Beginning generically, plus decltype considering local using-declaration

Question

C++0x's ranged-for loop has a special exception to handle arrays (FDIS §6.5.4), and there are two functions, std::begin and end, which are overloaded to handle arrays or to select begin/end methods. This leads me to believe a function accepting a generic sequence could be written to match a ranged-for loop's behavior:

template<class C>
void f(C &c) {
  using std::begin;
  using std::end;
  do_something_with(begin(c), end(c));
}

If there's a "more specific" begin/end in the namespace of C, it will be selected through ADL, otherwise the code "defaults" to std::begin/end.

However, there is a reason ranged-for has that special exception. If passing an array of a type in a namespace with a semantically-different begin/end which takes a pointer, the array forms of std::begin/end aren't selected:

namespace ns {
  struct A {};
  void begin(A*);  // Does something completely different from std::begin.
}

void f_A() {  // Imagine above f() called with an array of ns::A objects.
  ns::A c[42];
  using std::begin;
  begin(c);  // Selects ns::begin, not array form of std::begin!
}

To to avoid this, is there a better solution than writing my own begin/end wrappers (which use ADL internally) and calling them explicitly instead of either std::begin or an ADLized begin?

namespace my {
  template<class T>
  auto begin(T &c)  // Also overload on T const &c, as std::begin does.
  -> decltype(...)  // See below.
  {
    using std::begin;
    return begin(c);
  }

  template<class T, int N>
  T* begin(T (&c)[N]) {
    return c;
  }
}
// my::end omitted, but it is analogous to my::begin.

template<class C>
void f(C &c) {
  do_something_with(my::begin(c), my::end(c));
}

However, as shown by the ellipsis above, I don't even know how to write my::begin! How can I, for that decltype, select the type that will be selected through a local using-declaration and ADL?

It took me about 30 minutes to think about and write up this question, but it's something that's been brewing in my head for a while, and I think just writing it gave me an idea to solve the "decltype considering local using-declaration", but it's not pretty. I'll self-answer if no one mentions it sometime today. — Fred Nurk
I think there's another problem: wouldn't my::begin be considered for "return begin(c);", and, if selected, be infinite recursion? — Fred Nurk
@Fred: this second problem is simple and requires just an additional function wrapper in a separate "detail" namespace. — Alexandre C.
Personally, I'd look for a trait - something like typename range_traits<T>::iterator. That would require the definition of the range_traits class, but a range_traits class that handles the general case wouldn't be too difficult to write.. (too bad we don't have concepts in c++0x anymore) — rlc

Luc Danton Luc Danton · Accepted Answer · 2011-05-18T13:24:16

I've encountered the same situation while using tuples:

template<typename Tuple>
auto f(Tuple&& tuple)
-> /* ??? */
{
    using std::get;
    return get<Idx>(tuple);
}

which accepts both std::tuple and boost::tuple, and accepts both lvalues and rvalues as opposed to template<typename... Types> auto f(std::tuple<Types...>& tuple) -> /* ??? */.

This particular case was solved with a traits class, which is in fact provided by the Standard: std::tuple_element. As usual with traits classes, the idea is that tuple is a protocol and anything that want to conform to it will provide a specialization for e.g. tuple_element. So in my case the solution already existed.

In your case, if you were writing a library, I'd recommend writing (and documenting) such a traits class. In application code or other situations, I'm not so sure.

Beginning generically, plus decltype considering local using-declaration

2 Answers