Cpp Tutorial: Object Slicing in C++

In this tutorial we shall see what is object slicing in C++

1. Object Slicing occurs when a derived class object is assigned to the base class object.

2. Then, an instance of derived class object is copied into the base class object, omitting the additional attributes of derived class object.

3. It means the data members of the derived class will not be present

4. This is called as Object Slicing in C++

#include<iostream>
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

class Base 
{ 
    int x;
    int y; 
    
}; 
  
class Derived : public Base 
{ 
    int a;
    int b; 
    
}; 
  
int main()  
{ 
    Derived d; 
    Base b = d; // Object Slicing,  a and b of d are sliced off 
}

How to solve object slicing problem?

To solve the object slicing problem, use pointers or references.

Object Slicing will not occur if you pass pointers or reference to a function.

To know the difference, consider below 2 examples:

Example 1:

#include<iostream>
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

#include <iostream> 
using namespace std; 
  
class Base 
{ 

public:
    int i;
    Base(int a) 
    { 
        i = a; 
    } 
    virtual void display() 
    { 
        cout << "In Base class display(), i = " << i << endl; 
        
    } 
}; 
  
class Derived : public Base 
{ 
 
public: 
    int j; 
    Derived(int a, int b) : Base(a) 
    { 
        j = b; 
    } 
    
    void display() 
    { 
        cout << "In Derived class display(), i = "<< i << ", j = " << j << endl;  } 
}; 
  
//a function to display the values , using pass by value
void somefunc (Base obj) 
{ 
    obj.display(); 
} 
  
int main() 
{ 
    Base b(10); 
    Derived d(20, 30); 
    somefunc(b); 
    somefunc(d);  // Object Slicing, the member j of d is sliced off 
    return 0; 
}

Output:

In Base class display(), i = 10
In Base class display(), i = 20

As you can see, each time only the base class display() is getting called, and the member “j” is sliced off.

Example 2:

Now we shall pass as reference as arguments to the “somefunc()” function.

#include<iostream>
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

#include <iostream> 
using namespace std; 
  
class Base 
{ 

public:
    int i;
    Base(int a) 
    { 
        i = a; 
    } 
    virtual void display() 
    { 
        cout << "In Base class display(), i = " << i << endl; 
        
    } 
}; 
  
class Derived : public Base 
{ 
 
public: 
    int j; 
    Derived(int a, int b) : Base(a) 
    { 
        j = b; 
    } 
    
    void display() 
    { 
        cout << "In Derived class display(), i = "<< i << ", j = " << j << endl;  } 
}; 
  
//a function to display the values  pass by reference
void somefunc (Base& obj) 
{ 
    obj.display(); 
} 
  
int main() 
{ 
    Base b(10); 
    Derived d(20, 30); 
    somefunc(b); 
    somefunc(d);  // Object Slicing, the member j of d is sliced off 
    return 0; 
}

Output:

In Base class display(), i = 10
In Derived class display(), i = 20, j = 30

We can also achieve this by using pointers as shown below:

#include<iostream>
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

#include <iostream> 
using namespace std; 
  
class Base 
{ 

public:
    int i;
    Base(int a) 
    { 
        i = a; 
    } 
    virtual void display() 
    { 
        cout << "In Base class display(), i = " << i << endl; 
        
    } 
}; 
  
class Derived : public Base 
{ 
 
public: 
    int j; 
    Derived(int a, int b) : Base(a) 
    { 
        j = b; 
    } 
    
    void display() 
    { 
        cout << "In Derived class display(), i = "<< i << ", j = " << j << endl;  } 
}; 
  
//a functon to display the values  
void somefunc (Base *objp) 
{ 
    objp->display(); 
} 
  
int main() 
{ 
    Base *bp = new Base(10) ; 
    Derived *dp = new Derived(20, 30); 
    somefunc(bp); 
    somefunc(dp);  // No Object Slicing 
    return 0; 
}

Output:

In Base class display(), i = 10
In Derived class display(), i = 20, j = 30