C++ nested template classes syntax

Question

I have a class List which is a fixed size, linked List, and inside that class there is another class ListIterator which points to a specific element of that List. My problem is the c++ syntax I am not familiar with and the build errors from gcc are not helping me.

template<class T, int length>
class List {
public:
    //some stuff here
    //Row-struct

    template<class T>
    class ListIterator {
        public:
            ListIterator(int mPos, Row& mRow){pos = mPos; row = mRow;};
            T& operator *();
        private:
            int pos;
            Row& row;
    };

    typedef ListIterator<T> iterator;

    iterator begin();
private:
    int startIdx;
    Row storage[length];
};

template<class T, int length>
ListIterator List<T, length>::begin() {
    ListIterator *itor = new ListIterator(startIdx, storage[startIdx]);
    return itor;
}

I get the following errors:

error: shadows template parm 'class T'

How can I make this typedef ListIterator<T> work?
How can I use the same T I used for List for the Iterator?
Why does my begin() not work?

you don't have to do anything, just use it, it's known in this scope. — Sopel
– Sopel, Commented Nov 9, 2017 at 11:33
But I want to use ìterator in ListIterator as well as in List as typedef — binaryBigInt
– binaryBigInt, Commented Nov 9, 2017 at 11:34
class ListIterator inside template <typename T> class List has access to T, it doesn't need to be a template. You could even directly name it class iterator and skip the typedef — Caleth
– Caleth, Commented Nov 9, 2017 at 13:00

user7860670 · Accepted Answer · 2017-11-09 11:41:12Z

1

Rename ListIterator template parameter T to something else, for example to TInner. Or make it a regular class if it can use T from outer class. Or move ListIterator outside of List class which would be a better idea since ListIterator does not actually do anything list-specific.

To fix begin function you need to use proper types:

template<typename T, int length> typename List<T, length>::iterator
List<T, length>::begin(void)
{
    iterator item_iterator(startIdx, storage[startIdx]);
    return(item_iterator);
}

edited Nov 9, 2017 at 11:41

answered Nov 9, 2017 at 11:33

user7860670

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7 Comments

binaryBigInt Over a year ago

ListIterator has to know a few things about the List (pointer, next element, previous elemen etc.)

user7860670 Over a year ago

@binaryBigInt Your code does not demonstrate it though.

StoryTeller - Unslander Monica Over a year ago

Another options is a trailing return type

binaryBigInt Over a year ago

@VTT How do I access the object in my main.cpp? List<int,2> list; List::iterator *it = list.begin(); doesn't work.

user7860670 Over a year ago

@binaryBigInt begin is supposed to return iterator, not a pointer to iterator. And you need to use proper type names (again), it should be List<int, 2>::iterator it = list.begin();

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