Learning C/C++ Step-By-Step - Page 15

15. Step-by-Step C/C++ --- C++ Programming - Operator Overloading

Operator Overloading

1. Introduction
2. Operator
• Rules of Operator Overloading
• Restrictions on Operator Overloading
3. Overloading Unary Operators
4. Overloading Binary Operators
5. Operator Overloading with Strings

1. Introduction

// Assign a variable to another
#include <iostream>
using namespace std;
int main()
{
    int a = 10, b;
    b = a;                // valid
    cout << b;
    return 0;
}

// Assign an object to another
#include <iostream>
using namespace std;
class emp
{
    public:
    int eno;
    float sal;
};
int main()
{
    emp e1= { 1001, 2300.45 },e2 ;
    cout << endl << e1.eno << e1.sal;
    e2 = e1;                            // valid
    cout << endl << e2.eno << e2.sal;
    return 0;
}

Expressions are common in every language; an expression is a collection of operands and operators. Where as an operation is a collection of expressions.
The above two programs demonstrate how variables/objects were assigned together.

Both programs are valid, they demonstrates the use of equalto ( = ) operator.

Operator overloading is one of the most exciting feature of object-oriented programming. It is used to overcome the situation like the above illegal structure operation. It can transform complex, obscure program listing into intuitively obvious ones.

Through Operator overloading we can see how the normal C++ operators can be given new meanings when applied to user-defined data types. The keyword operator is used to overload an operator, and the resulting operator will adopt the meaning supplied by the programmer.

For example using object we can perform direct string assignment operation.

// Program to assign a string to other
#include <string.h>
#include <stdio.h>
#include <iostream>
using namespace std;
class string
{
     char *str;
     public:
          string() { }
          string(char *s) { str = s; }
          void putstring()
          {
               cout << str;
          }
};
int main()
{
     string s1("Computer");
     string s2;
     s2 = s1;
     s2.putstring();
     return 0;
}

2. Operator

type operator operator-symbol ( parameter-list )

The operator keyword declares a function specifying what operator-symbol means when applied to instances of a class. This gives the operator more than one meaning, or "overloads" it. The compiler distinguishes between the different meanings of an operator by examining the types of its operands.

Rules of Operator Overloading

You can overload the following operators:

+	-	*	/	%	^
!	=	<	>	+=	–=
^=	&=	\|=	<<	>>	<<=
<=	>=	&&	\|\|	++	––
( )	[ ]	new	delete	&	\|
~	*=	/=	%=	>>=	==
!=	,	–>	–>*

If an operator can be used as either a unary or a binary operator, you can overload each use separately.
You can overload an operator using either a non-static member function or a global function that's a friend of a class. A global function must have at least one parameter that is of class type or a reference to class type.
If a unary operator is overloaded using a member function, it takes no arguments. If it is overloaded using a global function, it takes one argument.

If a binary operator is overloaded using a member function, it takes one argument. If it is overloaded using a global function, it takes two arguments.

Restrictions on Operator Overloading

You cannot define new operators, such as **.
You cannot change the precedence or grouping of an operator, nor can you change the numbers of operands it accepts.
You cannot redefine the meaning of an operator when applied to built-in data types.
Overloaded operators cannot take default arguments.
You cannot overload any preprocessor symbol, nor can you overload the following operators:

The assignment operator has some additional restrictions. It can be overloaded only as a non-static member function, not as a friend function. It is the only operator that cannot be inherited; a derived class cannot use a base class's assignment operator.

3. Overloading Unary Operators

Let’s start off by overloading a unary operator. Unary operators act on only one operand. ( An operand is simply a variable acted on by an operator). Examples of unary operators are the increment and decrement operators ++ and --, and the unary minus.

Example:
The following example demonstrates the use of increment operator ++.

#include <iostream>
using namespace std;
class counter
{
      private:
            unsigned int count;
      public:
            counter(){ count = 0; }
            int get_count() { return count; }
            counter operator ++()
            {
                   count++;
                   counter temp;
                   temp.count = count;
                   return temp;
            }
};
int main()
{
      counter c1, c2;                            // c1 = 0, c2 = 0
      cout << "\nC1 = " << c1.get_count();       // display
      cout << "\nC2 = " << c2.get_count();

      ++c1;                                      // c1 = 1
      c2 = ++c1 ;                                // c1 = 2, c2 = 2

      cout << "\nC1 = " << c1.get_count();       // display again
      cout << "\nC2 = " << c2++.get_count();     // c2 = 3
      return 0;
}

One more example to overloading unary minus.

#include <iostream>
using namespace std;
class subtract
{
          int a;
          int b;
     public:
          void getdata(int x, int y)
          {
               a = x; b = y;
          }
          void putdata()
          {
               cout<< endl << "A = " << a <<"B = " << b;
          }
          void operator -()
          {
               a = -a; b = -b;
          }
};

int main()
{
     subtract s;
     s.getdata(34, -6);
     cout << endl << "S : ";
     s.putdata();
     -s;
     cout << endl << "S : ";
     s.putdata();
     return 0;
}

4. Overloading Binary Operators

But operators can be overloaded just as easily as unary operators. We will look at examples that overload arithmetic operators, comparison operators, and arithmetic assignment operators.

We have just seen how to overload a unary operator. The same mechanism can be used to overload a binary operator.

// Overloading + operator
#include <iostream>
using namespace std;
class time
{
          int hh; int mm; int ss;
     public:
          time( ) { }
          time(int h, int m, int s)
          {
               hh =h; mm = m; ss = s;
          }
          void disp_time()
          {
               cout << endl << hh<< " : "
               << mm << " : " << ss;
          }
          time operator+(time);
};

time time::operator+(time t)
{
     time temp;
     temp.hh = hh + t.hh;
     temp.mm = mm + t.mm;
     temp.ss = ss + t.ss;
     return temp;
}

int main()
{
     time t1(12,1,24) , t2(5, 23, 45), t3;
     t3 = t1 + t2;
     t3.disp_time();
     return 0;
}

5. Operator Overloading with Strings

C/C++ deals with strings quite differently; we never copy, concatenate, or compare strings using operators like other languages. C/C++ has built functions to perform the above operations. But C++ provides the facility to do every thing on strings using operators. That means we have to provide extra responsibility to operators to perform such things.

The following example demonstrates the comparison between two strings using comparison operator ==.

// Program to compare two strings using operator overloading
#include <string.h>
#include <stdio.h>
#include <iostream>
using namespace std;

enum boolean{ false, true };

class string
{
     char *str;
     public:
          string() { *str = NULL; }
          string(char *s) { str = s; }
          int operator ==(string ts)
          {
               if (strcmp(str, ts.str) >= 0)
                    return true;
               else
                    return false;
          }
};

int main()
{
     string s1("Computer");
     string s2("Computers");

     if(s1 == s2)
          cout << "Equal";
     else
          cout << "Not Equal";

     return 0;
}

// concatenation of two strings

#include <string.h>
#include <stdio.h>
#include <iostream>
using namespace std;

class string
{
          char *str;
     public:
          string()
          {
               str = new char[30] ;
               *str = NULL;
          }
          string(char *s) { str = s; }
          string operator +(string ts)
          {
               string t;
               strcat(t.str, str);
               strcat(t.str, ts.str);
               return t;
          }
          void putstring()
          {
               cout << endl << str;
          }
};

int main()
{
     string s1("Computer"); string s2("Institute");
     s1.putstring(); s2.putstring();
     string s3;

     s3 = s1 + s2;

     s3.putstring();
     return 0;
}