Circular template dependency

[Ghostcoder]

Hi

Can anyone help me work out a way around the following problem:

typedef SmartPtr<LinkedListObject<MyPointer> > MyPointer;

Unsurprisingly (I guess) gives me:

`MyPointer' was not declared in this scope
template argument 1 is invalid
ISO C++ forbids declaration of `type name' with no type


SmartPtr<T> is a smart pointer class that wraps type 'T' providing wrapped
operations of T* - dereference, equality, implicit conversion, and arithmetic.

Unfortunately the LinkedListObject _needs_ to store its next and prev pointers
as pointers of type MyPointer. This whole setup arises from defining different types of LinkedListObject that are passed as a template argument to a class that creates a singly linked list or a class that creates a doubly linked list and these classes must themselves keep internal pointers using the smart pointer 'MyPointer'.

Any help would be greatly appreciated!

Many thanks!

[raghuvamshi]

I dont see any circular dependency issues here. It would be a circular dependency issue only if the smartptr class needed to know the definition of your LinkedListObject class, which is not the case here.

All you need is:

in your linked list class header

#include "your smartptr header"

// use the smart ptr

in your main or SOME OTHER .cpp /.h file that needs to use the linked list:

#include "your smartptr header"
#include "your linked list class header"

typedef SmartPtr<LinkedListObject<MyPointer> > MyPointer;

hope this helps

[wien]

I dont see any circular dependency issues here.

Yes there is. Check out the MyPointer typedef. The SmartPtr template has a parameter which in turn has its own template parameter which is the SmartPtr type. Looks like circular dependecy to me.

Basically there is no way you can solve this. A template type cannot have itself as a template parameter. That's kinda like you being your own father.

I would say your design is a bit off if you need such a construct. Is there really a need for the pointer type to be a template parameter of the LinkedListObject class? How about something like this instead?

#include <memory>

template<typename T, template<typename> class P = std::auto_ptr>
class linked_list
    {
    private:
        class node;

        typedef node node_type;
        typedef P<node_type> pointer_type;

        class node
            {
            public:
                T value;
                pointer_type m_next;
            };

        pointer_type start;

    public:
        linked_list()
            :
            start(new node_type)

            {}
    };

int main()
    {
    linked_list<int> a_list;
    }