c++

Posted on 07 July 2015 at 05:33

�classes
�fields & methods
�objects
�representation invariant
2

Why objects?

At the end of the day...
computers just manipulate 0’s and 1’s

Figure by MIT OpenCourseWare.
But binary is hard (for humans) to work with

Towards a higher level of abstraction

binary code
assembly languages
procedural languages (C, Fortran, COBOL...)
OO languages (C++, Java, Python...)
declarative languages (Haskell, ML, Prolog...)
?
6
Figure by MIT OpenCourseWare.
There are always trade-offs

High-level languages
�simpler to write & understand
�more library support, data structures
Low-level languages
�closer to hardware
�more efficient (but also dangerous!)
What about C++?
7

What are objects?
Objects model elements of the problem context
Each object has:
�characteristics
�responsibilities (or required behaviors)
8

Example
Problem Design and build a computer hockey game
Object Hockey player
Characteristics Position, height, weight, salary, number
of goals

Responsibilities Pass the puck, shoot, skate forward,

skate backward, punch another player, etc.

Another example
Problem Computation modeling in biology
Write a program that simulates the growth of virus
population in humans over time. Each virus cell
reproduces itself at some time interval. Patients may
undergo drug treatment to inhibit the reproduction
process, and clear the virus cells from their body.
However, some of the cells are resistant to drugs and
may survive.
10

Write a program that simulates the growth of virus
population in humans over time. Each virus cell
reproduces itself at some time interval. Patients may
undergo drug treatment to inhibit the reproduction
process, and clear the virus cells from their body.
However, some of the cells are resistant to drugs and
may survive.
What are objects?
Characteristics?
Responsibilities?
11

Patient
characteristics
�virus population
�immunity to virus (%)

responsibilities
�take drugs
Virus
characteristics
�reproduction rate (%)

�resistance (%)
responsibilities
�reproduce
�survive
13

Questions
Why didn’t we model an object named Doctor?
Surely, most hospitals have doctors, right?
14

Questions
Why didn’t we model an object named Doctor?
Surely, most hospitals have doctors, right?
Doesn’t every patient have an age? Gender? Illness?
Symptoms? Why didn’t we model them as
characteristics?
15

Basic OOP in C++

Classes
A class is like a cookie cutter; it defines the shape of
objects
Objects are like cookies; they are instances of the class
17
Photograph courtesy of Guillaume Brialon on Flickr.
Classes: Declaration vs. definition

client
depends
declaration
implements
definition
Declaration (.h files)
�list of functions & fields
�including functions that the class promises to its client

�it’s like a “contract”
Definition (.cc files)
�implementation of functions
18
Class declaration

Class declaration

class Virus {
float reproductionRate; // rate of reproduction, in %
float resistance; // resistance against drugs, in %
static const float defaultReproductionRate = 0.1;
public:
Virus(float newResistance);

Virus(float newReproductionRate, float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);

};
20

class name field

class Virus {
float reproductionRate; // rate of reproduction, in %
float resistance; // resistance against drugs, in %
static const float defaultReproductionRate = 0.1;
public:

Virus(float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);
Virus(float newReproductionRate, float newResistance);
constructors
};
method
don’t forget the semi-colon!

21
Fields (characteristics)

Virus(float newReproductionRate, float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);

};
22

Methods (responsibilities)

Virus(float newReproductionRate, float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);

};
23

Constructors

Virus(float newReproductionRate, float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);

};
Note the special syntax for constructor (no return type!)

24
Access control: public vs. private

class Virus {
Virus(float newResistance);

Virus* reproduce(float immunity);
Virus(float newReproductionRate, float newResistance); public

bool survive(float immunity);

float reproductionRate; // rate of reproduction, in %

float resistance; // resistance against drugs, in %
private
static const float defaultReproductionRate = 0.1;
public:
};
private: can only be accessed inside the class
public: accessible by anyone
25
Client Interface
How do we decide private vs. public?

depends satisfies
Implementation
interface: parts of class that change infrequently
(e.g. virus must be able to reproduce)
implementation: parts that may change frequently

(e.g. representation of resistance inside virus)
26

Protect your private parts!

Client Interface Implementation
X

Why is this bad?
27

Access control: public vs. private

class Virus {
Virus(float newResistance);

Virus* reproduce(float immunity);
Virus(float newReproductionRate, float newResistance); public

bool survive(float immunity);

float reproductionRate; // rate of reproduction, in %

float resistance; // resistance against drugs, in %
private
static const float defaultReproductionRate = 0.1;
public:
};
In general,
�keep member fields as private
�minimize the amount of public par 28 ts

Access control: constant fields

Virus(float newReproductionRate, float newResistance);

Virus* reproduce(float immunity);

bool survive(float immunity);

};
Why make it a constant? Why not just declare it as a
normal field?
29
Class definition

Class definition

#<stdlib.h>
#include “Virus.h”
Virus::Virus(float newResistance) {
reproductionRate = defaultReproductionRate;
resistance = newResistance;
}
Virus::Virus(float newReproductionRate, float newResistance) {
reproductionRate = newReproductionRate;
resistance = newResistance;
}

// If this virus cell reproduces,

// returns a new offspring with identical genetic info.

// Otherwise, returns NULL.

Virus* Virus::reproduce(float immunity) {

float prob = (float) rand() / RAND_MAX; // generate number between 0 and 1
// If the patient's immunity is too strong, it cannot reproduce

if (immunity > prob)

return NULL;

// Does the virus reproduce this time?

if (prob > reproductionRate)

return NULL; // No!

return new Virus(reproductionRate, resistance);
}
// Returns true if this virus cell survives, given the patient's immunity
bool Virus::survive(float immunity) {
// If the patient's immunity is too strong, then this cell cannot survive
if (immunity > resistance)
return false;
return true;
}
const float Virus::defaultReproductionRate; 31
Header inclusion

#include <stdlib.h>

#include "Virus.h"

Virus::Virus(float newResistance) {
reproductionRate = defaultReproductionRate;
resistance = newResistance;
}
Virus::Virus(float newReproductionRate, float newResistance) {
reproductionRate = newReproductionRate;
resistance = newResistance;
}
32

Constructor definition

#include <stdlib.h>

#include "Virus.h"

33
Constructor definition

Can also do:
Virus::Virus(float newReproductionRate, float newResistance) {
reproductionRate = newReproductionRate;
resistance = newResistance;
}
Virus::Virus(float newReproductionRate, float newResistance) :
reproductionRate(newReproductionRate), resistance(newResistance)

{

}

Method definition

// Returns true if this virus cell survives,
// given the patient's immunity
bool Virus::survive(float immunity) {
// If the patient's immunity is too strong,
// then this cell cannot survive
if (immunity > resistance)

return false;

return true;
}
35

Working with objects

Patient class declaration

#include “Virus.h”
#define MAX_VIRUS_POP 1000
class Patient {
Virus* virusPop[MAX_VIRUS_POP];

int numVirusCells;

float immunity; // degree of immunity, in %

public:
Patient(float initImmunity, int initNumViruses);
~Patient();
void takeDrug();
bool simulateStep();
};
37

Patient class declaration

#include “Virus.h”
#define MAX_VIRUS_POP 1000 Array of pointers to objects

class Patient {
Virus* virusPop[MAX_VIRUS_POP];

int numVirusCells;

float immunity; Constructor // degree of immunity, in %

public:
Patient(float initImmunity, int initNumViruses);
~Patient();
void takeDrug();

bool simulateStep();

};
Destructor

Static object allocation

class Patient {
...
public:
Patient(float initImmunity, int initNumViruses);
...
};
int main() {
float initImmunity = 0.1;

int initNumVirusCells = 5;

Patient p(0.1, 5);
p.takeDrug();
}
39

Calling the constructor

class Patient {
...
public:
...
};
int main() {
p.takeDrug();
}
float initImmunity = 0.1;
int initNumVirusCells = 5;
Patient(float initImmunity, int initNumViruses);
Patient p(0.1, 5);
40

Deleting statically allocated objects

class Patient {
...
public:
Patient(float initImmunity, int initNumViruses);
...
};
int main() {
float initImmunity = 0.1;

int initNumVirusCells = 5;

Patient p(0.1, 5);
p.takeDrug();
}
Automatically destroyed at the end of scope

41
Objects on heap
To allocate an object on heap:
�use “new” keyword (analogous to “malloc”)

To deallocate:
�use “delete” keyword (analogous to “free”)

Patient* p = new Patient(0.1, 5);

...

delete p;

Dynamic object creation: Example

Patient::Patient(float initImmunity, int initNumVirusCells) {
float resistance;
immunity = initImmunity;
for (int i = 0; i < initNumVirusCells; i++) {
//randomly generate resistance, between 0.0 and 1.0
resistance = (float) rand()/RAND_MAX;
virusPop[i] = new Virus(resistance);
}
numVirusCells = initNumVirusCells;
}
43

Using dynamically allocated objects

bool Patient::simulateStep() {
Virus* virus;
bool survived = false;
...
for (int i = 0; i < numVirusCells; i++){
virus = virusPop[i];
survived = virus->survive(immunity);
if (survived) {
...
} else {
...
}
...
}
44

What happens during destruction?

The destructor is automatically called
Patient::~Patient(){
for (int i = 0; i < numVirusCells; i++){
delete virusPop[i];
}
}
Patient* p = new Patient(0.1, 5);

...

delete p;

But why didn’t we have a destructor forVirus?

45
Representation
invariant
Representation invariant
Statements about characteristics of objects
�defines what it means for an object to be valid
�e.g.“Every IKEA coffee table must have four legs”

Valid Invalid

Figures by MIT OpenCourseWare.
Example

class Patient {
Virus* virusPop[MAX_VIRUS_POP];

int numVirusCells;

float immunity; // degree of immunity, in %

public:
Patient(float initImmunity, int initNumViruses);

~Patient();

void takeDrug();

bool simulateStep();

};
What are the representation invariants for Patient?

48
Rep. invariant violation

void Patient::takeDrug(){
immunity = immunity + 0.1;
}
What’s wrong with this method?

Preserving rep. invariant

bool Patient::checkRep() {
return (immunity >= 0.0) && (immunity < 1.0) &&
(numVirusCells >= 0) &&
(numVirusCells < MAX_VIRUS_POP);
}
checkRep
�returns true if and only if the rep. invariants hold true

�call checkRep at the beginning and end of every
public method
�call checkRep at the end of constructors
50

Preserving rep. invariant

#include <cassert>
class Patient {
...
bool checkRep();
public:
...
};
void Patient::takeDrug() {
assert(checkRep());
...
assert(checkRep());
}
Patient::Patient(float initImmunity, int initNumViruses) {
...
assert(checkRep());
}
51
Preserving rep. invariant
Will calling checkRep() slow down my program?
Yes, but you can take them out once you are confident
about your code.
52

Until next time...
Homework #4 (due 11:59 PM Monday)

�implementing BSTs using classes
Next lecture
�inheritance & polymorphism
�templates

c++

About the author

maliX0024

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