Section 4.2 Overloading operators

C++ incorporates the option to use language standard operators between classes in addition to between fundamental types. For example:

int a, b, c;
a = b + c;

struct { char product [50]; float price; } a, b, c;
a = b + c;

But thanks to the C++ ability to overload operators, we can get to do that. Objects derived from composed types such as the previous one can accept operators which would not be accepted otherwise, and we can even modify the effect of operators that they already admit. Here is a list of all the operators that can be overloaded:

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

type operator sign (parameters);

// vectors: overloading operators example
#include <iostream.h>

class CVector {
  public:
    int x,y;
    CVector () {};
    CVector (int,int);
    CVector operator + (CVector);
};

CVector::CVector (int a, int b) {
  x = a;
  y = b;
}

CVector CVector::operator+ (CVector param) {
  CVector temp;
  temp.x = x + param.x;
  temp.y = y + param.y;
  return (temp);
}

int main () {
  CVector a (3,1);
  CVector b (1,2);
  CVector c;
  c = a + b;
  cout << c.x << "," << c.y;
  return 0;
}

If you are baffled seeing CVector so many times, consider that some of them make reference to the class name CVector and others are functions with that name. Do not confuse them:

CVector (int, int);            // function name CVector (constructor)
CVector operator+ (CVector);   // function operator+ that returns CVector type

c = a + b;
c = a.operator+ (b);

Notice also that we have incuded the empty constructor (without parameters) and we have defined it with a no-op block of instructions: CVector () { }; this is necessary, since there already exists another constructor, CVector (int, int); so none of the default constructors will exist in CVector if we do not explicitly declare one as we have done. Otherwise the declaration CVector c; included in main() would not be valid. Anyway, I have to warn you that a no-op block is not a recommended implementation for a constructor, since it does not fulfill the minimum functionality that a constructor should have, which is the initialization of all the variables in the class. In our case this constructor leaves variables x and y undefined. Therefore, a more advisable declaration would have been something similar to this: CVector () { x=0; y=0; }; that for simplicity I have not included in the code.

As well as a class includes by deafult an empty and a copy constructor, it also includes a default definition for the assignation operator (=) between two classes of the same type. This copies the whole content of the non-static data members of the parameter object (the one at the right side of the sign) to the one at the left side. Of course, you can redefine it to any other functionality that you want for this operator, like for example, copy only certain class members.

The overload of operators does not force its operation to bear a relation to the mathematical or usual meaning of the operator, although it is recommended. For example, it is not very logical to use operator + to subtract two classes or operator == to fill with zeros a class, although it is perfectly possible to do so.

Although the prototype of a function operator+ can seem obvious since it takes the right side of the operator as the parameter for the function operator+ of the left side object, other operators are not so clear. Here you have a table with a summary on how the different operator functions must be declared (replace @ by the operator in each case):

Expression	Operator (@)	Function member	Global function
@a	+ - * & ! ~ ++ --	A::operator@()	operator@(A)
a@	++ --	A::operator@(int)	operator@(A, int)
a@b	+ - * / % ^ & | < > == != <= >= << >> && || ,	A::operator@(B)	operator@(A, B)
a@b	= += -= *= /= %= ^= &= |= <<= >>= [ ]	A::operator@(B)	-
a(b, c...)	()	A::operator()(B, C...)	-
a->b	->	A::operator->()	-

* where a is an object of class A, b is an object of class B and c is an object of class C.

The keyword this

It can be used to check if a parameter passed to a member function of an object is the object itself. For example,

// this
#include <iostream.h>

class CDummy {
  public:
    int isitme (CDummy& param);
};

int CDummy::isitme (CDummy& param)
{
  if (&param == this) return 1;
  else return 0;
}

int main () {
  CDummy a;
  CDummy* b = &a;
  if ( b->isitme(a) )
    cout << "yes, &a is b";
  return 0;
}

It is also frequenty used in operator= member functions that return objects by reference (avoiding the use of temporary objects). Following with the vector's examples seen before we could have written an operator= function like this:

CVector& CVector::operator= (const CVector& param)
{
  x=param.x;
  y=param.y;
  return *this;
}

Static members

Static data members of a class are also known as "class variables", because their content does not depend on any object. There is only one unique value for all the objects of that same class.

For example, it may be used for a variable within a class that can contain the number of objects of that class that have been declared, as in the following example:

// static members in classes
#include <iostream.h>

class CDummy {
  public:
    static int n;
    CDummy () { n++; };
    ~CDummy () { n--; };
};

int CDummy::n=0;

int main () {
  CDummy a;
  CDummy b[5];
  CDummy * c = new CDummy;
  cout << a.n << endl;
  delete c;
  cout << CDummy::n << endl;
  return 0;
}

In fact, static members have the same properties as global variables but they enjoy class scope. For that reason, and to avoid that they may be declared several times, according to ANSI-C++ standard, we can only include the protype (declaration) in the class declaration but not the definition (initialization). In order to initialize a static data-member we must include a formal definition outside the class, in the global scope, as in the previous example.

Because it is a unique variable for all the objects of the same class, it can be referred to as a member of any object of that class or even directly by the class name (of course this is only valid for static members):

cout << a.n;
cout << CDummy::n;

Once again, I remind you that in fact it is a global variable. The only difference is its name outside the class.

Just as we may include static data within a class, we can also include static functions. They represent the same: they are global functions that are called as if they were object members of a given class. They can only refer to static data, in no case to nonstatic members of the class, as well as they do not allow the use of the keyword this, since it makes reference to an object pointer and these functions in fact are not members of any object but direct members of the class.

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