Question

I am fairly new to C++ and I keep getting segmentation fault with pointer assignment with code similar to the following, I know it means I’m accessing memory that hasn’t been allocated, but I don’t see where:

I have two classes:

class ClassA{ //class a decl.
  ClassB** oArray;
  unsigned int x;
 public:
  ClassA(unsigned int X);
  void oMember(ClassB* classb);
}


ClassA::ClassA(unsigned int X){ //Constructor for class a
  x = X;
  oArray = new ClassB* [x];

 for (unsigned int i = 0; i < x; i++){
        oArray = NULL; 
    }
}

class ClassB{ //rough decl of class B
  public:
   getId();
}

I have a class member function that takes in a pointer to another class like this:

void ClassA::oMember(ClassB* classb){
   unsigned int cID = classb.getId(); //defined in class b
   oArray[cID] = classb; //if cID is less than x defined in constructor, is this legal?
}

I just want to point the cIDth member of the array to classb.

I keep getting segmentation fault with an assignment similar to the above. I don’t quite know why, I printed the cID and it is definitely less than the size of the array we declared in ClassA’s constructor.

Why is that assignment illegal, or why am I getting a segmentation fault?

Answer 1

As we realized in the chat, I just record the problem here for the reference.

The problem lies in the piece

RegisteredVMs = new VendingMachine *[nVendingMachines];
for (unsigned int i = 0; i < nVendingMachines; i++){
   RegisteredVMs = NULL; 
}

RegisteredVMs is allocated and immediately set to NULL.
This pointer is accessed later in the VMregister() function which causes the seg. fault.

Pointers are hard and very error-prone. Use them only if there is no other way. Since this is a homework problem and since you says you have no say on the interface, I see that you have to use them.