Dynamic casting Failure in C++

Question

I have the following class hierarchy:

class A {
      virtual void blah() = 0;
};
class B {
      virtual void gah() = 0;
};
class C: public A, public B {};
class D: public C {
      gah() {}
      blah() {}
};

If I do the following:

A *something = new D();
auto a = dynamic_cast<B *>(something);
if(a == nullptr) std::cout << "NULL" << std::endl;

It prints "NULL" in the context of the program I am running (I simplified the code so that it is easily read). This suggests that the dynamic cast failed.

But if I do:

A *something = new D();
auto a = dynamic_cast<C *>(something);
auto b = dynamic_cast<B *>(a);
if(a == nullptr) std::cout << "NULL" << std::endl;

It won't print "NULL" and the dynamic_cast succeeds. Any idea why this would be the case? I thought about inlining, but I made sure every class had a virtual destructor and generated an object file.

Also please excuse any small syntax errors, the idea should be clear.

Yeah fixed class D: public D (should be C). Also the destructors are virtual, just not shown. — ggobieski, Nov 05 '18 at 21:13
Yes I know and that is what I would expect, I am saying that this is the structure I have (this is not my exact code) and I was wondering if someone could think of a reason why this would misbehave in a larger context. — ggobieski, Nov 05 '18 at 21:17
based on @Swordfish either this isnt your code (and you say it isnt) or you need to say what compiler you are using — pm100, Nov 05 '18 at 21:18
"this is not my exact code" - this is the problem with this question... so i'm voting to close because no mcve — user7860670, Nov 05 '18 at 21:20
Please don't oversimplify the code. Make a small example that compiles. — Ted Lyngmo, Nov 05 '18 at 21:20
This is not the exact code, that is correct. I just wanted to keep it abstract. Also the compile is g++ 6.3 — ggobieski, Nov 05 '18 at 21:20
@ggobieski Guessing at the issue is pointless. We'd need some code that reproduces the issue in order to help you. — George, Nov 05 '18 at 21:22
No output for gcc 6.3 either. You oversimplified your example. — Swordfish, Nov 05 '18 at 21:23
I can guess that one of those classes does not have a vtable so dynamic_cast does not work. — user7860670, Nov 05 '18 at 21:24
While I try to get a compiling example (easier said than done), if a vtbl didn't exist why would the second method work? — ggobieski, Nov 05 '18 at 21:49

score 0 · Answer 1 · answered Nov 05 '18 at 22:28

0

Thanks for the help.

Basically dynamic_cast was failing because two classes were conflicting, breaking the one definition rule. The linker, compiler, and I failed to spot this (project is pretty large and some of it is hidden in libraries that I didn't write). So yeah, make sure your classes don't conflict and then dynamic_cast will work.

answered Nov 05 '18 at 22:28

ggobieski

149
1
14

This answer isn't useful to anyone else. There's nothing in the question to point at this even being an issue. You should delete the entire question. – xaxxon Nov 05 '18 at 22:48

Dynamic casting Failure in C++

1 Answers1