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Asked: 2026-05-11T10:28:24+00:00

I have a base class and a derived class. Each class has an .h

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I have a base class and a derived class. Each class has an .h file and a .cpp file.

I am doing dynamic_cast of the base class object to the derived class in the following code:

h files:

class Base {   public:     Base();     virtual ~Base(); };  class Derived : public Base {   public:     Derived(){};     void foo(); };  class Another {   public:     Another(){};     void bar(Base* pointerToBaseObject); };

cpp files:

Base::Base() {     //do something.... } Base::~Base() {     //do something.... } void Derived::foo() {     Another a;     a.bar(this); } void Another::bar(Base* pointerToBaseObject) {     dynamic_cast<Derived*>(pointerToBaseObject) }

From some strange reason, the casting fails (returns NULL). However, the casting succeeds if I move the implementation of Derived class’s constructor from .h to the .cpp file.

What can cause it?

The compiler is gcc 3.1, on Linux-SUSE. BTW, I see this behavior only on this platform, and the same code works fine in Visual Studio.

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1 Answer

  1. The code, as posted, shouldn’t fail, provided you have a virtual function in the base class (as litb pointed out).

    But I believe every current compiler generates a ‘Base class is not polymorphic’ kind of error if you hadn’t, so that probably won’t be the problem.

    The only thing I can think of is that due to some weird bug everything gets inlined and no vtable gets generated. But if you put the constructor in the C++ file, the compiler decides not to inline everything, triggering the creation of a vtable, causing your cast to work.

    But this is very wild guesswork, and I don’t think any compiler would have such a mistake in it (?)

    If you want a definite answer, post more code. And the compiler / platform used.

    EDIT: Seeing the updated code

    I think you should at least derive Derived from Base 😉 (I suppose that’s a typo)

    But after seeing the code, the only thing I can think of is that gcc (wrongly) inlines everything and doesn’t generate a vtable for Derived. For what it’s worth, this runs fine compiled with gcc 4.0

    3.1 is over 7 years old by now… if there’s any possibility to upgrade I’d go for it.

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