Forward declaration of template class nested inside template class

Question

You can forward declare a template inner class inside a normal class, and use the defined type as any other forward declared type.

class Outer {
    template         


        

Answer 1

Try the following

template<typename T>
template <int N>
class Outer<T>::Inner {};

According to the C++ Standard (14.5.2 Member templates)

1 A template can be declared within a class or class template; such a template is called a member template. A member template can be defined within or outside its class definition or class template definition. A member template of a class template that is defined outside of its class template definition shall be specified with the template-parameters of the class template followed by the template-parameters of the member template.