CODEWITHSUNDEEP
c++c++14traitstemplate-meta-programming
Hans vesselgård
Hans vesselgård

Reputation: 317

Tag dispatching/enable_if - I am confused

I have the following structs:

struct A
{
}

struct B
{
tuple<string,string>  children{{"test1","test2"}};
}

I would like to create a template function that will overload the << operator on every class having a member variable called children. If possible- only on tuples named children.

When a class with a children tuple like B is met it should iterate the tuple members and call << on each of them.

Something like:

template<typename RECEIVERTYPE,typename SENDERTYPE>
typename std::enable_if<std::have_children_member<RECEIVER_TYPE>::value, void>::type
RECEIVERTYPE& operator<< (RECEIVERTYPE& streamReceiver, const SENDERTYPE&    streamSender)
    {
        for_each(streamSender.children, [&](const auto& child)
        {
            streamReceiver << child;
        }); 

        return streamReceiver;
    }

I have tried alot of examples- but I cant really get anything working in visual studio 2015.

Upvotes: 0

Views: 886

Answers (1)

Jonathan Wakely
Jonathan Wakely

Reputation: 171303

I would just do this to only match types with a tuple member called children:

template<typename S, typename T,
         std::size_t = std::tuple_size<decltype(T::children)>::value>
  S& operator<<(S& s, const T& t)
  { ... }

And might implement the body like this:

template<typename S, typename T, std::size_t... I>
  void
  print_tuple_like(S& s, const T& t, std::index_sequence<I...>)
  {
    void* unused[] = { &(s << std::get<I>(t))... };      
  }

template<typename S, typename T,
         std::size_t N = std::tuple_size<decltype(T::children)>::value>
    S& operator<<(S& s, const T& t)
{
    print_tuple_like(s, t.children, std::make_index_sequence<N>{});
    return s;
}

Or like:

template<std::size_t N, typename S, typename T>
  void
  print_tuple_like(S& s, const T& t, std::false_type)
  { }

template<std::size_t N, typename S, typename T>
  void
  print_tuple_like(S& s, const T& t, std::true_type)
  {
    s << std::get<N>(t);
    print_tuple_like<N+1>(s, t, std::integral_constant<bool, (N+1 < std::tuple_size<T>::value)>{});
  }

template<typename S, typename T,
         std::size_t N = std::tuple_size<decltype(T::children)>::value>
  S& operator<<(S& s, const T& t)
{
    print_tuple_like<0>(s, t.children, std::integral_constant<bool, (N != 0)>{});
    return s;
}

Upvotes: 4

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