Using array elements as non-type template parameters?

Question

In the follwing example code I use the object a0 as a non-type template-parameter, which works well, because it is an object with linkage (lvalue-ref). So I thought it must be possible to do so with an array element, but that fails:

constexpr uint8_t a[10] = {};
constexpr uint8_t a0 = {};

template<typename T, const T& V>
struct Test {};

using test = Test<uint8_t, a0>; // OK
using test = Test<uint8_t, a[0]>; //NOK

Is there a way to get this working?

Maxim Egorushkin · Accepted Answer · 2018-06-05 11:32:22Z

2

Binding a reference template parameter requires a name of an object with external linkage. Individual array elements have no name and no linkage in C++. This is why a reference template parameter cannot be bound to an array element. See 6.5 Program and linkage [basic.link] for full details.

You can make it compile if you drop the reference (if possible):

template<typename T, T V>
struct Test {};

Or access the element by index inside the template:

template<class T, T const& A, size_t Idx>
struct Test2 {
    static constexpr decltype(A[Idx]) element = A[Idx];
    static_assert(std::is_reference<decltype(element)>::value, "");
};

using test = Test2<decltype(a), a, 1>;

edited Jun 5, 2018 at 11:32

answered Jun 5, 2018 at 9:46

Maxim Egorushkin

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