dynamic_cast causing a bad_cast error

Question

I am trying to understand how dynamic_cast works. I have a class interfaceElement with a class menu derived from it. In my program, I create no objects of class interfaceElement. When executing the following code (executing the line with dynamic_cast, to be more precise), the bad_cast exception is thrown

void drawInterfaceElement(interfaceElement elem, int colour1, int colour2)
{
    if (elem.getType()=="MENU")
    {
        menu &cur = dynamic_cast<menu&>(elem);
        this->drawMenu(&cur,colour1,colour2);
    }
}

Why?

score 2 · Accepted Answer · edited May 23 '17 at 12:14

2

Take interfaceElement by reference. Currently you incur slicing by taking by value, causing the menu subobject to be lost. dynamic_cast cannot cast to that type, causing std::bad_cast to be thrown (if you were converting to a pointer, a null pointer would be returned).

See What is object slicing?

edited May 23 '17 at 12:14

Community

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answered May 08 '15 at 11:10

David G

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I have taken interfaceElement by reference, it didn't help. The same error is still thrown. – Vladgor May 08 '15 at 11:48
@Vladgor Your vector should hold `interfaceElement*` or `shared_ptr`. You have slicing going on there too. – David G May 08 '15 at 13:07

juanchopanza · Answer 2 · 2015-05-08T11:18:32.777

2

The problem is that you are passing elem by value. So it is an interfaceElement object and cannot be case to a derived type.

You need to pass by reference:

void drawInterfaceElement(interfaceElement& elem, int colour1, int colour2)
                                          ^

edited May 08 '15 at 11:18

answered May 08 '15 at 11:11

juanchopanza

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I have done it, didn't help. – Vladgor May 08 '15 at 11:46
@Vladgor Then the object you're passing isn't a `menu`. – juanchopanza May 08 '15 at 11:48
vector Interface; menu *Menu = new menu(&System); Interface.push_back(*Menu); artist.drawInterfaceElement(Interface[0],7,15); – Vladgor May 08 '15 at 11:53
@Vladgor You should post an [MCVE](http://stackoverflow.com/help/mcve). – juanchopanza May 08 '15 at 11:54

dynamic_cast causing a bad_cast error

2 Answers2