C++ Programming

The dynamic_cast keyword is used to casts a datum from one pointer or reference a of polymorphic type to another, similar to static_cast but performing a type safety check at runtime to ensure the validity of the cast. Generally for the purpose of casting a pointer or reference up the inheritance chain (inheritance hierarchy) in a safe way, including performing so-called cross casts.

Syntax

    TYPE& dynamic_cast<TYPE&> (object);
    TYPE* dynamic_cast<TYPE*> (object);

The target type must be a pointer or reference type, and the expression must evaluate to a pointer or reference.

If you attempt to cast to a pointer type, and that type is not an actual type of the argument object, then the result of the cast will be NULL.

If you attempt to cast to a reference type, and that type is not an actual type of the argument object, then the cast will throw a std::bad_cast exception.

When it doesn't fail, dynamic cast returns a pointer or reference of the target type to the object to which expression referred.

  struct A {
    virtual void f() { }
  };
  struct B : public A { };
  struct C { };
 
  void f () {
    A a;
    B b;
 
    A* ap = &b;
    B* b1 = dynamic_cast<B*> (&a);  // NULL, because 'a' is not a 'B'
    B* b2 = dynamic_cast<B*> (ap);  // 'b'
    C* c = dynamic_cast<C*> (ap);   // NULL.
 
    A& ar = dynamic_cast<A&> (*ap); // Ok.
    B& br = dynamic_cast<B&> (*ap); // Ok.
    C& cr = dynamic_cast<C&> (*ap); // std::bad_cast
  }