How to correctly check if an object implements an interface

Question

I want to implement custom behavior for certain objects.

For that, have my items (inheriting from QGraphicsItem) implement some interface.

class SomeParentItem 
{ 
    SomeParentItem(bool x) { x = true; } 
    void function1() {}
};
class SomeInterface
{
    virtual void function2() = 0;
};
class XYZItem : public QGraphicsXYZItem, public SomeParentItem, public SomeInterface
{
    XYZItem(bool x):SomeParentItem(x) {}
    virtual void function2() { x = false; }
};
class MPQItem : public QGraphicsMPQItem, public SomeParentItem
{
    MPQItem (bool x):SomeParentItem(x) {}
};

From outside, I was thinking that i just do

SomeInterface* item1 = dynamic_cast<SomeInterface*>(item);
if(item1 == NULL)
    item->function1(); 
else
    item1->function2();  

Unfortunately this crashes... usually... so I was creating a flag to test, and if the flag was true, only then dare to cast.

But I kept thinking, it shouldn't crash. So I got brave and tried it again, this time in a QWidget child. Instead of crash I got

QWidget::insertAction: Attempt to insert null action

It is the test if(item1 == NULL) that gives that message...

How do I check correctly if my item implements SomeInterface ?

Note: item cannot be null.

Answer 1

There may be a compiler switch which disables run-time-type-information, in which case it would explain behaviour. Otherwise item1 should only be non-null, if the dynamic_cast worked