SFINAE on template parameter + outside definition of nested class

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Hi all,
I have a class A, templated with a type T and an enable_if clause, which contains a nested class I (which is meant to be a user-defined iterator for A), like this:

template <typename T, std::enable_if_t<std::is_arithmetic<T>::value>>
class A {
   public:
      class I;
};

I can't find the proper way to defined I outside of A, I understand the error messages (see "What have you tried?" session) I get but what I miss is which is the correct way to do what I want..

What I have tried:

I tried this:

template <typename T, std::enable_if_t<std::is_arithmetic<T>::value>>
class A<T>::I {

};

but it gives the error

Quote:
cannot add a default template argument to the definition of a member of a class template

while this:

template <typename T>
class A<T>::I {

};

gives this error:

Quote:
too few template parameters in template redeclaration

Posted 27-Nov-18 2:39am

User 14036488

Updated 27-Nov-18 22:19pm

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CPallini 27-Nov-18 8:53am

I guess I should be in template argument list.

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User 14036488 · Accepted Answer · 2018-11-27T22:20:00

I answer myself, I solved this moving the enable_if check from the template parameters list to the constructor and everything works fine, like this:

template <typename T>
class A {
   public:
      A(std::initializer_list<typename std::enable_if<std::is_arithmetic<T>::value,T>::type> l) {}
      class I;
};

template <typename T>
class A<T>::I {
   public:
      int prova() { return 1; };
};