01 cpp classes

7/28/2019 01 Cpp Classes

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1April 13

Classes in C++

C++ originally called "C with classes": Swedish connection: Bjarne Stoustrup borrowed from Simula (67)

Simulating classes of real world objects

C++ continues to evolve: Version 1.0 released by AT&T in 1986

Version 2.0 in 1990 (added multiple inheritance)

Version 3.0 in 1992 (templates)

ANSI standard in 1996 (exception handling, run time type info)

C++ became dominant OOPL in early 90's

Now Java and C# challenge

A class extends the C++ type system:class Account { //creates a new type

}; //Note: you need this semi-colon in C++

Account a1,a2; //define variables of type Account


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2April 13

C structs

A data structure for dates in C:struct Date {

int month, day, year;

};

int set(struct Date*, int m, int d, int y);int print(struct Date*);

Whats the problem?

No information hiding

No way to control access to data (obscure side effects) No way to prevent assigning an illegal value to month

Changing representation of Date breaks all client code


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3April 13

C++ structs

C++ provides for closer coupling of data and functions:struct Date {


void set(int m, int d, int y);

void print(); //implement elsewhere

};

Invoke functions with variable . memberFunction():Date today; //In C++, structs automatically define types

today.set(9,29,1953); today.print();

Now we have data abstraction: Procedural abstraction hides details of code in functions

Data abstraction couples data structure and functions

Still, no information hiding: today.set(50,-10,0);


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C++ adds class

C++ adds new keywords to support information hiding:class Date {


public:

void set(int m, int d, int y);void print(); //implement elsewhere

};

Members afterpublic: are visible to clients:Date today; today.print();

today.month = 50; //is this legal? Members afterclass are by default private


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Member functions

What can we add to Date to allow access to month?class Date {


public:

void set(int m, int d, int y);

void print(); //implement elsewhere

int getMonth() { return month; }};

cout = 1 && m


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Inheritance

(class derivation)class Account generalizes many kinds of bankaccounts: checking, savings, etc.

C++ class derivation captures this generation:class Checking : public Account {

public:Checking(float balance);

Checking(); //default constructor

};

: denotes derivationChecking inherits from Account

publicderivation denotes subtype inheritance Accounts public methods accessible to instances ofChecking

Checking myChecking(200); //Checking constructor

myChecking.getBalance(); //Account function


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OOP = Data abstraction

+ inheritance

+ dynamic bindingPolymorphism: a function can mean different things at runtime

Dynamic binding: defer function binding to a subtype until runtime

Suppose we want to draw a heterogeneous collection of shapes?

class Point { ... }; //a Point has x and y coordinates

class Shape {protected: //accessible to subclasses but otherwise private

Point center; //all Shapes have a center Point

public:

Point where() { return center; }

virtual void move(Point to) //virtual can be overridden{ center = to; draw(); } // by derived classes

virtual void draw()=0; //a "pure" virtual function

//draw() must be implemented by derived classes

//...

}


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Subclasses override

virtual functionsclass Triangle: public Shape {

Point sw, se, top; //Three points define triangle

public:

Triangle(Point a, Point b, Point c) : sw(a), se(b), top(c) {}

draw() //implementing pure virtual function

{ put_line(sw,top); //draw line from sw to topput_line(top,se); //draw line from top to se

put_line(se,sw); //draw line from se to sw

}

};

class Circle : public Shape {

int radius;public:

Circle(Point a, int r) : center(a), radius(r) {}

draw(); //draw a circle using center and radius

};


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9April 13

Calling a virtual function

{ //Construct some shapes

Shape aShape; //illegal--why?

Circle c(Point(20,30),7); //legal--what does it do?

//Create an array of various shapes

Shape* shapes[10]; //Why is this legal?shapes[0] = new Circle(Point(20,30),7); //assign a Circle

shapes[1] = new Triangle(Point(50,50),Point(30,30),Point(40,40));

//... maybe assign other shapes, Rectangles, Squares, etc.

for (int i=0; i < 10; i++) //draw all the shapesshapes[i]->draw(); //each shape draws itself!

}

Why do we say that elements of shapes array are polymorphic?

How does polymorphic design support Open-Closed principle?


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Why dynamic binding?

What kind of code does dynamic binding avoid?

Avoids lots of switch statements, e.g.:

switch (shapes[i]->isa)

//each Shape derived class has an isa data member{ case(triangle) Triangle::draw(); //test enumeration

case(circle) Circle::draw(); //run specific draw()

// ...

}

Why is the dynamic binding version better for

big, growing programs?

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