pure virtual function

A pure virtual function is an abstract method in C++.  A pure virtual function is declared by assigning 0 to a virtual function. 
Pure virtual function is used to create abstract class in C++. A class is an abstract class if there is at least a single pure virtual function. A pure virtual function must override in the derived class, otherwise the derived class will also become abstract class. 
* Pure virtual function can has an implementation in base class outside class definition.
* Abstract methods may not be declared final. It can be private or protected, but cannot used by base pointer or reference.
* Implementing an abstract method in a subclass follows the same rules for overriding a method. The name and signature must be the same, and the visibility of the method in the subclass must be at least as accessible as the method in the parent class.
```
#include<iostream>  
using namespace std;  

class Base {  
protected:
    int i;
public:  
    Base():i(1) {}
    virtual void show()=0;
};  

void Base::show() { cout<<i<<endl; }  // pure virtual function can still have its implementation

class Derived:public Base {  
public:  
    void show() { cout<<2*i<<endl; }  
};  

int main()  {  
    Derived d;  
    Base &b=d;  

    b.show();  // 2
    b.Base::show(); // 1
    return 0;  
} 
```
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