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What does the virtual keyword do?
Hasham Ahmad
Virtual keyword
In C++, the virtual keyword is used to create a virtual function.
Virtual function
A virtual function is the member function of the class. This function is declared in the parent class and overridden by the child class.
This is true when a pointer from a parent class points to a child class object.
Importance of a virtual function
- Irrespective of the nature of the reference (or the pointer) that is used for a function call,
virtualfunctions make sure that the right function is called for an object. - Runtime polymorphism is achieved by the
virtualfunction.
Syntax
virtual returntype functionName(/*No of parameters*/)
{
// body of function
}
Code example
#include<iostream>
using namespace std;
class parent {
public:
virtual void virtualDisplayMessage()
{
cout << "Parnent class function virtual function."<<endl;
}
void simpleDisplayMessage()
{
cout << "Parent class simple function."<<endl;
}
};
class child : public parent {
public:
void virtualDisplayMessage()
{
cout << "Child class override virtual function. "<<endl;
}
void simpleDisplayMessage()
{
cout << "Child class simple function. "<<endl;
}
};
int main()
{
// declare class object
parent *parentClassPointer; // make parent class pointer
child childClassObject; // make dreived class object
parentClassPointer = &childClassObject;// assign derived class object
// calling class fucntions
parentClassPointer->virtualDisplayMessage(); // Runtime polymorphism is acheieved using virtual function
parentClassPointer->simpleDisplayMessage(); // Non virtual function,
return 0;
}Implementation of the "virtual" function
Code explanation
- Line 4: We create the class name
parent. - Line 6: We implement a public virtual function called
virtualDisplayMessage. - Line 11: We implement a non-virtual function called
simpleDisplayMessage. - Line 17: We create the class name
childand inherit it with theparentclass. - Line 19: We override the virtual function
virtualDisplayMessage. - Line 24: We implement a non-virtual function called
simpleDisplayMessage.
Note: We don’t need the
virtualkeyword in thechildclass when we have avirtualfunction in theparentclass that is being overridden in thechildclass. Functions are consideredvirtualfunctions in thechildclass.
RELATED TAGS
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virtual
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c++
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Hasham Ahmad
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