error: default argument for template parameter for class enclosing

[hbcpp]

I am trying to implement a std::list-like doubly linked list and when defining a constructor I am coming across an error that I don't quite understand.

The stripped down code:

#include <iostream>

template <typename T>
class List
{
public: 
   template<typename InputIt, typename = std::_RequireInputIter<InputIt>>
   List(InputIt first, InputIt last);

};

template<typename T>
template<typename InputIt, typename = std::_RequireInputIter<InputIt>>
List<T>::List(InputIt first, InputIt last)
{
   // some code here
}


int main()
{

}

When trying to compile an error is thrown:

error: default argument for template parameter for class enclosing 'List<T>::List(InputIt, InputIt)'|

The error message is not clear for me, anyone to shine a light?
Thanks!!

[highwayman]

template<typename T>
template<typename InputIt, typename = std::_RequireInputIter<InputIt>>
List<T>::List(I....

Why do you have two template things stacked on top of each other?

[hbcpp]

@highwayman

template<typename T> is for List<T>

template<typename InputI... is for List(InputIt first, InputIt last)

[highwayman]

Ahhh wait I get it now ok cool.

[mbozzi]

Default arguments only belong in the declaration:

template<typename T>
template<typename InputIt, typename>
List<T>::List(InputIt first, InputIt last)
{
   // some code here
}

[hbcpp]

@mbozzi oh I see, Thanks

Is there any workaround for that?

[mbozzi]

Yes, if it wasn't clear from the above, here's a version that compiles:

#include <iostream>

template <typename T>
class List
{
public: 
   template<typename InputIt, typename = std::_RequireInputIter<InputIt>>
   List(InputIt first, InputIt last);

};

template<typename T>
template<typename InputIt, typename>
List<T>::List(InputIt first, InputIt last)
{
   // some code here
}

int main()
{

}