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?

Not really, because your list holds `Base` objects. So unless the derived types set a member of `Base` to some value you can check, you're out of luck. But in general this is an indicator that you need a re-design. — juanchopanza, Mar 11 '14 at 11:50
Another problem is that your inheritance is private, so a derived type is-not-a `Base`. I assume that is just a typo and that you meant to use public inheritance. — juanchopanza, Mar 11 '14 at 11:51
You might want to read about [object slicing](http://en.wikipedia.org/wiki/Object_slicing), which is a possible problem with the objects stored in the list. — Some programmer dude, Mar 11 '14 at 11:54
Why does that code even compile? Does DeriA define a conversion constructor from Base or something? Also, afaik bad_cast is only thrown by dynamic_cast when casting a reference, not static_cast. — heinrichj, Mar 11 '14 at 11:55

score 1 · Accepted Answer · answered Mar 11 '14 at 12:00

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.

score 0 · Answer 2 · answered Mar 11 '14 at 11:59

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_

score -1 · Answer 3 · answered Mar 11 '14 at 11:53

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.

Identifying derived class type from a base class

3 Answers3