Run-time type information in C++

Question

Hi,

What is runtime type control in C++?

P.Gopalakrishnan.

Answer 1

It's dynamic_cast functionality - your code can detect at runtime if a given pointer or reference is really bound to an object of type you expect.

Answer 2

It enables you to identify the dynamic type of a object at run time. For example:

class A
{
   vurtual ~A();
};

class B : public A
{
}

void f(A* p)
{
  //b will be non-NULL only if dyanmic_cast succeeds
  B* b = dynamic_cast<B*>(p);
  if(b) //Type of the object is B
  {
  }
  else //type is A
  { 
  }
}

int main()
{
  A a;
  B b;

  f(&a);
  f(&b);
}

Answer 3

The correct name of this is Run-time type information (RTTI).

Answer 4

It is not just about dynamic_cast, but the entire RTTI is part of it. The best place to learn about RTTI is section 15.4 of the C++ Programming Language by Bjarne Stroustrup

Answer 5

You can take a Interface* and "ask" c++ to what type of object the pointer points. To my knowledge, this relies on runtime meta information, that needs a few cycles for storing and searching such information.

Look at the "typeid" keyword. It provides the most magic.

dynamic_cast only uses RTTI, typeid with std::type_info seems to me more like the "real thing".