range based for loop for custom linked list containing pointers in C++, only returns objects

Question

We wrote a custom container class that should be able to support rang based for loop, see here: c++ shell. When you run the example you can see that it is working.

Essentially the container is a linked list of pointers to Elements:

    LinkedList ll;
    ll.push_back(new Element(1));
    ll.push_back(new Element(2));
    ll.push_back(new Element(3));

    for(LinkedList::iterator it = ll.begin(); it != ll.end(); ++it){
        cout << it->some_value << endl;
    }

    for(Element ele : ll) {
        cout << ele.some_value << endl;
    }

will print123123. What we don't understand is: Why are the ele in the range based for loop not pointers? Or better why does this not work:

for(Element * ele : ll) {
    cout << ele->some_value << endl;
}

Basically we want to achieve the same with the custom linked list as can be achieved with the std vector:

vector<Element*> vec{new Element(1),new Element(2),new Element(3)};
for(Element * ele : vec)
{
    cout<<ele->some_value<<endl;
}

Look at what LinkedList::begin() and LinkedList::end() return. Range based for loops will dereference those iterators for you. Note that each ele is a copy of the elements in ll. Use a reference to avoid that copy. To support your suggestion, you would need to be able to convert Element& to Element * implicitly. — François Andrieux
– François Andrieux, Commented Jun 19, 2017 at 19:20
I'm guessing you want for (const auto& ele : ... like one usually does... — Jesper Juhl
– Jesper Juhl, Commented Jun 19, 2017 at 19:22
No, what we really want is for(Element * ele : vec). I mean it is possible with the std vector, it should be possible with our custom linked list. — newandlost
– newandlost, Commented Jun 19, 2017 at 19:50

newandlost · Accepted Answer · 2017-06-20 13:47:20Z

6

The standard defines the ranged based for to be equivalent to:

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
             __end = end-expr;
        __begin != __end;
        ++__begin )
  {
    for-range-declaration = *__begin;
    statement
  }
}

The reason ele shouldn't be a pointer is because of the dereference of the iterator (for-range-declaration = *__begin;), so the for-range-declaration needs to be a reference type or the actual value type.

Hence you need to change your iterator class such that the operator* returns Element* or Element*&.

edited Jun 20, 2017 at 13:47

newandlost

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answered Jun 19, 2017 at 19:24

Nullrelation

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3 Comments

newandlost Over a year ago

That is kind of a bit too abstract... what do we have to change such that for(Element * ele : ll) is working?

Benjamin Lindley Over a year ago

@newandlost: You need to change your iterator class such that operator* returns Element* or Element*&.

newandlost Over a year ago

The way in which the anser is written like for-range-declaration = *__begin; is that c++ documentation language?

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