Data structure with get, returning a constexpr (C++)

Question

I am currently looking for a data-structure that encapsulates data for compile-time access. Thereby, the accessed values should returned as constexpr.

While a tuple does have a constexpr constructor, the get function of the tuple does not return a constexpr.

Does such a data structure exist or is it possible to manually define such a data structure?

The final goal is to pack compile time known values within some kind of object, pass it (via template) to a function, access the elements there and have the compile time known values directly pasted inplace in the binary as constants. For my purpose the encapsulation part is crucial.

Perhaps [`std::array` if you do it correctly](https://stackoverflow.com/questions/14267399/can-stdarray-be-used-in-a-constexpr-class) — Cory Kramer, Aug 20 '15 at 12:13
Using `std:array` in the example above, the access to the array in the struct is compiled to `callq std::array::operator[](unsigned long) const` which is not the constant at all, but a memory access I assume... — smoes, Aug 21 '15 at 07:32
To be clear: It may be constant, but it is not the integer constant that is stored within the array, as I want it to be! — smoes, Aug 21 '15 at 07:39

score 1 · Accepted Answer · answered Sep 23 '15 at 10:00

As of C++14, std::tuple does accept a constexpr std::get

#include <tuple>

int main()
{
   constexpr std::tuple<int, int, int> t { 1, 2, 3 };
   static_assert(std::get<0>(t) == 1, "");
}

Live Example

Similarly, you can use std::array as well with std::get (and also the operator[] is now constexpr). Raw C-arrays can also be done.

#include <array>

int main()
{
   constexpr std::array<int, 3> a {{ 1, 2, 3 }};
   static_assert(std::get<0>(a) == 1, "");
   static_assert(a[0] == 1, "");
}

Live Example

Data structure with get, returning a constexpr (C++)

1 Answers1