Object Slicing in C++

2.6

In C++, a derived class object can be assigned to a base class object, but the other way is not possible.

class Base { int x, y; };

class Derived : public Base { int z, w; };

int main() 
{
    Derived d;
    Base b = d; // Object Slicing,  z and w of d are sliced off
}

Object slicing happens when a derived class object is assigned to a base class object, additional attributes of a derived class object are sliced off to form the base class object.

#include <iostream>
using namespace std;

class Base
{
protected:
    int i;
public:
    Base(int a)     { i = a; }
    virtual void display()
    { cout << "I am Base class object, i = " << i << endl; }
};

class Derived : public Base
{
    int j;
public:
    Derived(int a, int b) : Base(a) { j = b; }
    virtual void display()
    { cout << "I am Derived class object, i = "
           << i << ", j = " << j << endl;  }
};

// Global method, Base class object is passed by value
void somefunc (Base obj)
{
    obj.display();
}

int main()
{
    Base b(33);
    Derived d(45, 54);
    somefunc(b);
    somefunc(d);  // Object Slicing, the member j of d is sliced off
    return 0;
}

Output:

I am Base class object, i = 33
I am Base class object, i = 45

We can avoid above unexpected behavior with the use of pointers or references. Object slicing doesn’t occur when pointers or references to objects are passed as function arguments since a pointer or reference of any type takes same amount of memory. For example, if we change the global method myfunc() in the above program to following, object slicing doesn’t happen.

// rest of code is similar to above
void somefunc (Base &obj)
{
    obj.display();
}           
// rest of code is similar to above

Output:

I am Base class object, i = 33
I am Derived class object, i = 45, j = 54

We get the same output if we use pointers and change the program to following.

// rest of code is similar to above
void somefunc (Base *objp)
{
    objp->display();
}

int main()
{
    Base *bp = new Base(33) ;
    Derived *dp = new Derived(45, 54);
    somefunc(bp);
    somefunc(dp);  // No Object Slicing
    return 0;
}

Output:

I am Base class object, i = 33
I am Derived class object, i = 45, j = 54

Object slicing can be prevented by making the base class function pure virtual there by disallowing object creation. It is not possible to create the object of a class which contains a pure virtual method.

This article is contributed by Pravasi Meet. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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