dynamic_cast

In C++, dynamic_cast is mainly used for safe downcasting at run time. To work on dynamic_cast there must be one virtual function in the base class. A dynamic_cast works only polymorphic base class because it uses this information to decide safe downcasting.
```
#include <iostream>
  
using namespace std;
class Base {
    virtual void print() { cout << "Base" << endl; }
};
  
class Derived : public Base { 
    void print() { cout << "Derived" << endl; }
};

int main()
{
    Derived d;
    Base* bp = &d;
    Derived* dp = dynamic_cast<Derived*>(bp);
    if (dp == nullptr)
        cout << "null" << endl;
    else
        cout << "not null" << endl;
  
    return 0;
}
// not null
```
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