Thursday, October 13, 2011

C++ Tutorial: Operator Overloading

C++ incorporates the option to use standard operators to perform operations with classes in addition to with fundamental types. For example:
int a, b, c;
a = b + c;
This is obviously valid code in C++, since the different variables of the addition are all fundamental types. Nevertheless, it is not so obvious that we could perform an operation similar to the following one:
struct {
string product;
float price;
} a, b, c;
a = b + c; 
In fact, this will cause a compilation error, since we have not defined the behavior our class should have with addition operations. However, thanks to the C++ feature to overload operators, we can design classes able to perform operations using standard operators. All operators except the following six operators are overloadable:
  1. Scope resolution operator(::)
  2. Dot operator (.)
  3. Sizeof operator (sizeof())
  4. Type cast operator (typeid())
  5. Conditional operator (?:)
  6. Pointer to member operator (.*)
To overload an operator in order to use it with classes we declare operator functions, which are regular functions whose names are the operator keyword followed by the operator sign that we want to overload. The format is: type operator sign (parameters) { /*...*/ } Here you have an example that overloads the addition operator (+). We are going to create a class to store two dimensional vectors and then we are going to add two of them: a(3,1) and b(1,2). The addition of two  two dimensional vectors is an operation as simple as adding the two x coordinates to obtain the resulting x coordinate and adding the two y coordinates to obtain the resulting y. In this case the result will be (3+1,1+2) = (4,3).
// vectors: overloading operators example
#include <iostream>
using namespace std;

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;
}
It may be a little confusing to see so many times the CVector identifier. But, consider that some of them refer to the class name (type) CVector and some others are functions with that name (constructors must have the same name as the class). Do not confuse them:
CVector (int, int);            // function name CVector (constructor)
CVector operator+ (CVector);   // function returns a CVector 
The function operator+ of class CVector is the one that is in charge of overloading the addition operator (+). This function can be called either implicitly using the operator, or explicitly using the function name:
c = a + b;
c = a.operator+ (b);
Both expressions are equivalent. Notice also that we have included the empty constructor (without parameters) and we have defined it with an empty block:
CVector () { };
This is necessary, since we have explicitly declared another constructor:
CVector (int, int);
And when we explicitly declare any constructor, with any number of parameters, the default constructor with no parameters that the compiler can declare automatically is not declared, so we need to declare it ourselves in order to be able to construct objects of this type without parameters. Otherwise, the declaration:
CVector c;
included in main() would not have been valid. Anyway, I have to warn you that an empty block is a bad implementation for a constructor, since it does not fulfill the minimum functionality that is generally expected from a constructor, which is the initialization of all the member variables in its class. In our case this constructor leaves the variables x and y undefined. Therefore, a more advisable definition would have been something similar to this:
CVector () { x=0; y=0; };
which in order to simplify and show only the point of the code I have not included in the example. As well as a class includes a default constructor and a copy constructor even if they are not declared, it also includes a default definition for the assignment operator (=) with the class itself as parameter. The behavior which is defined by default is to copy the whole content of the data members of the object passed as argument (the one at the right side of the sign) to the one at the left side
CVector d (2,3);
CVector e;
e = d;           // copy assignment operator 
The copy assignment operator function is the only operator member function implemented by default. Of course, you can redefine it to any other functionality that you want, like for example, copy only certain class members or perform additional initialization procedures. 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, the code may not be very intuitive if you 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 what is at the right side of the operator as the parameter for the operator member function of the object at its left side, other operators may not be so obvious. Here you have a table with a summary on how the different operator functions have to be declared (replace @ by the operator in each case):
Points to be remembered while overloading operators
  1. All operators are not overloadable. Any attempt to make these operators overloadable will case compiler error.
  2. The ‘+’ operator cannot be made as ‘-‘ due to overloading.
  3. Overloading only extends the meaning of operator and their use but does not change their identity. For example, ‘+’ operator can be extended to add two complex numbers.
  4. Although C++ programmers can write program that use ‘+’ operator to subtract two complex numbers but it is very bad programming practice. Programmer should be aware of this.
  5. Overloading can be used very efficiently in data conversion between user defined and basic or between user defined themselves.

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