Question

I am attempting to output functions common to a set of objects that share a base class and I am having some difficulty. When the objects are instantiated they are stored in an array and then I am attempting with the following code to execute functionality common to all the objects in this loop:

  if ( truck <= v ) // all types of trucks

vptr is an array of objects and the functions in the loop are common to all the objects. The code compiles fine but when I run it I get a segmentation fault when it enters this loop. I’m fairly confident that the call to the first function in this loop is what is causing the problem.

this is how I have instantiated the objects in a previous loop:

        vptr[ i ] = new Vehicle( sn, pc );

I should also mention, I’m sorry I forgot to be clear from the beginning, that in this array each object is of a different class. They all share a base class but they are derived objects of that class. Sorry for forgetting that probably important piece of information.

thanks
nmr

Answer 1

dynamic_cast to a pointer type returns a null pointer (aka 0, NULL) if the object isn’t of the specified type. You must check the pointer before using it, or use a reference type (which throws an exception on failure instead):

if (Truck* p = dyanmic_cast<Truck*>(vptr[i])) {
  // use the pointer here
}
else {
  // vptr[i] doesn't point to a Truck
}

(Notice the nice effect of the correctly-typed pointer being scoped for you, take advantage of this when you can to improve readability.)