How to determine actual object type at runtime in C++;

Question

Lets say we have a class hierarchy. At the bottom we have Base and at the top Derived. How to determine object class even if it is converted to base class pointer.

Answer 1

If all you want to do is find if b actually points to Derived, just use dynamic_cast():

if (dynamic_cast<Derived*>(b)) { ... }

dynamic_cast returns a null pointer if the actual runtime type of the object pointed to by b is not Derived (or a class derived from Derived). Unlike the name() member of std::type_info, this is compiler-invariant.

Note that this only works if Base has at least one virtual member functions. Which it should anyway, since you're manipulating types derived from it through a base pointer, so it should have a virtual destructor.

Answer 2

make sure the base class has at least one virtual method, include <typeinfo> and use your current code just with an additional dereferencing, typeid(*b).name().

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in passing, note that a typeid call is the one place in C++ where you can dereference a nullpointer with well-defined behavior, which implies that it can throw an exception:

C++11 §5.2.8/2:
“If the glvalue expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value (4.10), the typeid expression throws the std::bad_typeid exception (18.7.3).”