c++ - Generating one class member per variadic template argument

Question

I have a template class where each template argument stands for one type of value the internal computation can handle. Templates (instead of function overloading) are needed because the values are passed as boost::any and their types are not clear before runtime.

To properly cast to the correct types, I would like to have a member list for each variadic argument type, something like this:

template<typename ...AcceptedTypes> // e.g. MyClass<T1, T2>
class MyClass {
    std::vector<T1> m_argumentsOfType1;
    std::vector<T2> m_argumentsOfType2; // ...
};

Or alternatively, I'd like to store the template argument types in a list, as to do some RTTI magic with it (?). But how to save them in a std::initializer_list member is also unclear to me.

Thanks for any help!

Answer 1

As you have already been hinted, the best way is to use a tuple:

template<typename ...AcceptedTypes> // e.g. MyClass<T1, T2>
class MyClass {
    std::tuple<std::vector<AcceptedTypes>...> vectors;
};

This is the only way to multiply the "fields" because you cannot magically make it spell up the field names. Another important thing may be to get some named access to them. I guess that what you're trying to achieve is to have multiple vectors with unique types, so you can have the following facility to "search" for the correct vector by its value type:

template <class T1, class T2>
struct SameType
{
    static const bool value = false;
};

template<class T>
struct SameType<T, T>
{
    static const bool value = true;
};

template <typename... Types>
class MyClass
{
     public:
     typedef std::tuple<vector<Types>...> vtype;
     vtype vectors;

     template<int N, typename T>
     struct VectorOfType: SameType<T,
        typename std::tuple_element<N, vtype>::type::value_type>
     { };

     template <int N, class T, class Tuple,
              bool Match = false> // this =false is only for clarity
     struct MatchingField
     {
         static vector<T>& get(Tuple& tp)
         {
             // The "non-matching" version
             return MatchingField<N+1, T, Tuple,
                    VectorOfType<N+1, T>::value>::get(tp);
         }
     };

     template <int N, class T, class Tuple>
     struct MatchingField<N, T, Tuple, true>
     {
        static vector<T>& get(Tuple& tp)
        {
            return std::get<N>(tp);
        }
     };

     template <typename T>
     vector<T>& access()
     {
         return MatchingField<0, T, vtype,
                VectorOfType<0, T>::value>::get(vectors);
     }
};

Here is the testcase so you can try it out:

int main( int argc, char** argv )
{
    int twelf = 12.5;
    typedef reference_wrapper<int> rint;

    MyClass<float, rint> mc;
    vector<rint>& i = mc.access<rint>();

    i.push_back(twelf);

    mc.access<float>().push_back(10.5);

    cout << "Test:
";
    cout << "floats: " << mc.access<float>()[0] << endl;
    cout << "ints: " << mc.access<rint>()[0] << endl;
    //mc.access<double>();

    return 0;
}

If you use any type that is not in the list of types you passed to specialize MyClass (see this commented-out access for double), you'll get a compile error, not too readable, but gcc at least points the correct place that has caused the problem and at least such an error message suggests the correct cause of the problem - here, for example, if you tried to do mc.access<double>():

 error: ‘value’ is not a member of ‘MyClass<float, int>::VectorOfType<2, double>’