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Operator Overloading and Type Conversions

Posted By: Harrison Evans     Category: C++ Programming     Views: 23667

This article explains about Operator Function, Rules for Overloading Operators, Overloading operator, Overloading Using a Friend, Overloading in Vector, Manipulating Strings, Type Conversions, Basic to Class Type, Class to Basic Type, One Class to Another Class Type, Data Conversion, Data Conversion.

-  Giving a special additional meaning to operator
-  Overloading is not possible for the following
  • Class member operators (.,.*)
  • Scope resolution operator (::)
  • Size operator:(sizeof)
  • Conditional Operator(?:)
-  Semantics of an operator is changed but neither the syntax nor the precedence is changed
-  The original meaning is not lost

Operator Function

  • return-type classname :: operator <op>(arg list)
  • operator <op> is the function name
  • Operator functions must be either (non static) member functions or friend functions
  • Objects used to invoke operator is passed implicitly the operator function
  • So friend need one more argument!
  • Passing by value or passing by variable accepted
  • The interpretation is like a function only

Overloading minus operator (-)

class space
{ int x,y,z;
     void getdata(int a, int b, int c);
     void display(void);
     void operator-(); }
void space :: operator –()
    { x = -x; y = -y; z = -z; }
main ()
{ space s; s.getdata(10,-20,30); s.display();
  -s; s.display(); }

Overloading Using a Friend

friend void operator – (space & s); //declaration
void operator –(space & s)  // definition
 { s.x = -s.x; s.y = -s.y; s.z = -s.z; }
  • If we do not use reference parameter, the changes in the operator function will not be reflected
  • For some situations, friends are more useful
    • When manipulating an object with a pure data
    • When dealing with different type of objects
  • Some operators { =,(),[],-> } can not be overloaded as friends, though allowed as member

Overriding plus operator (+)

class complex
{ float x,y;
   public : ….
// complex sum (complex c)
   complex operator+(complex c)
{ complex temp;
   temp.x = x + c.x;   temp.y = x + c.y; return temp}
{ complex c1,c2,c3;
 // c3 = c1.sum(c2);
    c3 = c1 + c2;   }

Using Friend For +

complex() {}// a default constructor
complex(float r, float i)//constructor
  { real = r; imag = i; }
//friend complex sum (complex c1,complex c2)
friend complex +(complex c1,complex c2)
{ return complex((a.x+b.x),(a.y+b.y)) }
{ complex c1,c2,c3;
   // c3 = sum(c1,c2);
      c3 = c1 + c2;

Overloading in Vector

class vector
{ int v[3];
   public :
    vector() {for v[i] = 0; }
    vector(int *x) { for (int i=0;i<3;i++) v[i] = x[i]}
    // friends declarations come here };
vector operator *(int a, vector b)
  {vector c; for (int i=0;i<3;i6++) {c.v[i]=b.v[i]*a;}
vector operator *(vector b, int a) // same code
main() { vector m; vector n = {1,2,3}; int num=5;
   vector p = m * num; vector q = 5 * n;

Overloading << and >>

istream & operator >> (istream & din, vector & b)
{for (int i=0;i<3;i++) din >> b.v[i]; return din; }
ostream & operator << (ostream& dout,vector& b)
{dout <<“(“ << b.v[0]; 
for (int i=1;i<3;i++)   dout << “,” << b.v[i]; 
                                                 dout << “)”; }
cin >> p;   // here p is a vector
cout << “p=“ << p;
  • istream and ostream are classes defined in iostream.h file, they are input and output streams

Manipulating Strings

class string
{char *p; int len;
  public :   string() {p = NULL, len = 0}
  string(char *x) { len= strlen(char *x);
                              p = new char(len + 1); }
  string (const string & s)  {len = strlen(s);
                            p = new char(len+1); strcpy(s,p);}
  friend string operator +(string s1, string s2) 
 { string temp; temp.len = s1.len + s2.len; 
    temp.p = new char(len + 1); strcpy(s1.p,temp.p); strcat(temp.p,s2.p); return temp }

friend int operator <=(string s1,string s2)
  { return (s1.len <= s2.len); }
ostream & operator <<(ostream & dout, string s)
  { dout << s.p ; return dout; }
{ string s1 = “New” //second constructor
   string s2 = string(“York”) // second constructor
   string t1,t2,t3,t4; // first constructor
   t1 = s1; t2 = s2; t3 = t1+t2; t4 = t1+t3;
  if (t3 <= t4) cout << t3 << “ < or = “ << t4;

Rules for Overloading Operators

  • The oo must have one userdefined type as arg
  • Basic meaning of an op can not be changed (+-)
  • Only existing operators can be overloaded, new ones can not be created
  • When using binary oo by member functions, the lhs operator must be object of the relevant class
  • Binary arithmetic ops must explicitly return a value, and not change values of arguments
  • Some ops can not be simply overloaded 

Type Conversions

  • C’s and C++’s built-in data conversion routines for expressions and assignment
  • int to float and float to int conversions
  • Copying bit by bit for objects of compatible type
  • Compiler does not support for incompatible types
  • Three situations for incompatible matches
    • From built in type to a class type
    • From class type to built-in type
    • From one class type to another class type

Basic to Class Type

  • Constructors to convert from basic types to class types are quite common
class time
{ int hrs; int mins;
   public : time (int t)
       { hrs = t / 60;
          mins = t % 60; } };
main() {
int duration  = 85;
time T1 = duration; // conversion}

Class to Basic Type

  • Overloading a casting operator function
  • operator <type name> () { … }
vector::operator double()
{ double sum = 0;
   for (int i =0; i <size; i++)
     sum +=v[i]*v[i];
     return sqrt(sum)   }
double length = double(v1) or v1// con to double
operator char*() {return (p);}
string s = “New”; char *q=s; //converts to char*

One Class to Another Class Type

  • Obj_X = Obj_Y; here Obj_Y is a source class and Obj_X is a destination class
  • If conversion to take place in a source class, it is done as a conversion fun. otherwise cons is used
  • In the casting op function the type name may be built-in or may be user defined (class name)
  • In operator type_name(), the type_name will be the destination class
  • For the cons case the arg is the source class

Data Conversion, the Source Class

class invent1
{ int code; int items; float price;
   public invent1(int a, int b, float c)
   { code = a; items = b; price = c; }
  void putdata() {//print all data}
   int getcode() { return code;}
   int getitems() { return items;}
   int getprice()  { return price;}
   operator invent2()
   { int c = code; float v = price * itmes; 
      return invent2(c,v);        }

Data Conversion, the Dest Class

class invent2
{ int code; float value;
   public : invent2() {code = 0; value = 0;}
           invent2(int c, float v) {code = c; value = v;}
           void putdata(){//putting all values!}
     invent2(invent1 in) // this is conversion function
     { code = in.getcode(); 
       value = in.getitems() * in.getprice(); };
{ invent1 s1(100,5,140.0); invent2 d1;
   d1 = s1; // here the conversion takes place!

Note for Overloading ++ and --

  • In old C++, for overloading prefix and postfix versions of ++ and - -, there was no diff.
  • operator - -() or ++() is to be used for prefix version now
  • operator - -(int not_used) and ++(int not_used) for postfix version
  • i.e. it is possible to overload a++ and ++a to do different jobs now! but that will be against the spirit of C++


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Harrison Evans
Harrison Evans author of Operator Overloading and Type Conversions is from London, United Kingdom.
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