Class partial template specialization for single function, how to solve undefined error of other members

Question

I have the following template class. I need to specialize the alloc function for some specific outT case.

template <typename inT, typename outT>
class Filter {
public:

  typedef inT  InputType;
  typedef outT OutputType;

  struct Options {
    int a, b, c;
  };

  virtual void alloc();

};



// Partial template specialization for SpecificOutputType
template < typename inT>
class Filter<inT, SpecificOutputType> {

  virtual void alloc();

};

This results in the Options class and OutputType being undefined for gcc, example:

using FilterType = Filter<SomeIn, SpecificOutputType>:
FilterType::Options options;

Results in

 error: ‘Options’ is not a member of `Filter<SomeIn, SpecificOutputType>`

This error does not happen if I use some type other than SpecificOutputType.

How can I solve this?

Answer 1

Every template specialization is independent, they're irrelevant with the primary template, so you have to also define Options, OutputType and other necessary members in the template specialization explicitly.

Members of partial specializations are not related to the members of the primary template.

You could make a common base class template to avoid code duplication.

template <typename inT, typename outT>
class FilterBase {
public:

  typedef inT  InputType;
  typedef outT OutputType;

  struct Options {
    int a, b, c;
  };

};

template <typename inT, typename outT>
class Filter : public FilterBase<inT, outT> {
public:
  virtual void alloc();
};


// Partial template specialization for SpecificOutputType
template <typename inT>
class Filter<inT, SpecificOutputType> : public FilterBase<inT, SpecificOutputType> {
  virtual void alloc();
};