Question

I am attempting to create a template “AutoClass” that create an arbitrary class with an arbitrary set of members, such as:

AutoClass<int,int,double,double> a;
a.set(1,1);
a.set(0,2);
a.set(3,99.7);
std::cout << "Hello world! " << a.get(0) << " " << a.get(1) << " " << a.get(3) << std::endl;

By now I have an AutoClass with a working “set” member:

class nothing {};

template <  typename T1 = nothing, typename T2 = nothing, typename T3 = nothing,
            typename T4 = nothing, typename T5 = nothing, typename T6 = nothing>
class AutoClass;

template <>
class AutoClass<nothing, nothing, nothing,
                nothing, nothing, nothing>
{
    public:
    template <typename U> void set(int n,U v){}
};

template <  typename T1, typename T2, typename T3,
            typename T4, typename T5, typename T6>
class AutoClass: AutoClass<T2,T3,T4,T5,T6>
{
    public:
    T1 V;
    template <typename U> void set(int n,U v)
    {
        if (n <= 0)
            V = v;
        else
            AutoClass<T2,T3,T4,T5,T6>::set(n-1,v);
    }
};

and I started to have problems implementing the corresponding “get”. This approach doesn’t compile:

template <  typename T1, typename T2, typename T3,
            typename T4, typename T5, typename T6>
class AutoClass: AutoClass<T2,T3,T4,T5,T6>
{
    public:
    T1 V;
    template <typename U> void set(int n,U v)
    {
        if (n <= 0)
            V = v;
        else
            AutoClass<T2,T3,T4,T5,T6>::set(n-1,v);
    }
    template <typename W> W get(int n)
    {
        if (n <= 0)
            return V;
        else
            return AutoClass<T2,T3,T4,T5,T6>::get(n-1);
    }
    template <> T1 get(int n)
    {
        if (n <= 0)
            return V;
        else
            return AutoClass<T2,T3,T4,T5,T6>::get(n-1);
    }
};

Besides, it seems I need to implement get for the <nothing, nothing, nothing, nothing, nothing, nothing> specialization. Any Idea on how to solve this?

Answer 1

First of all, I prefer Boost.Fusion to Boost.Tuple as it supports a better mixin of template metaprogramming and runtime algorithms I think.

For example, I’d like to present you a little marvel:

struct Name {}; extern const Name name;
struct GivenName {}; extern const GivenName givenName;
struct Age {}; extern const Age age;

class Person
{
public:
  template <class T>
  struct value
  {
    typedef typename boost::fusion::result_of::at_key<data_type const,T>::type type;
  };

  template <class T>
  struct has
  {
    typedef typename boost::fusion::result_of::has_key<data_type,T>::type type;
  };

  template <class T>
  typename value<T>::type
  get(T) { return boost::fusion::at_key<T>(mData); }

  template <class T>
  Person& set(T, typename value<T>::type v)
  {
    boost::fusion::at_key<T>(mData) = v; return *this;
  };

private:
  typedef boost::fusion::map <
    std::pair<Name, std::string>,
    std::pair<GivenName, std::string>,
    std::pair<Age, unsigned short>
  > data_type;
  data_type mData;
};

It’s really fun to use:

Person p;
p.set(name, "Rabbit").set(givenName, "Roger").set(age, 22);

Well, I myself prefer indexing by classes than by indices, because I can convey meaning as well as adding type checking 😉