lab 2: polymorphism zone name, many purposes ztwo forms: yfunction overloading yoperator overloading
TRANSCRIPT
Lab 2: Polymorphism
One Name, Many Purposes
Two forms: Function Overloading Operator Overloading
Function Overloading
Two or more functions can share the same name provided that: Type of their arguments differsOR Number of their arguments differs
To overload a function, simply declare and define all required versions.
Exampe: abs()
int abs(int n) {return n < 0 ? –n : n;}long abs(long n) {return n < 0 ? –n : n;}double abs(double n) {return n < 0 ? –n : n;}int main() {
cout<<“Abs. value of –10:” << abs(-10) << “\n”;cout<<“Abs. value of –10L:” << abs(-10L) <<“\n”;cout<<“Abs. value of –10.1:” << abs(-10.1) <<“\n”;return 0;
}
Operator Overloading
Operator overloading is just a type of function overloading.
An operator is always overloaded relative to a user-defined type, such as a class.
To overload an operator, you create an operator function.
Member Operator function
An operator function is often a member of the class for which it is defined.
Syntax:return-type class-name::operator#(arg-list){
// operation to be performed}
Binary Operator
When a member operator function overloads a binary operator, the function will have only one parameter.
Consider: x + yx: object that generates the call to the operator functiony: parameter of the operator function
Example: coord class
class coord {int x, y; // coordinate values
public:// Constructor function is overload
edcoord() {x = 0; y = 0;}coord(int i, int j) {x = i; y = j;} coord operator+(coord ob2);
}
Example: operator+()
// Overload + relative to coord classcoord coord::operator+(coord ob2) {
coord temp;temp.x = x + ob2.x;temp.y = y + ob2.y;return temp;
}
Example: overloaded +
int main() {coord o1(10,10), o2(5,3), o3, o4;
o3 = o1 + o2;// a string of additions is allowed.o4 = o1 + o2 + o3;return 0;
}
Example: coord classclass coord {
int x, y; // coordinate valuespublic:
coord() {x = 0; y = 0;}coord(int i, int j) {x = i; y = j;} coord operator+(coord ob2);coord operator– (coord ob2);coord operator=(coord ob2);
}
Overloading – and =
The operator –() can be implemented similarly to operator+(), and we omit the details.
We focus on the assignment operator function, operator=().
Example: operator=()
void coord::operator=(coord ob2) {x = ob2.x;y = ob2.y;
}int main() {
coord o1(4,6), o2;o2 = o1;return 0;
}
Assignment Operator
In C++, the following type of statement is syntactically correct:a = b = c = d = 0;
However, in our example, the following statement is invalid (why?):o3 = o2 = o1;
How to modify our program so as to support the above statement?
Introducing this pointer
this is a pointer: automatically passed to any member
function when it is called. pointing to the object that generates the
call. For example, consider:
ob.f1(); // assume ob is an objectf1() is automatically passed a pointer
to ob. This pointer is referred to as this.
Example: operator=()
coord coord::operator=(coord ob2) {x = ob2.x;y = ob2.y;return *this; // return the object
}int main() {
coord o1(4,6), o2, o3;o3 = o2 = o1; // this statement is now validreturn 0;
}
Overload + for (object + integer)
Suppose you want to overload operator+() so that you can add an integer value to a coord object.
For example,use the statement o1 + 3 to represent
o1.x + 3 and o1.y + 3
Example: coord class
class coord {int x, y; // coordinate values
public:coord() {x = 0; y = 0;}coord(int i, int j) {x = i; y = j;} coord operator+(coord ob2);coord operator+ (int i);
}
Overload + for (object + integer)
coord::operator+(int i) {coord temp;
temp.x = x + i;temp.y = y + i;return temp;
}
How about integer + object?
What happens when the compiler sees the following statement?
o3 = 19 + o1;This statement generates a compile-
time error.How to solve this problem?
Introducing Friend Functions
A friend is NOT a member of a class but still has access to its private elements.
Situations when friend functions are useful: Operator overloading Creation of certain types of I/O functions One function has to access the private
members of two or more different classes.
Example: friend function
class myclass {int n, d;
public:myclass(int i, int j) {n = i; d = j;}//declare a friend of myclassfriend int isfactor(myclass ob);
}
Example: friend function
/* This function returns true if d is a factor of n. Notice that the keyword friend is not used in the definition of isfactor(). */
int isfactor (myclass ob) {if (! (ob.n % ob.d)) return 1;else return 0;
}
Example: friend function
int main() {myclass ob1(10,2);
if (isfactor(ob1))cout << “2 is a factor of 10\n”;
return 0;}
Friend Function
It is important to understand that a friend function is NOT a member of the class.
For example, // wrong; isfactor() is not a member functionob1.isfactor()
Friends are called just like regular functions.
integer + object (revisited)
class coord {int x, y; // coordinate values
public:coord() {x = 0; y = 0;}coord(int i, int j) {x = i; y = j;} friend coord operator+(coord ob1, coord ob2);friend coord operator+(coord ob1, int i);friend coord operator+(int i, coord ob1);
}
Overload for integer + object
coord operator+ (int i, coord ob1) {coord temp;
// private variables of ob1 can be accessedtemp.x = ob1.x + i;temp.y = ob1.y + i;return temp;
}
C++ I/O Basics
C++ I/O system operates through streams, e.g. cin, cout, …etc.
The ios class contains many member functions and variables that control the operation of a stream.
Each C++ stream has associated with it a number of format flags that determine how data is displayed.
Some Format Flags
enum {skipws = 0x0001;
:dec = 0x0010,oct = 0x0020,hex = 0x0040,
:};
Formatted I/O: Hexadecimal
Using setf() and unsetf() to set and unset a flag.Example:
cout.unsetf(ios::dec); cout.setf(ios::hex);cout << 10;
In the above example, “a” will be displayed, which is the hexadecimal representation of 10.
width() and fill()
A minimum field width can be specified by using the width() function.
Example:cout.width(10);cout << “hello” << “\n”;cout.fill(‘%’);cout.width(10);cout << “hello” << “\n”;
width() and fill() (cont’d)
The program displays the following output:
hello
%%%%%hello
Overload Insertion Operator
In C++, the output operation is called an insertion and the << is called the insertion operator.
In the next example, you’ll learn how to overload C++’s insertion operator <<.
Example: Overload <<
class coord {int x, y; // coordinate values
public:coord() {x = 0; y = 0;}coord(int i, int j) {x = i; y = j;} friend ostream &operator << (ostream &out, coord ob);
}
Example: Overload <<
ostream &operator<<(ostream &out, coord ob){
out << ob.x << “, ” << ob.y << ‘\n’;return out;
}
Example: Overload <<
int main() {coord a(1,1);cout << a;return 0;
}This program displays the following:
1, 1