Question

this is my first time using the list STL and i’m not sure if what i’m trying to do is possible.
I have class_B which holds a list of class_A, I need a function in class_B that takes an ID, searches the list for an instance with the same ID, and gets a pointer form the list to the instance in that list:

bool class_B::get_pointer(int ID,class_A* pointer2A){

   list<class_A>::iterator i;
   for(i=class_A.begin();i!=class_A.end();i++){
       if((*i).get_id()==ID) {
           \\pointer2A=(i);<---------------this is what I'm trying to do
           return true;
       }
   }
   pointer2A=NULL;
   return false;
}

how do I perform this, is it possible to convert from iterator to instance ?

EDIT:

I’m using this function in a multi-threaded program and I can’t return an iterator to the calling function since another thread might delete an element of the list.

Now that I have a pointer to my element(and lets say it’s locked so it can’t be deleted), and a different thread removed another element and performed a sort on the list, what will happen to the pointer I’m holding ? (I don’t know how the list rearranges the elements, is done by copying the elements using a copy c’tor, or by another mean?).

Useless answer was the most helpful in my case (BIG thanks), and yes I should use a reference to the pointer since I’m planing to change it.

Answer 1

if(i->get_id()==ID) {
    pointer2A=&*i;
    return true;
}

iterators are designed to have similar semantics to pointers, so for example you can write i->get_id() just as if you had a pointer to A.

Similarly, *i yields a reference A&, and &*i converts that back into a pointer – it looks a bit clunky (it would be an identity operation if i were really a pointer), but it’s idiomatic.

Note that this won’t do what you presumably want anyway – the caller’s class_A* pointer2A is passed by value, so only get_pointer‘s copy of the pointer is modified, and the caller won’t see that value. Try this:

bool class_B::get_pointer(int ID, class_A *& pointer2A)
{
    list<class_A>::iterator i;
    for(i=class_A.begin();i!=class_A.end();i++) {
        if(i->get_id()==ID) {
            pointer2A=&*i;
            return true;
        }
    }
    pointer2A=NULL;
    return false;
}   

Now pointer2A is passed by reference, so the caller’s copy gets modified inside your function.

BTW, you can read the parameter declaration class_A * & pointer2A right-to-left, as “pointer2A is a reference to a pointer to class_A”.