11. Introduction to C++

Introduction to C++

Brief History and Overview
C++ vs. Smalltalk
Classes and Objects in C++

Declaring classes
Data hiding: Public, Private, Protected
Multiple Inheritance
Looking up Functions at Run-Time: Virtual

Slightly More Advanced Topics in C++

Overloading functions and operators
Constructors and Destructors
Class Templates
Pointers in C/C++
References in C++

Brief History

C was developed in early 1970's by Dennis Ritchie at Bell Labs

Based on BCPL - Assembler-as-algol
Used by Ritchie and Ken Thompson for writing and maintaining UNIX

C++ developed in early 1980's by Bjarne Stroustrup

C++ is a superset of C
C programs over about 100,000 lines of code become extremely difficult to read, understand, and maintain
Goals: Add OOP to C and fix some C shortcomings

C++ is...

-Strongly typed: Declare everything, don't mix types
-Compiled: Faster, smaller, less flexible
-Mixed object-oriented and procedural: You can cheat

Your First C++ Program

#include <iostream.h> \// Access the runtime
main() \{ \// Must be main, not MAIN, not Main

cout << "The volume of the box car is ";
cout << 11 * 9 * 40;
cout << endl;

}

Declaring variables

#include <iostream.h>
main() \{

int result, height = 11, width = 9;
int length = 40;
result = height * width * length;
cout << "The volume of the box car is"
<< result << endl;

}

char	A single character
short	A small integer (two bytes)
int	Integer of machine's word size
long	Large integer (four bytes)
float	Floating-point number
double	Double-precision floating point number
char *	A string (array of characters; array = pointer)
void	Typeless (typically, used to declare functions as procedures)
type []	An array (= a pointer)

Handling input, too

#include <iostream.h>
main() \{

int height, width, length;
cout << "Please type three integers:" << endl;
cin >> height;
cin >> width;
cin >> length;
cout << "The volume of the box is " <<
height * width * length << endl;

}


\{cleon:guzdial:240} g++ input.cc -o input
\{cleon:guzdial:241} input
Please type three integers:
 12 456
789
The volume of the box is 4317408

C++ is not Smalltalk

-Smalltalk is not typed
-C++ has base types that are not objects, and allows functions that are not methods
-C++ enforces a difference between an object and a pointer to that object
-There is no base Object class in C++
-C++ syntax is more complex than Smalltalk's
-C++ does not handle memory management

Terminology in C++

C++	Smalltalk
Class (but classes aren't objects)	Class
Class objects or compound objects	Instances
Derived class	Subclass
Base class	Superclass
Member function (sometimes, method)	Method
Overloading or Shadowing or Overriding (more ways to do it)	Overriding
Member data	Instance variables

Declaring Classes in C++

#include <iostream.h>
#include <string.h>
class book \{
public:

book(char *, char *, int);
void show_book(void);

private:

char title[64];
char author[64];
int pages;

};
class catalog_card : public book \{
public:

catalog_card(char *, char *, int, char *, int);
void show_card(void);

private:

char catalog[64];
int checked_out; \// 1 if checked out, otherwise 0

};

Filling in the functions: Constructors

book::book(char *newtitle, char *newauthor, int newpages)
\{

strcpy(title,newtitle);
strcpy(author,newauthor);
pages = newpages;

}
catalog_card::catalog_card(char *title, char *author, int pages, char * newcatalog, int newchecked_out) : book(title, author, pages)
\{

strcpy(catalog, newcatalog);
checked_out = newchecked_out;

}

Filling in the functions: Output

void book:show_book(void)
\{

cout << "Title: " << title << endl;
cout << "Author: " << author << endl;
cout << "Pages: " << pages << endl;

}
void catalog_card::show_card(void)
\{

show_book();
cout << "Catalog: " << catalog << endl;
if (checked_out)

cout << "Status: Checked out" << endl;

else

cout << "Status: Available" << endl;

}

Using the new classes

void main(void)
\{

catalog_card card("Tale of Two Cities","Dickens",1000,"Fiction Row 1",1); \// Declaration

card.show_card();
}

Data hiding in C++

public Anyone can access (data or function) through use of the dot (.) operator.
private No one but the class can access - not even derived classes!
protected Derived classes and the declaring class can access, but no one else.

Accessing Member Data

class person_statistics \{

public:
int month_of_birth;
int year_of_birth;
int married;
void initialize_ssn(char *); \// function protoype
void show_dob(void) \{ \// inline member function
cout << month_of_birth << "/" << year_of_birth << endl;
};

private:

char ssn[12];
int children;

};
void main(void)
\{

person_statistics guzdial, ericson;
guzdial.month_of_birth = 9;
guzdial.year_of_birth = 62;
guzdial.married = 1;
guzdial.children = 1; \// DEFECTIVE!!!
guzdial.show_dob();

}

Smalltalk Data Hiding in C++ Terms

-Instance variables are protected (only class and subclasses can access)
-Smalltalk methods are public (anyone can access)

Multiple Inheritance in C++: Declaring two base classes

class computer_screen \{
public:

computer_screen(char *, long, int, int);
void show_screen(void);

private:

char type[32];
long colors;
int x_resolution;
int y_resolution;

};
class mother_board \{
public:

mother_board(int, int, int);
void show_mother_board(void);

private:

int processor;
int speed;
int RAM;

};

Multiple Inheritance in C++: Using both classes in a derived class

class computer: public computer_screen, public mother_board
\{
public:

computer(char *, int, float,
char*, long, int,
int, int, int, int);
void show_computer(void);

private:

char name[64];
int hard_disk; \//size of
float floppy;

};

Deriving from a Base Class other than Public

class box\{

public: double height, width, length;
box() \{}; \//Default constructor
box (double h, double w, double l) \//with arguments
\{height = h; width = w; length =l;};
double volume ()
\{return height * width * length;}

};
class box_car : public railroad_car, private box \{
public: box_car() : box (10.5, 9.5, 40.0)\{}; \// Default constructor
}
-box_car in this example cannot access box private members.
-box's public and protected members are private within box_car: Accessible, but not public, and even derived classes can't access

Roles for Multiple Inheritance

Implementation Inheritance

-Inheriting for functionality but not for semantics

"Mix-Ins": CLOS, Python.

-Adding in some additional functionality

Hiding Inherited Classes

	Public Derivation (e.g., public box)	Protected Derivation (e.g., protected box)	Private Derivation (e.g., private box)
Public Member Function	Remains public	Remains protected	Remains private
Protected Member Function	Becomes protected	Remains protected	Remains private
Private Member Function	Becomes private (to derived)	Becomes private (to derived)	Remains private

Creating objects at run-time and overriding

First: Let's do it obvious, but wrong
#include <iostream.h>
class railroad_car \{

public:
void display_short_name() \{cout << "rrc";}

};
class box_car: public raiload_car \{

public:
void display_short_name() \{cout << "box";}

};
\//..Similar definitions of tank_car, engine, and caboose go here

Main

\//Define raiload car array
railroad_car *train[100];
main() \{

int car_count, type_code, n;

for (car_count = 0; cin >> type_code; ++car_count)

if (type_code == 0) train[car_count] = new engine;
else if (type_code == 1) train[car_count] = new box_car;
else if (type_code == 2) train[car_count] = new tank_car;
else if (type_code == 3) train[car_count] = new caboose;

cout << "There are " << car_count

<< " cars in the array." << endl;

for (n=0; n < car_count; ++n) \{

train[n] -> display_short_name();
cout << endl;

}

}

Running the wrong version

Input:

0 1 1 2 3

Output:
There are 5 cars in the array.
rrc
rrc
rrc
rrc
rrc

Why didn't each car use its own display_short_name?

Because C++ can't do the lookup on the fly?
If it thinks an object is a railroad_car, it thinks it will always call railroad_car functions
Unless we tell it otherwise...

Fixing the Class Declarations

#include <iostream.h>
class railroad_car \{

public:
virtual void display_short_name() \{cout << "rrc";}

};
class box_car: public raiload_car \{

public:
virtual void display_short_name() \{cout << "box";}

};
\//..Similar definitions of tank_car, engine, and caboose go here
Input:

0 1 1 2 3

Output:
There are 5 cars in the array.
eng
box
box
tnk
cab

If you never expect to use the base function...

(Sort of like self error: subclassResponsibility)
#include <iostream.h>
class railroad_car \{

public:
virtual void display_short_name() = 0

};
class box_car: public raiload_car \{

public:
virtual void display_short_name() \{cout << "box";}

};
\//..Similar definitions of tank_car, engine, and caboose go here

Slightly More Advanced Topics in C++

Overloading functions and operators
Constructors and Destructors
Class Templates
Pointers in C/C++
References in C++

Overloading Functions in C++

Multiple signatures can be distinguished if input is distinguishable.
#include <iostream.h>
int add_values(int a, int b)
\{

return (a + b);

}
int add_values(int a, int b, int c)
\{

return(a + b + c);

}
void main(void)
\{cout << "200 + 801=" << add_values (200, 801) << endl;
cout << "100+201+700=" << add_values(100,201,700) << endl;
}

Overloading operators in C++

#include <iostream.h>
#include <string.h>
class string \{

public:
string(char *); \//Constructor
void operator +(char *);
void show_string(void);

private:

char data[256];

};
string::string(char *str)
\{strcpy(data,str);}
void string::operator +(char *str)
\{strcat(data,str);}
void string::show_string(void)
\{cout << data << endl;}
void main(void)
\{

string title("CS 2390");
string name("Modeling and Design");
title.show_string();
title + name;
title.show_string();

}

Constructors and Destructors

Constructor functions run for each object you create

-You load up the member data as you want it -- like Smalltalk initialize:
-Can have several with different signatures, to initialize in different ways

Destructor functions run when an object is discarded (explicitly or at end of program)

-Good time to get rid of connections, free up allocated non-object memory, free-up I/O devices

Class templates: Parameterizing types!

First Version
class array\{

public:
array(int size);
long sum(void);
int average_value(void);
void show_array(void);
int add_value(int);

private:

int *data;
int size;
int index;

};
long array::sum(void)
\{ long sum=0; int i;

for (i=0; i< index; i++)
sum = sum + data[i]; //sum += data[i];
return(sum);

}

Parameterized Version

Class T is type of array elements
Class T1 is type of sum of array elements
template<class T, class T1>
class array\{

public:
array(int size);
T1 sum(void);
T average_value(void);
void show_array(void);
int add_value(T);

private:

T *data;
int size;
int index;};

template<class T, class T1>
T1 array<T, T1>:sum(void)
\{

T1 sum = 0; int i;
for (i = 0; i < index; i++)
sum += data[i];
return(sum);}

Pointers in C/C++

#include <iostream.h>
void show_string(char *string)
\{

while (*string != `/0')
\{

cout << *string;
string++;

}

}
void main(void)
\{

show_string("Hello, World!");

}

Can also be written:

while (*string)
\{

cout << *string;
string++;

}
Or

even:

while (*string)

cout << *string++;

References in C++

References are not variables: You can't change them, add them, assign a reference to a pointer, etc.
They're a second name for a variable -- hopefully, a little safer
Pointer way:
void swap_values(float *a, float *b)
\{

float temp;
temp = *a;
*a = *b;
*b = temp;

}
C++ references way
#include <iostream.h>
void swap_values(float& a, float& b)
\{

float temp;
temp = a;
a = b;
b = temp;

}
void main(void)
\//

float big = 10000.0;
float small = 0.00001;
float& big_alias = big;
float& small_alias = small;

swap_valus(big_alias,small_alias);

\// swap_value(float& big, float& small);

cout << "Big:"<<big<<endl;
cout << "Small:"<<small<<endl;

}

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Page last updated 5/14/98; 9:59:52 AM

11. Introduction to C++

First: Let's do it obvious, but wrong