enable class's member depending on template

I already know that you can enable (or not) a class's method using std::enable_if

for exemple:

template<size_t D, size_t E>
class Field
{
  ...

  size_t offsetof(const std::array<float,D>& p) const
  {
    ...
  }

  template<typename TT = size_t>
  typename std::enable_if<D!=E, TT>::type
  offsetof(const std::array<float,E>& p) const
  {
    return offsetof(_projection(p));
  }

  ...
};

This helps not being able to call function that are invalid in a specific case as well as removing overloading errors ... which, to me, is very nice !

I'd like to go further and make some of my class's members being present only if the are needed. That way I would get an error if I try to use an objected which would have otherwise not been initiated

I tried to do

template<size_t D, size_t E>
class Field
{
  ...

  template<typename TT = projectionFunc>
  typename std::enable_if<D!=E, TT>::type _projection;
}

But the compiler tells me :

erreur: data member ‘_projection’ cannot be a member template

Is there any way to achieve what I want ?