration constantvalues; the first is less than the second, which isless than the third, and so forth


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Specifying different values in

enumerations By default, the first enumeration constant

has the value 0, the second 1, the third 2,

etc. The default behavior can be overridden by

assigning explicit values to one or more ofthe constants

The next several examples illustrate thisoption


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Example 1 (enum)

The following enumeration uses explicit

assignment to specify values for the

symbols used in the Roman numeralsystem:

enum RomanNum { I = 1, V = 5, X = 10, L = 50,

C = 100, D = 500, M = 1000};


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Example 2 (enum)

The following enumeration type creates constants

that stand for the months of the year:

enum MonthType {JAN=1, FEB, MAR, APR, MAY,JUN, JUL, AUG, SEP, OCT, NOV, DEC};

Only the first constants value is specified; since it

is 1, the second is 2, the third is 3, and so on until

the last (DEC) with the value 12


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Using enumerated types

Enumerations only create new data types; toactually store and use values of the the new types,you must declare variables

Variables of each enum type can hold only thosevalues specified by the enumeration

For example, with the MonthType enumeration,you could declare variables and assign them

values like the following:MonthType thisMonth = APR;

MonthType nextMonth = MAY;

MonthType birthMonth = nextMonth;


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Operations on enumerations

Since enumerations are not built-in data types, only someof the most common operations can be performed usingvariables of these types

The allowable operations include: logical comparison using the relational operators (, =, ==,!=)

simple arithmetic (but not arithmetic/assignment operations like ++or --)

enumerations can be parameters to, and/or return values from,

functions enumerations can be used as switch expressions and/or case labels

in switchesexample on next slide


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MonthType thisMonth;

switch ( thisMonth ) // using enum type switch expression

{ case JAN :

case FEB :

case MAR : cout


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Incrementing enum variables using

type cast mechanism The operators ++ and -- are not available for use

with enum-type variables, as previously noted

However, enum-type variables can appear inmixed-type expressions with, for example,

integers

This provides a mechanism for

increment/decrement of enumeration type

variables


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Using enum type Control

Variable with for LoopMonthType month ;

for (month = JAN ; month


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Simple arithmetic with enumeration

constants Previously, we defined an enumeration of

the symbols used in the Roman numeral

system

We will use this enumeration to illustrate

arithmetic with enums, and see another

example of type casting


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Example#include

#include

enum RomanNum {I=1, V=5, X=10, L=50, C=100, M=1000};

int main(){

cout


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Enumerated types and I/O

The insertion ()

operators are not defined for enum-type

variables

To perform input or output operations on

user-defined types, you must provide your

own functions


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Databases

A database is a collection of informationorganized for ease of storage and retrieval

A library catalog is a good example of a database;such a catalog contains information about eachbook in the librarys collection

Although each book is unique, books share

common characteristics, such as author, title,publisher, etc.

We store information in the database based onthese common characteristics


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Database components

At the macroscopic level, a database consists of one ormore filescontaining information

The information in the files is stored in the form of

records; in our example, each record would contain all ofthe information concerning a specific book Within the records, individual data items are stored in

fields; for library books, each record would have separatefields for author, title, publisher, etc.

All the records contain the same set of fields; they differ inthe actual data stored in the fields


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Record structures

If we wanted to create our own library database, we wouldwant to consider how people use books, and whatinformation they might want to look up

We could come up with a preliminary list of fields toinclude, such as: Author

Title

Subject(s)

Call number (the Dewey Decimal or Library of Congressclassification that determines where the book is shelved)

ISBN (for those who delight in reading bar codes

and many more, but this is enough to start with


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Record structures in C++

The C++ mechanism for creating a recorddata type is the struct

A struct consists of a block of codecontaining list ofmember variabledeclarations

The member variables represent the fieldswhere data can be stored when variables ofthe struct type are declared


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Syntax for struct declarationstruct StructureName

{

dataType field1;

dataType field2;

.

.

.

dataType fieldn;

};

StructureName must be a

valid identifier

Member variable declaration

looks the same as any other

variable declaration but the

struct declaration, including

declaration of member

variables, doesnt allocateany memory must declare

variables of the new type to

allocate memory


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Example

struct BookRecord

{

string title;string author;

string subject1, subject2, subject3;

string callNo;

string ISBN;

};


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Storing information in records

A struct declaration, like the previous example,just provides a blueprint for a recordit doesntallocate memory, so there isnt anyplace to storedata

To create an actual record, we need to declare avariable of the struct type; example:

BookType myBook, yourBook;

This statement allocates memory for twoBookType records; the next step is to assignvalues to these variables


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Assigning data to struct variables

In order to assign data to a struct variable, we

must first access the individual member variables

We do this by using the member-selectionoperator, or dot operator

This is the same operator we use to call member

functions, and for good reason: in both instances,

we are accessing a member of a larger structure in

order to perform some operation


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Example

myBook.title = Seabiscuit;myBook.author = Hillenbrand, Laura;

myBook.subject1 = Horse racing;

myBook.subject2 = Great Depression 1929-1939, U.S.;

myBook.subject3 = Biscuits, Aquatic;

myBook.ISBN = 0-345-46508-3;

myBook.callNo = 939.41 H42;

yourBook.title = Geeks;

yourBook.author = Katz, Jon;

yourBook.subject1 = Computer technicians U.S.case studies;

yourBook.subject2 = Electronic data processing personnel U.S.case studies;yourBook.ISBN = 0-375-50298-X;

yourBook.callNo = TK7885.54 K38;


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Memory allocation and structs

Memory for struct-type is allocated when avariable is declared

Each struct variable contains all of themember variables listed in the structdeclaration

You can think of a struct variable as a singlebox with several compartments, each ofwhich can contain a data item


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Operations on struct variables

The operations that can be performed on

individual member of a struct variable depend on

the members data type For example, all of the following statements are

valid for a BookType variable:

getline(cin, myBook.title);

cout


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Aggregate operations

An aggregate operation is an operation performed on anentire struct variable (as opposed to one of its membervariables)

The following are allowable aggregate operations on structvariables: assignment to another struct variable of the same type (example:

yourBook = myBook;)

pass to a function as an argument (either by value or by reference)

use as return value from a function

On the other hand, the following aggregate operations areNOT allowed: I/O, arithmetic, logical comparison


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Notes on structs as parameters

As previously mentioned, we can pass a struct

variable as a value parameter

Although this operation is permitted, it is costly:pass by value requires copying original argument to

new memory location

struct variables are rather large objects to copy

It is therefore much more common to pass structvariables by reference, because then only a single

copy of the large object is held in memory


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A third way: const reference

parameters The advantage of pass by value is the guarantee

that an arguments value will not be changed by

the action of a function Passing by reference could lend itself to side

effects

Fortunately, there is an alternative that combines

the advantages of passing by value and passing by

reference: passing by const reference


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Using const reference parameters

To pass by const reference, label your parameter with theampersand (&), as usual, but add the keyword const beforethe parameters data type

For example, suppose you had a function for checking outbooks that took a BookType variable as its argument; thisfunction could take either a value of const reference

parameter:

void checkout (BookType b); // value parameter; must

// copy whole object

void checkout (BookType &b); // const ref parameter;

// uses original memory,

// but doesnt change it


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Hierarchical structures

A struct can contain an enum type as one of

its member variables

It is also possible for a struct to contain amember variable of a struct type

Such an arrangement makes possible a

greater level of detail in each record


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Example: struct MachineRec Information about each machine in a shop

contains:

an idNumber,

a written description,

the purchase date,

the cost,

and a history (including failure rate, number of

days down, and date of last service).


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struct DateType

{ int month ; // Assume 1 . . 12

int day ; // Assume 1 . . 31

int year ; // Assume 1900 . . 2050

};

struct StatisticsType

{ float failRate ;

DateType lastServiced ; // DateType is a struct type

int downDays ;} ;

struct MachineRec

{ int idNumber ;

string description ;

StatisticsType history ; // StatisticsType is a struct type

DateType purchaseDate ;

float cost ;

} ;


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struct type variable machine

7000

.idNumber .description . history .purchaseDate .cost

.month .day .year

5719 DRILLING 3 21 1995 8000.0

.failrate .lastServiced .downdays

.02 1 25 1999 4.month .day .year

machine.history.lastServiced.year has value 1999

defining new data types in c++

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