Lookup table with constexpr

Question

I'm looking to create a lookup table of coordinates, something like:

int a[n][2] = {{0,1},{2,3}, ... }

For a given n, to be created at compile time. I started looking into constexpr, but is seems like a function returning a constexpr std::vector<std::array <int, 2> > isn't an option, as I get:

invalid return type 'std::vector<std::array<int, 2ul> >' of constexpr function

How can create such a compile time array?

Answer 1

With C++14, you do not need too much template magic. Here an example of how to have a lookup table for f(x) = x^3 with the first coordinate being the x value and the second coordinate being the f(x) value:

#include <iostream>

template<int N>
struct Table
{
    constexpr Table() : values()
    {
        for (auto i = 0; i < N; ++i)
        {
            values[i][0] = i;
            values[i][1] = i * i * i;
        }
    }
    int values[N][2];
};

int main() {
    constexpr auto a = Table<1000>();
    for (auto x : a.values)
        std::cout << "f(" << x[0] << ") = " << x[1] << '\n';
}