Downcasting and copying a const object

Question

My method receives an object as a parameter and it must be accepted as a base type (though it is sent as a Derived type). I want to make a copy of this object AND down cast it to a derived type, so that I can call a method of the derived type (which modifies properties of the derived object).

I've been messing with variations of dynamic_cast, but can't make the compiler happy, OR, it compiles and I get a sigABRT on the dynamic_cast line (as per below):

void mymethod(Base message) {
    if (message.messageType() == "D") {
        Derived& derivedMessage = dynamic_cast<Derived&>(message);
        Derived derivedCopy = derivedMessage;
        derivedCopy.derMethod();

In case it matters, mymethod is a Qt SLOT, which receives the signal across threads. I don't think one can (or at least safely) pass a pointer to an object across threads, which is why I'm passing by value.

Answer 1

void mymethod(const Base message)

If you pass a parameter of type Base, then the type of the object is Base and not something else - i.e. it will never be an object of a derived type.

If you want to have runtime polymorphism, then you need to use indirection. Typical solution is to pass a reference to a base class subobject:

void mymethod(const Base& message)

The referred base can be a subobject within any derived class.