Object Oriented Programming
C++
ADT vs. Class
• ADT: a model of data AND its related functions
• C++ Class: a syntactical & programmatic
element for describing how the functions and
data are related
• An ADT implementation has several parts:
– interface function-names (methods)
– operators (possibly re-defined symbols)
– data
• Any/all of these may be public or private
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Classes
• Collection of data elements and functions
• Functions are called "methods" or members of
the class
– Tied to the data
– May be hidden from other functions
• Member access
– exposed by "public" names
– Hidden by "private" names
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Classes - example
class Complx
{public:
float my_real_part;
float my_imag_part;
};
// note the semicolon!!!!!
• Using "public" makes all elements "visible"
• Using "private", would make them visible ONLY to
methods (functions) inside the class (not shown).
• NOTE: looks a lot like a struct
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Program structure – pt 2 - C++
• Usually, put the class definition in a header file
• #include it later in BOTH :
– the implementation file
• where the methods are defined
– the user-program file
• where methods get used
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Objects
• An object is just a struct with members
• Default operations (members)
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–
–
–
Constructor
Destructor
Assignment
Copy
• YOU must provide any initialization code
– in a constructor
• YOU must provide return values (if any)
• YOU must free any dynamic storage
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Constructors
• C++ provides a "default" constructor
• Only works if no parameters: Complx X;
• Fails for uses like: Complx X(3,4);
– variables get initialized
– other setup must be done
• IF you create a constructor, you MUST specify
the Default Constructor too or it won't exist
– Complx( ) { };
– no actual code needed inside the { }
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Class Constructors
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Assign initial values to an object
Never returns a value
Same name as the class
Guaranteed to be called (automatic) upon
instantiation of an object of type class
Destructors
• Automatically run when an object of type class
goes out of scope
• use to clean up after the object (return
dynamically allocated storage to the storage
management system
• Same name as the class, but preceded by a ~
• no overloading allowed
• never returns a value
• not needed for statically allocated storage
C++ console I/O
• basic operators
– << the "insertion" operator
– >> the "extraction" operator
• stream names for standard I/O
– cin
– cout
is the input "file"
is the output "file"
• usage
– cin>>x;
– cout<<x;
// get a value
// output a value
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Operators
• Ordinary variables have basic operators built-in
+ - * / % | & || &&
• New objects (structs, classes) may need more
• How do you "add" 2 complex numbers?
• Given: complex#'s a and b;
– what does a+b mean???
– compiler knows nothing about complex #'s
• Define a NEW action to perform "+"
– but continue to use the "+" operator!!!!
– add the real parts, add the imaginary parts
– store each result in the proper answer-part.
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Operators – 2
• Often, we "want" to re-use an operator
– i.e.; give a new meaning to "+"
– this is called "overloading"
• let "+" mean "add complex numbers"
as well as add integers, floats, etc.
• c=a+b; // a, b & c are Complex
• May need a "friend" function, due to flaws in the
C++ object model
• "friend" functions do not get the leading
"classname::" syntax
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More on Overloading
• = overload this as a member function
• == != <= >= <> overload as a non-member
(friend function) returning a boolean
• >> << (insertion and extraction) overload
as non-members (friends) returning type
iostream
• + - * / % (arithmetic) as overload nonmembers
• += and -= ... overload same as + and • cannot overload :: . sizeof ?:
Polymorphism (multiple forms)
• x=sqrt(y); // what data type is "y"?
– could be: float, int, double, real
• How does compiler know how to handle the
incoming data?
• An ADT allows us to create MULTIPLE methods
– all have the same name
– all have different parameter types
– compiler links caller's code & the "right one" based on
parameter types.
– Object writer creates these multiple methods
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Operators
• Ordinary variables have basic operators built-in
+ - * / % | & || &&
• New objects (structs, classes) may need more
• How do you "add" 2 complex numbers?
• Given: Complex#'s a and b;
– what does a+b mean???
– compiler knows nothing about Complex #'s
• Define a NEW action to perform "+"
– add the real parts, add the imaginary parts
– store each result in the proper answer-part.
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Declare the methods
Complx.h
#include <iostream>
class Complx
{ private:// hidden member data
double my_real_part, my_imag_part;
// below are prototypes for the member functions (i.e.; "methods")
public:
Complx
(double a, double b);
// initializing constructor
Complx( ) { };
// default constructor
double get_real(Complx);
// not shown
Complx operator + (Complx); // defines "+" as a non-member function
// define what stream output means for a “Complx” variable
friend std::ostream &operator<<(std::ostream & , const Complx & )
};
// orange: declare parts of the class, green: use it, red: return-value
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Implement the methods
Complx.cpp
#include <iostream>
program need to know about “ostream”
#include Complx.h
program needs to know what the names mean
// :: means the name after :: is a member of the class "Complx"
Complx :: Complx (double a, double b) // implement constructor
{my_real_part=a; my_imag_part=b;}
Complx :: Complx () {} // default constructor ( required because of initializing constructor)
Complx Complx ::operator+ (Complx C) // define what “plus” means
{Complx tmp; // need a place for output
tmp.my_real_part = my_real_part + C.my_real_part;
tmp.my_imag_part = my_imag_part + C.my_imag_part;
return tmp; }
// define << for a Complx element
friend std::ostream &operator<<(std::ostream & xout, const Complx &C )
{ xout << C.my_real_part <<"+"<< C.my_imag_part<<"j";
return xout; // actually do the output operation
}
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"friend"s
friend ostream &operator<<(ostream &xout, const Complx &C )
{ xout << C.my_real_part <<"+"<< C.my_imag_part<<"j";
return xout;
}
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What does it do?
Remember that we needed to define + for complex numbers?
Now need to define how to output them
Output functions don't understand class data
So we have now overloaded:
– the + operator
– the << operator
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"friend"s - pt-2
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Can access private and protected members
NOT a member of the class
Allows operations across multiple classes
<< is a member of the iostream class
Want << able to output private data in Complx
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Our program to add Complx’s
#include <iostream>
#include “Complx.h”
using namespace std;
void main()
{
Complx A(2,2);
Complx B(1,1),C;
C=A+B;
}
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