typedef of array of typedefs of array

It's bool[4][2] You can verify it by static_assert:

static_assert(std::is_same<decltype(what_am_i), bool[4][2]>::value, "");
static_assert(std::is_same<decltype(what_am_i), bool[2][4]>::value, ""); // failed

foo is an array with 2 elements of type bool, i.e. bool[2].

bar is an array with 4 elements of type foo, i.e. foo[4], each element is a bool[2].

Then what_am_i is bool[4][2].

In order to complete @Slardar Zhang's C++ answer for C:

It's bool[4][2].

You can verify it by either one of the following:

• sizeof(what_am_i)/sizeof(*what_am_i) == 4
• sizeof(*what_am_i)/sizeof(**what_am_i) == 2

After inspecting the variable through the debugger, I found that I was right - what_am_i is a [4][2] dimensional array.

When you don't know the type of a variable, one of the easy ways is this trick:

 template<class T> struct tag_type{using type=T;};
 template<class T> constexpr tag_type<T> tag{};

then do this:

 tag_type<void> t = tag<some_type>;

almost every compiler will spit out a reasonably readable "you cannot assign tag_type<full_unpacked_type> to tag_type<void>" or somesuch (presuming your type isn't void that is).

You can also verify you have a good guess with

tag_type<your_guess> t = tag<some_type>;

sometimes you'll want to generate some_type via decltype(some_expression).