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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.

Base* b = new Derived():

typeid(b).name(); // i want this to tell me that this is actually derived not base object

is there any way other than manual implementation of string field or such and virtual get function?

PS: I talking about compiler-independent solution

spinkus
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user1079475
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    This is possible as explained in the answer, but this is almost always a wrong thing to do. What are you trying to achieve here? – n. m. could be an AI Apr 09 '13 at 09:28
  • Nothing exactly it is just for the purpose of argueing :) – user1079475 Apr 09 '13 at 09:40
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    More application oriented version: http://stackoverflow.com/questions/351845/finding-the-type-of-an-object-in-c || More generic version that does not mention inheritance: http://stackoverflow.com/questions/11310898/how-do-i-get-the-type-of-a-variable – Ciro Santilli OurBigBook.com Jun 18 '15 at 15:11

2 Answers2

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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().

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).”

Cheers and hth. - Alf
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16

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.

Angew is no longer proud of SO
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