static compile time table with floating point values

Question

Is it possible to generate an array at compile time, like in this nice answer of G. Fritzsche: Georg Fritzsche

but with floating point values?

Answer 1

It wasn't possible in standard-compliant C++ when Georg wrote his answer, but with C++11 we can now do compile-time computation with floating-point and other numeric types.

The trick is to use constexpr functions instead of non-type template parameters.

Answer 2

Here is a solution creating an array of doubles which takes an initialization class to map an int index to a suitable value. The example create an array with 20 value approximating sin(x) for 20 values in the range [0, 2π]. Most of the code is devoted to producing a sequence of integers [0...N): if that operation is readily available, the code becomes fairly trivial (it is being added to C++14; see n3493).

#include <iostream>
#include <cmath>

template <int...> struct indices {};
template <int N, typename> struct make_list;
template <int... Indices>
struct make_list<0, indices<Indices...> > {
    typedef indices<0, Indices...> type;
};

template <int N, int... Indices>
struct make_list<N, indices<Indices...> > {
    typedef typename make_list<N-1, indices<N, Indices...> >::type type;
};

template <typename Init, int N, typename> struct array_aux;
template <typename Init, int N, int... Indices>
struct array_aux<Init, N, indices<Indices...> >
{
    static double const values[N];
};

template <typename Init, int N, int... Indices>
double const array_aux<Init, N, indices<Indices...> >::values[N] = {
    Init::value(Indices)...
};

template <typename Init, int N>
struct array
    : array_aux<Init, N, typename make_list<N-1, indices<> >::type>
{
};



struct init_sin
{
    static constexpr double value(int index) {
        return std::sin(index * 2.0 * 3.1415 / 20.0);
    }
};

int main()
{
    for (int i = 0; i != 20; ++i) {
        std::cout << array<init_sin, 20>::values[i] << "\n";
    }
}

If the generator function, i.e., Init::value() is a constexpr function actually return constant values (probably not quite the case for std::sin()) the values can be computed at compile time. Otherwise they will be computed during static initialization.