Thread: Dereferencing Class Pointer List

Whoa, you definitely have code that should be throwing warnings and/or errors. If your compiler isn't rejecting this then I'm very surprised.

Let's take a look at your main function:

Code:

int main(int argc, char *argv[])
{
   //mynewList->Add();
   cout<<"BEFORE: "<<endl;
   ShowAll(*mynewList->GetList());

   // There is no need to make this a pointer!  It's ridiculous.
   medList::iterator *it = &mynewList->GetList()->begin();

   while( (*it) != mynewList->GetList()->end())
   {
      Results->push_back(&(**it));   
      (*it)++;
   }
   system("PAUSE");  // You really should #include <cstdlib> if you're going to use this...

   medSearchResultList::iterator iterate = Results->begin();
   while(iterate != Results->end())
   {
      // Whoa!  Why are you using 'it' here!?
      // Furthermore, you are trying to dereference the return value
      // from a function that returns void!!!
      *(*it)->SetName("FROM POINTER!");

      // Oh jeez, you forgot to increment your iterator...
      // Can you say infinite loop?
   }
   cout<<"MODIFIED THROUGH POINTER LIST CLASS"<<endl;
   ShowAll(*mynewList->GetList());

   system("PAUSE");
   return(0);
}

Here's a slightly fixed up version:

Code:

int main(int argc, char *argv[])
{
   //mynewList->Add();
   cout<<"BEFORE: "<<endl;
   ShowAll(*mynewList->GetList());
   medList::iterator it = mynewList->GetList()->begin();

   while (it != mynewList->GetList()->end())
   {								
      Results->push_back(&*it);
      it++;
   }
   system("PAUSE");

   medSearchResultList::iterator iterate = Results->begin();
   while(iterate != Results->end())
   {
      (*iterate)->SetName("FROM POINTER!");
      iterate++;
   }
   cout<<"MODIFIED THROUGH POINTER LIST CLASS"<<endl;
   ShowAll(*mynewList->GetList());

   system("PAUSE");
   return(0);
}

Your compiler is hopefully also throwing numerous warnings about how you are passing const char pointers into your constructors/functions and assigning them to non-const char pointers. This is very dangerous. You might be tempted to try modifying those internal strings at some point, at which time you will most likely experience crashes.

I understand that you're trying to learn about pointers, but your usage of them is pretty ridiculous at times, and in several places you would be better off not using pointers. I find this code very hard to follow.

My small experience with Visual Studio is that whenever you're in debug mode, the error comes up when you're trying to de-reference a NULL pointer or delete an object of a foo pointer.

>> Pointers are confusing..
Yep, and you're having a hard time with them. You're also using them in many places where it is not necessary, and now you have a program full of pointer errors. Unfortunately you've got a lot more to fix up besides the mismatched iterators.

The first thing I see is that you don't have a copy constructor or copy assignment operator for your SingleListClass even though you have a member variable allocated with dynamic memory. The rule of three says that if you need a destructor, copy assignment operator or copy constructor, you probably need all three. Here you have a destructor and you need the other two. A better solution is to not use a pointer here. There's no benefit and if you don't use dynamic memory you won't need the destructor, copy assignment operator or copy constructor.

The second thing is that you leak memory in SingleListClass::Add(). You allocate a temp _medContainer dynamically, then you push_back a copy of the object. You never delete the temp one (the copy will be cleaned up automatically by the list).

There's probably a lot more errors lying around, I haven't looked closely. I'd suggest using pointers only where they're necessary and learning how to use them correctly that way first.

As to your original problem, did you try the main() program example I posted? Did it work or fail? In Release mode it might work because the extra checks done by the compiler to help you are skipped, you want to make sure it works in debug mode and release mode.