Think before you code, Virtual Functions in C++

Introduction

A few days back, I was doing a job, and unintentionally, I made a mistake in the code (What mistake? That I will explain in the detailed section of the article), and when I was caught by a bug and started de-bugging it, I was amazed how a little mistake can give a programmer a whole lot of pain. Yes, I made a mistake in the virtual function area. How? Let's find out........

Using the code

So, why do we need a virtual function? Everyone knows that. Let's say I have a base class and a few derived class as well; and all the derived classes shares a common function, and in the driver program, I do not want to make a big huge switch/if block. I want to iterate through all the derived types and want to execute the common member function. Like this:

#include <iostream>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
class CommunicationDevices
{
 //Base class has some property, for this article I dont need those
 public:
   inline virtual void which(){
     cout<<"This is a common device..."<<endl;
   }
};

class MobilePhoneWithGSMSupport:public CommunicationDevices
{
  //Derived class also has some extended property, GSM related
  public:
    inline virtual void which(){
     cout<<"This is a Mobile Phone...GSM Supported"<<endl;
    }
};

class MobilePhoneWithCDMASupport:public CommunicationDevices
{
  //Derived class also has some extended property, CDMA related
  public:
    inline void which(){
      cout<<"This is a Mobile Phone....CDMA Supported"<<endl;
    }
};

class Landline:public CommunicationDevices
{
  //Derived class also has some extended property
  public:
    inline void which(){
      cout<<"This is a Landline Phone..."<<endl;
    }
};

class Iphone:public MobilePhoneWithGSMSupport
{
  //More specific IPhone Feature here
  public:
    inline void which(){
      cout<<"This is apple Iphone with AT&T connection, GSM Support only..."
          <<endl;
    }
};


void whichPhoneUserIsUsing(CommunicationDevices &devices){
  devices.which();
}

int main(){
 MobilePhoneWithGSMSupport user1;
 MobilePhoneWithCDMASupport user2;
 Landline user3;
 Iphone user4;
 whichPhoneUserIsUsing(user1);
 whichPhoneUserIsUsing(user2);
 whichPhoneUserIsUsing(user3);
 whichPhoneUserIsUsing(user4);
 return 0;
}

Here, the idea is simple. Since we are using a virtual function in the base class, the “whichPhoneUserIsUsing()” method can take a generic base class argument, and the proper method from the derived class gets accessed depending upon the actual type of the object. This is the beauty of virtual functions. Note that in the method “whichPhoneUserIsUsing()”, we used a reference to the base class as the argument: “CommunicationDevices &devices”, and from the driver (main()), we are passing the derived class' object while calling this function. This is normally called as Upcasting in C++. That is, we are going from the more specific type to the more generic type. And, this casting is type-safe always. As you expected, this code will produce the following o/p:

bash-3.2$ g++ -g -o hello code1.cpp

bash-3.2$ ./hello

This is a Mobile Phone...GSM Supported
This is a Mobile Phone....CDMA Supported
This is a Landline Phone...
This is apple Iphone with AT&T connection, GSM Support only...

Now, consider the following code, only a single character (believe me, just a single character) has been changed here from the previous code:

We just modified our method whichPhoneUserIsUsing() like this:

#include <iostream>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
class CommunicationDevices
{
 //Base class has some property, for this article I dont need those
 public:
   inline virtual void which(){
     cout<<"This is a common device..."<<endl;
   }
};

class MobilePhoneWithGSMSupport:public CommunicationDevices
{
  //Derived class also has some extended property, GSM related
  public:
    inline virtual void which(){
     cout<<"This is a Mobile Phone...GSM Supported"<<endl;
    }
};

class MobilePhoneWithCDMASupport:public CommunicationDevices
{
  //Derived class also has some extended property, CDMA related
  public:
    inline void which(){
      cout<<"This is a Mobile Phone....CDMA Supported"<<endl;
    }
};

class Landline:public CommunicationDevices
{
  //Derived class also has some extended property
  public:
    inline void which(){
      cout<<"This is a Landline Phone..."<<endl;
    }
};

class Iphone:public MobilePhoneWithGSMSupport
{
  //More specific IPhone Feature here
  public:
    inline void which(){
      cout<<"This is apple Iphone with AT&T connection, GSM Support only..."
          <<endl;
    }
};


void whichPhoneUserIsUsing(CommunicationDevices devices){
  devices.which();
}

int main(){
 MobilePhoneWithGSMSupport user1;
 MobilePhoneWithCDMASupport user2;
 Landline user3;
 Iphone user4;
 whichPhoneUserIsUsing(user1);
 whichPhoneUserIsUsing(user2);
 whichPhoneUserIsUsing(user3);
 whichPhoneUserIsUsing(user4);
 return 0;
}

We just modified our method whichPhoneUserIsUsing() like this:

void whichPhoneUserIsUsing(CommunicationDevices devices){

 devices.which();

}

and bang.................given below is the output:

bash-3.2$ g++ -g -o hello code2.cpp 

bash-3.2$ ./hello

This is a common device...
This is a common device...
This is a common device...
This is a common device...

bash-3.2$ vim code2.cpp

So, what gets wrong here?

Yes, you guessed it correctly, it's a famous copy-constructor problem. When the arguments are just a “CommunicationDevices” instead of a reference to it, the function says:

Hey Mr. Programmer, I am bound to create only a temporary object for this function (whichPhoneUserIsUsing()). I am no more responsible to take a reference, so I don't care what kind of actual object you are passing through; I will create a concrete “CommunicationDevices” object, and will copy only those segments from the actual object which are meaningful to me (i.e., which are part of the base class). And, will only invoke the “which” method for this temporary object. And hence, every time you call me, I will call the base class version (i.e., CommunicationDevices version) of the which() method.

This famous property is called Object Bisection, or Object Slicing. Cutting down the desired property from one object and copying it to a concrete base class object.

References:

1. C++ Programming Language: Bjarne Stroustrup