Pure Virtual Functions or Abstract Class

A pure virtual function (or abstract function) in C++ is a virtual function for which we don’t have implementation, we only declare it. A pure virtual function is declared by assigning 0 in declaration.
1) A class is abstract if it has at least one pure virtual function

/ pure virtual functions make a class abstract

#include

using namespace std;

 

class Test

{

   int x;

public:

    virtual void show() = 0;

    int getX() { return x; }

};

 

int main(void)

{

    Test t;

    return 0;

}

Output:

Compiler Error: cannot declare variable 't' to be of abstract
 type 'Test' because the following virtual functions are pure 
within 'Test': note: 	virtual void Test::show() 

2) We can have pointers and references of abstract class type.

#include using namespace std;   class Base { public:     virtual void show() = 0; };   class Derived: public Base { public:     void show() { cout << "In Derived \n"; } };   int main(void) {     Base *bp = new Derived();     bp->show();     Derived d;     d.show(); //In Derived.     return 0; }

Output:

In Derived 

3) If we do not override the pure virtual function in derived class, then derived class also becomes abstract class.

#include using namespace std; class Base { public:     virtual void show() = 0; };   class Derived : public Base { };   int main(void) {   Derived d;   return 0; }

Compiler Error: cannot declare variable 'd' to be of abstract type 
'Derived'  because the following virtual functions are pure within
'Derived': virtual void Base::show(

4) An abstract class can have constructors.
For example, the following program compiles and runs fine.

#include using namespace std;   // An abstract class with constructor class Base { protected:    int x; public:   virtual void fun() = 0;   Base(int i) { x = i; } };   class Derived: public Base {     int y; public:     Derived(int i, int j):Base(i) { y = j; }     void fun() { cout << "x = " << x << ", y = " << y; } };   int main(void) {     Derived d(4, 5);     d.fun();     return 0; }

Output:

x = 4, y = 5

In C++, an interface can be simulated by making all methods as pure virtual. In Java, there is a separate keyword for interface.
In Java, a class can be made abstract by using abstract keyword. Similarly a function can be made pure virtual or abstract by using abstract keyword.