#OOP_D_ITS - 4th - C++ Oop And Class Structure

C++ OOP :: Class Structure
29/09/2009
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Hadziq Fabroyir - Informatics ITS

C++ Structures: Classes
In C++, the concept of structure has been generalized in an object-oriented sense:
classes are types representing groups of similar instances
each instance has certain fields that define it (instance variables)
instances also have functions that can be applied to them (represented as function fields) -- called methods
the programmer can limit access to parts of the class (to only those functions that need to know about the internals)
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A Motivating Example
You declare a simple structure:
struct Robot {
float locX;
float locY;
float facing;
};
Robot r1;
What if you never want locX or locY to be negative ?
Someone unaware of your restriction could set:
r1.locX = -5;
and you have a problem !
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General Class Definition in C++
class ClassName {
access_modifier1:
field11_definition;
field12_definition;
…
access_modifer2:
field21_definition;
field22_definition;
…
};
ClassName is a new type (just as if declared as struct with typedef in C)
Possible access modifiers:
public
private
Protected
Fields can be variables (as in C structures) or functions
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A Simple Class
class Robot {
public:
float getX() { return locX; }
float getY() { return locY; }
float getFacing() { return facing; }
void setFacing(float f) { facing = f; }
void setLocation(float x, float y);
private:
float locX;
float locY;
float facing;
};
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private Access Modifier
Fields marked as private can only be accessed by functions that are part of that class
In the Robot class, locX, locY, and facing are private float fields, these fields can only be accessed by functions that are in class Robot (getX, getY, getFacing, setFacing, setLocation)
Example:
void useRobot() {
Robot r1;
r1.locX = -5; // Error
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publicAccess Modifier
Fields marked as public can be accessed by anyone
In the Robot class, the methods getX, getY, etc. are public
these functions can be called by anyone
Example:
void useRobot() {
Robot r1;
r1.setLocation(-5,-5); // Legal to call
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structVS class
In C++ struct and class can be used interchangeably to create a class with one exception
What if we forget to put an access modifier before the first field?
struct Robot { OR class Robot {
float locX; float locX;
In a class, until an access modifer is supplied, the fields are assumed to beprivate
In a struct, the fields are assumed to bepublic
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What about protected?
C++ allows users to create classes based on other classes:
a FordCar class based on a general Car class
idea: the general Car class has fields (variables and functions that describe all cars)
the FordCar class then uses the fields from the general Car class and adds fields specific to FordCars
done with inheritance
public fields are inherited (as public)
private fields are not inherited
protected fields are like private, but can be inherited
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Class Methods
Functions associated with a class are declared in one of two ways:
ReturnType FuncName(params) { code }
function is both declared and defined (code provided)
ReturnType FuncName(params);
function is merely declared, we must still define the body of the function separately
To call a method we use the . form:
classinstance.FuncName(args);
FuncName is a field just like any other field in the structured variable classinstance
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Defined Methods
class Robot {
public:
float getX() { return locX; }
};
Robot r1;
The function getX is defined as part of class Robot
To call this method:
cout << r1.getX() << endl; // prints r1’s locX
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Defining Methods Separately
For methods that are declared but not defined in the class we need to provide a separate definition
To define the method, you define it as any other function, except that the name of the function is ClassName::FuncName
:: is the scope resolution operator, it allows us to refer to parts of a class or structure
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A Simple Class
class Robot {
public:
private:
float locX;
float locY;
float facing;
};
void Robot::setLocation(float x, float y) {
if ((x < 0.0) || (y < 0.0))
cout << “Illegal location!!” << endl;
else {
locX = x;
locY = y;
}
}
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Types of Class Methods
Generally we group class methods into three broad categories:
accessors - allow us to access the fields of a class instance (examples: getX, getY, getFacing), accessors do not change the fields of a class instance
mutators - allow us to change the fields of a class instance (examples: setLocation, setFacing), mutators do change the fields of a class instance
manager functions- specific functions (constructors, destructors) that deal with initializing and destroying class instances (more in a bit)
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Accessors & Mutators
Why do we bother with accessors and mutators?
Why provide getX()?? Why not just make the locX field publicly available?
By restricting access using accessors and mutators we make it possible to change the underlying details regarding a class without changing how people who use that class interact with the class
Example: what if I changed the robot representation so that we no longer store locX and locY, instead we use polar coordinates (distance and angle to location)?
If locX is a publicly available field that people have been accessing it, removing it will affect their code
If users interact using only accessors and mutators then we can change things without affecting their code
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Rewritten Robot Class
class Robot {
public:
float getX() { return dist * cos(ang); }
float getY() { return dist * sin(ang); }
private:
float dist;
float ang;
};
if ((x < 0.0) || (y < 0.0))
cout << “Illegal location!!” << endl;
else {
dist = sqrt(x * x + y * y);
ang = atan2(y,x);
}
}
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Object-Oriented Idea
Declare implementation-specific fields of class to be private
Provide public accessor and mutator methods that allow other users to connect to the fields of the class
often provide an accessor and mutator for each instance variable that lets you get the current value of that variable and set the current value of that variable
to change implementation, make sure accessors and mutators still provide users what they expect
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Static Instance Variables
C++ classes may also contain, static instance fields -- a single field shared by all members of a class
Often used when declaring class constants (since you generally only need one copy of a constant)
To make a field static, add the static keyword in front of the field
can refer to the field like any other field (but remember, everybody shares one copy)
static variables are also considered to be global, you can refer to them without an instance
static fields can be initialized (unlike other fields)
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Static Fields Example
class Robot {
public:
static int numRobots = 0;
static const float minX = 0.0;
static const float maxX = 100.0;
void initializeRobot() {
numRobots++;
}
}
if ((x < minX) || (x > maxX))
…
}
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Static Fields as Global Variables
One can examine public static fields of a class outside of that class using the form:
ClassName::StaticFieldName
Example:
void main() {
Robot r1, r2;
r1.initializeRobot();
r2.initializeRobot();
cout << Robot::numRobots << “ robots. ”;
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The Need for Manager Functions
To make the previous program work we have to explicitly call the routine initializeRobot, wouldn’t it be nice to have a routine that is automatically used to initialize an instance -- C++ does
C++ provides for the definition of manager functions to deal with certain situations:
constructors (ctor) - used to set up new instances
default - called for a basic instance
copy - called when a copy of an instance is made (assignment)
other - for other construction situations
destructors (dtor) - used when an instance is removed
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When Managers Called
void main() {
Robot r1; // default ctor (on r1)
Robot r2; // default ctor (on r2)
Robot* r3ptr;
r3ptr = new Robot; // default ctor on
// Robot object r3ptr points to
r2 = r1; // copy ctor
delete r3ptr; // dtor
} // dtor (on r1 and r2) when main ends
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The Default Constructor (ctor)
A default constructor has no arguments, it is called when a new instance is created
C++ provides a default ctor that initializes all fields to 0
To write your own, add following to your class:
class MyClass {
public:
…
MyClass() { // repeat class name, no
code here // return type
}
}
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Example Default ctor
class Robot {
public:
static int numRobots = 0;
Robot() {
numRobots++;
locX = 0.0;
locY = 0.0;
facing = 3.1415 / 2;
}
private:
float locX;
float locY;
float facing;
}
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Destructor (dtor)
A destructor is normally not critical, but if your class allocates space on the heap, it is useful to deallocate that space before the object is destroyed
C++ provides a default dtor that does nothing
can only have one dtor
class MyClass {
public:
…
~MyClass() {
code here
}
}
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Example dtor
class Robot {
public:
char *robotName;
Robot() {
robotName = 0;
}
void setRobotName(char *name) {
robotName = new char[strlen(name)+1];
strcpy(robotName,name);
}
~Robot() {
delete [] robotName;
}
}
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The Copy ctor
A copy constructor is used when we need a special method for making a copy of an instance
example, if one instance has a pointer to heap-allocated space, important to allocate its own copy (otherwise, both point to the same thing)
class MyClass {
public:
…
MyClass(const MyClass& obj) {
code here
}
}
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Example Copy ctor
class Robot {
public:
char *robotName;
void setRobotName(char *name) {
robotName = new char[strlen(name)+1];
strcpy(robotName,name);
}
Robot(const Robot& obj) {
robotName = new char[strlen(obj.robotName)+1];
strcpy(robotName,obj.robotName);
}
}
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Other ctors
It is often useful to provide constructors that allow the user to provide arguments in order to initialize arguments
Form is similar to the copy ctor, except parameters are chosen by programmer:
class MyClass {
public;
…
MyClass(parameters) {
code here
}
}
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Example ctor
class Robot {
public:
Robot(float x, float y, float face) {
locX = x;
locY = y;
facing = face;
}
}
calling:
Robot r1(5.0,5.0,1.5);
Robot r2(5.0,10.0,0.0);
Robot* rptr;
rptr = new Robot(10.0,5.0,-1.5);
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A Combination ctor
Can combine a ctor that requires arguments with the default ctor using default values:
class Robot {
public:
Robot(float x = 0.0, float y = 0.0,
float face = 1.57075) {
locX = x; locY = y; facing = face;
}
}
calling:
Robot r1; // ctor called with default args
Robot r2(); // ctor called with default args
Robot r3(5.0); // ctor called with x = 5.0
Robot r4(5.0,5.0); // ctor called with x,y = 5.0
…
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Hiding the Default ctor
Sometimes we want to make sure that the user gives initial values for some fields, and we don’t want them to use the default ctor
To accomplish this we declare an appropriate ctor to be used in creating instances of the class in the public area, then we put the default ctor in the private area (where it cannot be called)
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Example ctor
class Robot {
public:
Robot(float x, float y, float face) {
locX = x;
locY = y;
facing = face;
}
private:
Robot() {}
}
calling:
Robot r1(5.0,5.0,1.5);
Robot r2; // ERROR, attempts to call default ctor
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For your practice …
Exercise 9.5
Complex Class
Properties: float realPart, float imaginaryPart
Method (Mutators): Complex addBy(Complex), ComplexSubstractBy(Complex)
Lab Session for upper order (Senin, 15.00-17.00)
Lab Session for lower order (Senin, 19.00-21.00)
Please provide:
the softcopy(send it by email – deadline Sunday 23:59)
the hardcopy(bring it when attending the lab session)
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☺~ Next: C++ OOP :: Overloading ~☺
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29/09/2009