Get the return type of begin on a c array

Question

I would like to get the return type of std::begin in a generic way. My current solution is:

using type = decltype(std::begin(std::declval<T>()));

and it works when T = std::vector<int>. But I don't understand why the following does not work:

using type = decltype(std::begin(std::declval<int[3]>()));

I get the error:

example.cpp:83:60: error: no matching function for call to ‘begin(int [3])’
     using type = decltype(std::begin(std::declval<int[3]>()));

How to get the return type of std::begin in a generic way?

Answer 1

The overload for arrays is:

template< class T, std::size_t N > 
constexpr T* begin( T (&array)[N] );

And std::declval<int[3]>() gives you a int(&&)[3], which doesn't match that overload. It also doesn't match the normal container overload, because those are SFINAE-ed on having c.begin(). So you're left with no matching function.

What you need instead is to pass an lvalue reference to array to begin(), to get the iterator back out. So either you need to manually provide a that lvalue reference when you use your alias:

template <class T>
using type = decltype(std::begin(std::declval<T>()));

using arr = type<int(&)[3]>; // int*

or have the alias itself provide the lvalue reference for you:

template <class T>
using type = decltype(std::begin(std::declval<T&>()));

using arr = type<int[3]>; // int*

The former seems more correct to me, but YMMV.