Re: Variadic function to propagate array indexing doesn't work correctly

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Mon, 16 Apr 2012 23:32:11 -0700 (PDT)
Am 16.04.2012 16:00, schrieb Daryle Walker:

Note that, for now, I just working with built-in arrays. I checked
my copy of C++2003, and it says that pointer indexing (which
conceptually includes array indexing) works with one pointer(/array)
operand and one integer or enumeration operand. If that's changed
for C++2011, let me know.

This is correct, but there is an issue in progress,

that will extend the concept of integral constant expressions. In
particular, literal class types with conversions will be legal, like

struct L {
    int v;
    constexpr L(int v) : v(v) {}
    constexpr operator int() { return v; }

constexpr L c(2);

int arr[c];

gcc 4.8 already accepts this, which - strictly speaking - is an
extension versus the current wording.

It also works when I want to stop indexing a nested array after one
layer. I want to keep the "noexcept" specification, so I don't want
possibly-throwing conversions from UDTs to discrete-scalars. I
enforce this by requiring the index types C++ allows:

template< typename T>
struct can_index_array
      : std::integral_constant< bool, std::is_integral<T>::value ||

If anyone has a way to make this work with class-types indices, let
us know. (The class-type must have an unambiguous (non-explicit?)
conversion to either one enum-type or one built-in integer type.
And we need to know if that conversion is no-throw.) The limitation
in index types lets me pass them around by-value, without

If you want to pass them around by-value, you also need to test for
no-throw copy/move, I think. To test for the nothrow-convertibility
you may want to use something like

#include <utility>

template<class T, class U>
struct is_nothrow_convertible
    template<class V>
    static void sink(V) noexcept;

    template<class V>
    static void test(int) noexcept(noexcept(sink<V>(std::declval<T>())));

    static void test(...) noexcept(false);

    static const bool value = noexcept(test<U>(0));

and test for is_nothrow_convertible<T, std::size_t> in the
array-subscription context. Combine this with
is_nothrow_convertible<T, T> and is_nothrow_convertible<T&, T>
(depending on whether the argument is forwarded as lvalue or rvalue)
to respect the copy-initialization context of by-value passing.

HTH & Greetings from Bremen,

Daniel Kr?gler

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