CODEWITHSUNDEEP
c++templatesiterator
Dan Brenner
Dan Brenner

Reputation: 898

Unqualified IDs and Template Classes

I've been writing a custom class for a circular list, called CList. I've been basing it off a homework assignment I completed a long while ago with a lot of copy paste so I don't exactly remember exactly everything I've been writing works.

Anyway, simply trying to include the .h file causes me the error on the using namespace std line:

main.cpp:11: error: expected unqualified-id before ';' token

As well as two errors pointing to a function in my code:

In file included from main.cpp:9:
CList.h:119: error: non-template 'CIterator' used as template
CList.h:119: note: use 'CList<T>::template CIterator' to indicate that it is a template

This is the function in question:

template <class T> 
typename CList<T>::CIterator<T> CList<T>::push(T const& v) 
{
    size++;  
    Node<T>* p = new Node<T>(v);

    if (this -> size_ == 1)
    {
        head = p;
        tail = p;
        p -> next = p;
        p -> prev = p;
    }
    else
    {
        tail -> next = p;
        p -> next = head;
        p -> prev = tail;
        head -> prev = p;
        tail = p;
    }

    return CIterator(p);
}

I don't really understand what the error here is. I am telling the function to return a CIterator of a CList, and am denoting that this function is part of the CList class. That is what I understand when I read the line

typename CList<T>::CIterator<T> CList<T>::push(T const& v)

Why does it think that CIterator is the template when clearly T is the template? I'm just confused.

Upvotes: 3

Views: 229

Answers (1)

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 154025

The error message states that CIterator<T> isn't considered a template. This is because it is a dependent name and the compiler can only really determine at instantiation time that it, indeed, is a nested template. The fix for the problem is to tell the compiler that it actually is a template:

template <class T> 
typename CList<T>::template CIterator<T> CList<T>::push(T const& v) 
{
    ...
}

The reason the compiler doesn't know if CList<T>::CIterator<T> is a template is that the class CList<X> could be specialized for some type X to not have a nested template inside. However, the compiler only finds out about these specializations when the template is instantiated.

Upvotes: 1

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