Identifying derived class type from a base class

Question

In this scenario where classes DeriA and DeriB inherit from Base:

class Base
class DeriA : public Base
class DeriB : public Base

std::list<Base> objects;

Is it possible to check what type of class is being inherited from each member of the objects list? I've tried using static_cast within a try/catch statement in order to check if a certain member of objects is a particular type as seen here:

try
{
    DeriA tempA = static_cast<DeriA>(*objects_iterator);
    std::cout << "Found A" << std::endl;
} catch(std::bad_cast e)
{
    // Dealing with the exception
    std::cout << "Found B" << std::endl;
}

However this always outputs "Found A" regardless of whether or not the object that was being accessed is DeriA or DeriB. Could anyone help shed some light as to why?

Answer 1

If you have a Base* pointer you can dynamic_cast<DeriA*>. The result will be the cast object or NULL if it is not a DeriA.

However, you are dealing with values. A Base (not Base*) that you store in the list will always only be a Base and not a derived object. If you ever had a DeriA that you put in there, you probably sliced the rest of at some point.

Answer 2

From what little examples I saw, most using std::bad_cast exception are using dynamic_cast. Check this example out and see if dynamic_cast can help you. There is a requirement with dynamic_cast to have polymorphic functions, but you'll figure that out.

http://en.cppreference.com/w/cpp/types/bad_cast

-AP_

Answer 3

No you cannot properly do this. C++ has no build in concept of runtime type information, therefore it is not possible to cast around with static casts and see what happens.

Some compilers albeit implement a (horrible) RTTI system which allows you to make use of dynamic_cast. How ever the RTTI on most systems has unwanted side effects which causes large frameworks like Qt, etc to implement their own MACRO Magic RTTI system.