Invoking begin and end via using-directive?

Question

The established idiom for invoking swap is:

using std::swap
swap(foo, bar);

This way, swap can be overloaded for user-defined types outside of the std namespace.

Should we invoke begin and end in the same fashion?

using std::begin;
using std::end;
some_algorithm(begin(some_container), end(some_container));

Or should we just write:

some_algorithm(std::begin(some_container), std::end(some_container));

Answer 1

Using a using-declaration like that is the correct way IMO. It's also what the standard does with the range for loop: if there is no begin or end members present then it will call begin(x) and end(x) with std as an associated namespace (i.e. it will find std::begin and std::end if ADL doesn't find non-member begin and end).

If you find that writing using std::begin; using std::end; all the time is tedious then you can use the adl_begin and adl_end functions below:

namespace aux {

using std::begin;
using std::end;

template<class T>
auto adl_begin(T&& x) -> decltype(begin(std::forward<T>(x)));

template<class T>
auto adl_end(T&& x) -> decltype(end(std::forward<T>(x)));

template<class T>
constexpr bool is_array()
{
    using type = typename std::remove_reference<T>::type;
    return std::is_array<type>::value;
}

} // namespace aux

template<class T,
         class = typename std::enable_if<!aux::is_array<T>()>::type>
auto adl_begin(T&& x) -> decltype(aux::adl_begin(std::forward<T>(x)))
{
    using std::begin;
    return begin(std::forward<T>(x));
}

template<class T,
         class = typename std::enable_if<!aux::is_array<T>()>::type>
auto adl_end(T&& x) -> decltype(aux::adl_end(std::forward<T>(x)))
{
    using std::end;
    return end(std::forward<T>(x));
}

template<typename T, std::size_t N>
T* adl_begin(T (&x)[N])
{
    return std::begin(x);
}

template<typename T, std::size_t N>
T* adl_end(T (&x)[N])
{
    return std::end(x);
}

This code is pretty monstrous. Hopefully with C++14 this can become less arcane:

template<typename T>
concept bool Not_array()
{
    using type = std::remove_reference_t<T>;
    return !std::is_array<type>::value;
}

decltype(auto) adl_begin(Not_array&& x)
{
    using std::begin;
    return begin(std::forward<Not_array>(x));
}

decltype(auto) adl_end(Not_array&& x)
{
    using std::end;
    return end(std::forward<Not_array>(x));
}

template<typename T, std::size_t N>
T* adl_begin(T (&x)[N])
{
    return std::begin(x);
}

template<typename T, std::size_t N>
T* adl_end(T (&x)[N])
{
    return std::end(x);
}