CODEWITHSUNDEEP
c++templatestemplate-specialization
Chuck Fry
Chuck Fry

Reputation: 397

Class template specialized on generic class template

I have a family of classes for which I want to write an abstract factory. The minimal example below will give you the general idea.

My problem is that I can't figure out how to define member functions of ConcreteFactory >. clang++ reports this error:

template-class-of-template-class.cc:36:39: error: nested name specifier 'ConcreteFactory<Derived<NUM> >::' for
      declaration does not refer into a class, class template or class template partial specialization
Base* ConcreteFactory<Derived<NUM> >::construct() const

I've only been able to define them for fully specified classes, e.g. ConcreteFactory >. There will be an awful lot of duplicated code if I have to do it that way. Is there any way to avoid writing this boilerplate through intelligent use of templates?

#include <cstdlib>

class Base
{
};

template <typename NUM>
class Derived : public Base
{
public:
  Derived(NUM const &thing) : m_thing(thing) {}
  ~Derived() {}
private:
  NUM m_thing;
};

class AbstractFactory
{
public:
  virtual Base *construct() const = 0;
};


template <class Y>
class ConcreteFactory
{
public:
  Base* construct() const
  {
    return new Y();
  }
};

template <typename NUM>
template <>
Base* ConcreteFactory<Derived<NUM> >::construct() const
{
  return new Derived<NUM>(rand());
}

int main(int argc, char *argv[])
{
  ConcreteFactory<Base> baseFact;
  ConcreteFactory<Derived<int> > intFact;
  ConcreteFactory<Derived<double> > doubleFact;
  Base* a = baseFact.construct();
  Base* b = intFact.construct();
  Base* c = doubleFact.construct();
  delete c;
  delete b;
  delete a;
}

Upvotes: 0

Views: 41

Answers (1)

Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42899

  • You can't partially specialize on a member function of a template class.

  • You have to partially specialize the whole template class.

  • See corrected code below:

// partial specialization of class `ConcreteFactory`
template<typename NUM>
class ConcreteFactory<Derived<NUM>> {
  public:
    Base* construct() const { return new Derived<NUM>(rand());}
};

see Display.

Upvotes: 1

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