1 chapter 11 structured types, data abstraction and classes dale/weems
TRANSCRIPT
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Chapter 11
Structured Types,Data Abstraction and Classes
Dale/Weems
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Chapter 11 Topics
Meaning of a Structured Data Type Declaring and Using a struct Data Type C++ union Data Type Meaning of an Abstract Data Type Declaring and Using a class Data Type Using Separate Specification and
Implementation Files Invoking class Member Functions in Client
Code C++ class Constructors
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C++ Data TypesC++ Data Types
structured
array struct union class
address
pointer reference
simple
integral enum
char short int long bool
floating
float double long double
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Structured Data Type
A structured data type is a type in which each value is a collection of component items The entire collection has a single name Each component can be accessed individually Used to bundle together related data of various
types for convenient access under the same identifier
For example . . .
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thisAnimal
5000
.id 2037581
.name “giant panda”
.genus “Ailuropoda”
.species “melanoluka”
.country “China”
.age 18
.weight 234.6
.health Good
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anotherAnimal
6000
.id 5281003
.name “llama”
.genus “Lama”
.species “peruana”
.country “Peru”
.age 7
.weight 278.5
.health Excellent
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struct AnimalType
enum HealthType { Poor, Fair, Good, Excellent };
struct AnimalType // Declares a struct data type{ // does not allocate memory long id; string name; string genus; string species; struct members string country;
int age; float weight; HealthType health;};// Declare variables of AnimalType
AnimalType thisAnimal; AnimalType anotherAnimal; 7
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struct type Declaration
SYNTAX struct TypeName // Does not allocate memory {
MemberList };
MemberList SYNTAX
DataType MemberName;
DataType MemberName; . . .
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struct type Declaration
The struct declaration names a type and names the members of the struct
It does not allocate memory for any variables of that type!
You still need to declare your struct variables
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More about struct type declarations
Scope of a struct• If the struct type declaration precedes all functions, it
will be visible throughout the rest of the file • If it is placed within a function, only that function can
use it
It is common to place struct type declarations in a (.h) header file and #include that file
It is possible for members of different struct types to have the same identifiers; also a non-struct variable may have the same identifier as a structure member
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Accessing struct Members
Dot (period) is the member selection operator
After the struct type declaration, the various members can be used in your program only when they are preceded by a struct variable name and a dot
EXAMPLES
thisAnimal.weight
anotherAnimal.country
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Operations on struct Members
The type of the member determines the allowable operations
thisAnimal.age = 18;thisAnimal.id = 2037581;cin >> thisAnimal.weight;getline (cin, thisAnimal.species);thisAnimal.name = “giant panda”;thisAnimal.genus[0] = toupper(thisAnimal.genus[0]);thisAnimal.age++;
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Aggregate Operation
An aggregation operation is an operation on a data structure as a whole, as opposed to an operation on an individual component of the data structure
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Aggregate struct Operations
Operations valid on struct type variables are Assignment to another struct variable of the same
type Pass as an argument (by value or by reference)
Return as value of a function I/O, arithmetic, and comparisons of entire
struct variables are NOT ALLOWED!
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Aggregate struct Operations
anotherAnimal = thisAnimal; // Assignment
WriteOut(thisAnimal); // Value parameter
ChangeWeightAndAge(thisAnimal); // Reference parameter
thisAnimal = GetAnimalData(); // Function return value
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void WriteOut( /* in */ AnimalType thisAnimal)// Prints out values of all members of thisAnimal// Precondition: all members of thisAnimal are assigned// Postcondition:all members have been written out{ cout << “ID # “ << thisAnimal.id << thisAnimal.name << endl;
cout << thisAnimal.genus << thisAnimal.species << endl;
cout << thisAnimal.country << endl;
cout << thisAnimal.age << “ years “ << endl;
cout << thisAnimal.weight << “ lbs. “ << endl;
cout << “General health : “;
WriteWord (thisAnimal.health);}
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void ChangeAge(/* inout */ AnimalType& thisAnimal)
// Adds 1 to age// Precondition: thisAnimal.age is assigned // Postcondition:thisAnimal.age == // thisAnimal.age@entry + 1
{
thisAnimal.age++;
}
Passing a struct Type by Reference
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AnimalType GetAnimalData ()
// Obtains all information about an animal from keyboard// Postcondition:// Return value == AnimalType members entered at kbd{
AnimalType thisAnimal;
char response;
do
{
// Have user enter members until they are correct.
.
.
} while (response != ‘Y’);
return thisAnimal;
} 18
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Hierarchical Structures
The type of a struct member can be another struct type
This is called nested or hierarchical structures
Hierarchical structures are very useful when there is much detailed information in each record
For example . . .
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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 DateType purchaseDate;
float cost;};MachineRec machine;
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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
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Unions in C++
DEFINITION
A union is a struct that holds only one of its members at a time during program execution.
EXAMPLE
union WeightType
{
long wtInOunces; int wtInPounds; Only one at a time
float wtInTons;
};
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Using Unions
union WeightType // Declares a union type{ long wtInOunces; int wtInPounds; float wtInTons; };
WeightType weight; // Declares a union variable
weight.wtInTons = 4.83;
// Weight in tons is no longer needed// Reuse the memory space
weight.wtInPounds = 35;24
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Abstraction
Abstraction is the separation of the essential qualities of an object from the details of how it works or is composed
Focuses on what, not how
Is necessary for managing large, complex software projects
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Control Abstraction
Constrol abstraction separates the logical properties of an action from its implementation
Search (list, item, length, where, found);
The function call depends on the function’s specification (description), not its implementation (algorithm)
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Data Abstraction
Data abstraction separates the logical properties of a data type from its implementation
LOGICAL PROPERTIES IMPLEMENTATION
What are the possible values? How can this be done in C++?
What operations will be needed? How can data types be used?
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Data Type
set of values(domain)
allowable operationson those values
FOR EXAMPLE, data type int has
domain
-32768 . . . 32767
operations
+, -, *, /, %, >>, <<
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Abstract Data Type (ADT)
An abstract data type is a data type whose properties (domain and operations) are specified (what) independently of any particular implementation (how)
For example . . .
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ADT Specification Example
TYPETime
DOMAINEach Time value is a time in hours, minutes, and seconds.
OPERATIONSSet the timePrint the timeIncrement by one secondCompare 2 times for equalityDetermine if one time is “less than” another
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Another ADT Specification
TYPEComplexNumber
DOMAIN
Each value is an ordered pair of real numbers (a, b) representing a + bi
OPERATIONSInitialize the complex numberWrite the complex numberAdd SubtractMultiplyDivideDetermine the absolute value of a complex number
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ADT Implementation
ADT implementation Choose a specific data representation
for the abstract data using data types that already exist (built-in or programmer-defined)
Write functions for each allowable operation
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10 45 27
Several Possible Representations of ADT Time
3 int variables
3 strings
3-element int array
Choice of representation depends on time, space, and algorithms needed to implement operations
10 45 27
“10” “45” “27”
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Some Possible Representationsof ADT ComplexNumberstruct with 2 float members
2-element float array
-16.2 5.8
-16.2 5.8
.real .imag
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C++ Data TypesC++ Data Types
structured
array struct union class
address
pointer reference
simple
integral enum
char short int long bool
floating
float double long double
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class Time Specification// Specification file (Time.h)
class Time // Declares a class data type{ // does not allocate memory
public : // Five public function members
void Set (int hours , int mins , int secs); void Increment (); void Write () const; bool Equal (Time otherTime) const; bool LessThan (Time otherTime) const;
private : // Three private data members
int hrs; int mins; int secs;
};
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C++ classType
Facilitates re-use of C++ code for an ADT
Software that uses the class is called a client
Variables of the class type are called class objects or class instances
Client code uses class’s public member functions to manipulate class objects
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Client Code Using Time#include “time.h” // Includes specification of the classusing namespace std;
int main (){ Time currentTime; // Declares two objects of Time Time endTime; bool done = false;
currentTime.Set (5, 30, 0); endTime.Set (18, 30, 0); while (! done) { . . .
currentTime.Increment (); if (currentTime.Equal (endTime))
done = true; };}
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class type Declaration
The class declaration creates a data type and names the members of the class
It does not allocate memory for any variables of that type!
Client code still needs to declare class variables
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Remember … Two kinds of class members:
data members and function members
Class members are private by default
Data members are generally private
Function members are generally declared public
Private class members can be accessed only by the class member functions (and friend functions), not by client code
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Aggregate class Operations
Built-in operations valid on class objects are
Member selection using dot (.) operator ,
Assignment to another class variable using (=),
Pass to a function as argument
(by value or by reference),
Return as value of a function
Other operations can be defined as class member functions
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Separate Specification and Implementation
// Specification file “time.h”// Specifies the data and function membersclass Time{ public: . . .
private: . . .};
// Implementation file “time.cpp”// Implements the Time member functions { . . .
}
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Implementation File for Time// Implementation file “time.cpp”
// Implements the Time member functions. #include “ time.h” // Also must appear in client code#include <iostream>
. . .bool Time::Equal(/* in */ Time otherTime) const // Postcondition: Return value == true, // if this time equals otherTime,// otherwise == false{ return ((hrs == otherTime.hrs) && (mins == otherTime.mins) && (secs == otherTime.secs));}
. . .
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Should be familiar … The member selection operator (.) selects either data members or
function members
Header files iostream and fstream declare the istream, ostream,and ifstream, ofstream I/O classes
Both cin and cout are class objects and get and ignore are function members
cin.get (someChar);cin.ignore (100, ‘\n’);
These statements declare myInfile as an instance of class ifstream and invoke function member open
ifstream myInfile;
myInfile.open (“mydata.dat”);
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Information Hiding
Information hiding - Class implementation details are hidden from the client’s view
Public functions of a class provide the interface between the client code and the class objects
clientcode
specification implementation
abstraction barrier
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Selection and Resolution C++ programs typically use several class types
Different classes can have member functions with the same identifier, like Write()
Member selection operator is used to determine the object to whom member function Write() is applied
currentTime.Write(); // Class TimenumberZ.Write(); // Class ComplexNumber
In the implementation file, the scope resolution operator is used in the heading before the function member’s name to specify its class
void Time::Write () const{ . . .
}
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Time Class Instance Diagrams
Private data:
hrs
mins
secs
Set
Increment
Write
LessThan
Equal
Private data:
hrs
mins
secs
Set
Increment
Write
LessThan
Equal
17
58
2
18
30
0
currentTime endTime
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Use of const with Member Functions
When a member function does not modify the private data members, use const in both the function prototype (in specification file) and the heading of the function definition (in implementation file)
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Example Using const with a Member Function
void Time::Write () const
// Postcondition: Time has been output in form// HH:MM:SS{
if (hrs < 10) cout << ‘0’; cout << hrs << ‘:’; if (mins < 10) cout << ‘0’; cout << mins << ‘:’; if (secs < 10) cout << ‘0’; cout << secs;
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Separate Compilation and Linking of Files
time.h
client.cpp time.cpp
client.obj
client.exe
time.obj
Compiler Compiler
Linker
#include “time.h”
implementation file
specification file
main program
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Often several program files use the same header file containing typedef statements, constants, or class type declarations--but, it is a compile-time error to define the same identifier twice within the same namespace
This preprocessor directive syntax is used to avoid the compilation error that would otherwise occur from multiple uses of #include for the same header file
#ifndef Preprocessor_Identifier#define Preprocessor_Identifier .
. .
#endif
Avoiding Multiple Inclusion of Header Files
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Example Using Preprocessor Directive #ifndef
// time .h For compilation the class declaration in// Specification file File time.h will be included only once
#ifndef TIME_H #define TIME_H // time .cpp // client.cpp
// IMPLEMENTATION FILE // Appointment program
class Time{ #include “time.h” #include “time.h”
public: . . . . . . int main (void)
{
private: . . . . . . }}; #endif
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Class Constructors
A class constructor is a member function whose purpose is to initialize the private data members of a class object
The name of a constructor is always the name of the class, and there is no return type for the constructor
A class may have several constructors with different parameter lists
A constructor with no parameters is the default constructor
A constructor is implicitly invoked when a class object is declared--if there are parameters, their values are listed in parentheses in the declaration
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Specification of Time Class Constructors
class Time // Time.h{public : // 7 function members
void Set(int hours, int minutes, int seconds); void Increment(); void Write() const; bool Equal(Time otherTime) const; bool LessThan(Time otherTime) const;
// Parameterized constructor Time (int initHrs, int initMins, int initSecs); // Default constructor Time(); private : // 3 data members
int hrs; int mins;
int secs;};
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Implementation of Time Default Constructor
Time::Time ()
// Default Constructor
// Postcondition:
// hrs == 0 && mins == 0 && secs == 0
{
hrs = 0;
mins = 0;
secs = 0;
}
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Parameterized Constructor
Time::Time( /* in */ int initHrs, /* in */ int initMins, /* in */ int initSecs)
// Constructor// Precondition: // 0 <= initHrs <= 23 && 0 <= initMins <= 59// 0 <= initSecs <= 59// Postcondition:// hrs == initHrs && mins == initMins // && secs == initSecs{ hrs = initHrs; mins = initMins; secs = initSecs;}
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Automatic invocation of constructors occurs
Time departureTime; // Default constructor invoked
Time movieTime (19, 30, 0);// Parameterized constructor
departureTime movieTime
Private data:
hrs
mins
secs
Set
Increment
Write
LessThan
Equal
0
0
0
Private data:
hrs
mins
secs
Set
Increment
Write
LessThan
Equal
19
30
0