Hello. I'm trying to implement double linked list. I have a linked_list class and DListNode class as a shell for elements in my container and I prefer DListNode as a class rather than as a struct, because I want node fields to be inaccessible outside, but I also need to get an access to them in a linked_list class, so instead of making getters and setters I have chosen to make linked_list as a friend class for DLinstNode to improve encapsulation.
Problem: I did what I planned like this:
template <
class Dty_
>
class DListNode {
public:
// ...
template <class ...> friend class linked_list; // line 168
// template <class ...> friend class linked_list<Dty_, DListNode<Dty_>, std::allocator<Dty_>>; full specialization doesn't work too
protected:
// ...
};
template <
class Ty_,
template <class ...> class List_node_ = DListNode,
class Alloc_ = std::allocator<Ty_>
>
class linked_list { // line 13
// ...
};
And got an error in line 13: "redeclared with 3 template parameters"
The full error text:
C:\Users\averu\github-local\base\ds\list\list.cpp:13:7: error: redeclared with 3 template parameters
13 | class linked_list { // biderectional double linked list
| ^~~~~~~~~~~
In file included from C:\Users\averu\github-local\base\ds\list\iterator.cpp:2,
from C:\Users\averu\github-local\base\ds\list\list.cpp:1:
C:\Users\averu\github-local\base\ds\list\node.cpp:168:39: note: previous declaration 'template<class ...> class linked_list' used 1 template parameter
168 | template <class ...> friend class linked_list;
|
As you can see there I'm trying to use variadic template, but it doesn't work. So, perhaps I need to declare class linked_list forward via using declaration how then do it?
You declaration of the friend class uses different template arguments than the definition of linked_list. You need to declare it as a friend like so:
class DListNode {
public:
template <class, template <class...> class, class> friend class linked_list;
};
becaus linked_list has one class template argument, followed by a template template argument, followed by another class argument.