C ++部分模板专门化和模板参数中的类继承

Question

Although here is several questions with similar topic, I didn't make my code to work. I have following classes:

template<class T, class DataGetter>
class LogableTemplate
{
  public:
    //logs data got by dataGetter
    static DataGetter dataGetter;
};
template<class T, class DataGetter> 
DataGetter LogableTemplate<T, DataGetter>::dataGetter;

template<class T, class DataGetter>
class Logable : public LogableTemplate<T, DataGetter>
{
};

I use this class like this:

class ADataGetter;
class A : public Logable<A, ADataGetter> {}
class ADataGetter {  //generic external functor returning some data
  public:
    int operator()(const A&a) { return 3; }
};

But there is a special case, when I have some class converting other class to required type, like:

template <class T>
class IntClassMapper {
  public:
    int mapToInt(const T &t);    
};

It is fine, but it doesn't have required operator(). I made helper class:

template<class T, class Mapper>
class IntMapperDataGetter {
  public:
    int operator()(const T &t) {
      return mapper.mapToInt(t);
    }
  static Mapper mapper;
};
template<class T, class Mapper>
Mapper IntMapperDataGetter<T, Mapper>::mapper;

And not I need a partial specialization for case that second template argument of Logable is inherited from IntClassMapper. I've tried following code (and hundred of similar), but I always get some compile error - usually:

error: template parameters not deducible in partial specialization

Here's the code:

template<class T, class Mapper>
class Logable<T, typename std::enable_if<std::is_base_of<IntClassMapper<T>, Mapper>::value, IntMapperDataGetter<T, Mapper>>::type> : public LogableTemplate<T, IntMapperDataGetter<T, Mapper>>
{
};

You can check and test the code on https://ideone.com/qz9jIt

Is it possible to do it this way? Thank you, Martin

Answer 1

As far I understand, you want your Logger to have different behaviour, based on properties of the template parameter. Something like this can help maybe?

template
<
  class T, 
  class DataGetter, 
  bool enable = std::is_base_of<QVariantClassMapper<T>, DataGetter>::value
>
class Logable 
  : public LogableTemplate<T, DataGetter>
{
};

template<
  class T, 
  class Mapper
>
class Logable<T, Mapper, true> 
  : public LogableTemplate<T, QVariantMapperDataGetter<T, Mapper> >
{
};