Function overloading

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• What is function overloading?

• What is the ‘this’ keyword?

Objectives

• Add a member function to only display part of our Vector.

It might be nice to have a way to display only part of our vector, particularly if it is long.

Function overloading allows you to declare multiple functions with the same name that are distinguished by the type and number of parameters passed to the function. We could overload our display function to take an extra int argument to indicate how many elements we want to output.

$ cp member_functions.cpp function_overload.cpp
$ nano function_overload.cpp

and modify it to look like this:

#include <iostream>

class Vector{
public:
  int size;
  int* data;
  
  void display(int num){
    int numToDisplay=size;
    if(num<size && num>=0){
      numToDisplay=num;
    }
    
    std::cout<<"Vector: size="<<size;
    std::cout<<", contents=(";
    for(int i=0;i<numToDisplay-1;++i){
      std::cout<<data[i]<<",";
    }
    if(numToDisplay<size){
      std::cout<<data[numToDisplay-1]<<"...)\n";
    }
    else{
      std::cout<<data[numToDisplay-1]<<")\n";
    }
  }
  void display(){
    display(size);
  }
};

int main(){
  Vector a;
  a.size=10;
  a.data=new int[a.size];
  a.data[9]=15;
  a.display();
  a.display(5);
  delete [] a.data;
}

To avoid having two functions with fairly similar duplicate code, we also now call our new function from the old function but just pass the size of the vector. This will help with maintainability in that we only need to make changes to the display function in one place.

We can now distinguish between which of these two function we wish to call, by either including an int parameter or not in the function call, e.g.

a.display();

vrs.

a.display(5);

Lets build and run it

$ g++ function_overload.cpp -o function_overload
$ ./function_overload

Vector: size=10, contents=(0,0,0,0,0,0,0,0,0,15)
Vector: size=10, contents=(0,0,0,0,0...)

The elements 0-8 are zeros, but we didn’t set them to zero. C and also C++ don’t initialize variables to zero so we just got lucky that all these values where zero. The only one value that we can rely on is the one we actually set, the 15 at index 9.

Function overloading works not just for class member functions, but also for regular independent functions.

The this keyword

There is a special keyword that can be used in class member functions this, which is a pointer to the class object that the function was invoked from (e.g. the a object in the a.display(); line). We could have written our class member function like this:

class A{
public:
  int foo;
  void display(){
    std::cout<<"foo="<<this->foo<<"\n";
  }
};

You might like to use the this keyword to make it clear when you are and aren’t referencing the class object, but it isn’t needed in this example. In a later episode we will use the this keyword to solve a problem we wouldn’t have been able to without it.

Key Points

• Any function can be overloaded, class member function or stand alone function

• Overloaded functions are only distinguished by their parameters and not their return types.

• The this keyword allows you to explicitly reference the object within a member function.