How to allow certain types in a template?

Question

C++ / C#

Hello! The question is, how to allow, for example, the types int and double in the template, and put asserts on the rest?

template <class T>
struct Test {
    T workingOnlyWithIntegerOrDouble();
private:
    T num_;
}

Answer 1

template <class T>
struct is_permitted 
{
  static constexpr bool value = false;
};

template <>
struct is_permitted<int>
{
  static constexpr bool value = true;
};

template <>
struct is_permitted<double>
{
  static constexpr bool value = true;
};

template <class T>
struct test
{
  test()
  {
    static_assert(is_permitted<T>::value);
  }
};