Getting "source type is not polymorphic" when trying to use dynamic_cast

Question

struct A {};

struct B : A {};

int main()
{
    A* a = new B();

    B* b = dynamic_cast<B*>(a);
}

gives:

cannot dynamic_cast 'a' (of type 'struct A*') to type 'struct B*' (source type is not polymorphic)

How can I make A polymorphic? I want to safely cast it to B.

(One way is to add a dummy virtual function, but is there a better way?)

If you don't need virtual functions, why do you want polymorphic types? `static_cast` works just fine here. — R. Martinho Fernandes, Feb 27 '13 at 14:09
Adding to R. Martinho comment... if you feel that you need to perform casts you might already be in the wrong track. — David Rodríguez - dribeas, Feb 27 '13 at 14:12
Virtual destructor for polymorphic type is not dummy - it saves you from UB if you decide to fix your memory leak by writing `delete a;` — Tadeusz Kopec for Ukraine, Feb 27 '13 at 14:14
@user1131467: That is why I said *might already be*... although in general casts tend to indicate flaws in the design. — David Rodríguez - dribeas, Feb 27 '13 at 14:39
@TadeuszKopec That's true but the compiler should really complain already at "A* a = new B();" — TamaMcGlinn, Aug 03 '18 at 07:54
@all above: There are lots of reasons to have polymorphic types without virtual functions. In my case I have a class that returns an event object to a callback handler. This is subclassed depending on what the event is. Some events contain a message (info/warning/error), some events contain a % progress value, and so on. The base class contains a type value so that the handler knows which sub-class to dynamically cast the event to. — AlastairG, Jul 07 '23 at 12:26

score 146 · Accepted Answer · edited Dec 06 '17 at 14:10

146

You need to make A polymorphic, which you can do by adding a virtual destructor or any virtual function:

struct A {
  virtual ~A() = default;
};

or, before C++11,

struct A {
  virtual ~A() {}
};

Note that a polymorphic type should have a virtual destructor anyway, if you intend to safely call delete on instances of a derived type via a pointer to the base.

edited Dec 06 '17 at 14:10

Bathsheba

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answered Feb 27 '13 at 14:07

juanchopanza

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score 16 · Answer 2 · answered Feb 27 '13 at 14:07

16

You need at least a virtual function - typically, if no others are suitable, the destructor:

struct A {
   virtual ~A() {}
};

answered Feb 27 '13 at 14:07

Luchian Grigore

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I thought I don't need a destructor because the class has no member data. Turns out it needs one, thanks! – Leedehai Jul 20 '18 at 06:50

score 8 · Answer 3 · edited Nov 29 '18 at 09:11

8

As your compiler says, your type A is not polymorphic. You should add a virtual function to it. For instance, a virtual destructor could be a good choice:

struct A { virtual ~A() { } };
//         ^^^^^^^ This makes A a polymorphic type

struct B : A {};

int main()
{
    A* a = new B();

    B* b = dynamic_cast<B*>(a); // Should work now
}

edited Nov 29 '18 at 09:11

ChrisF

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answered Feb 27 '13 at 14:07

Andy Prowl

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I thought I don't need a destructor because the class has no member data. Turns out it needs one, thanks! – Leedehai Jul 20 '18 at 06:50

Getting "source type is not polymorphic" when trying to use dynamic_cast

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