This relates to a problem I'm trying to solve which has already been addressed a couple of times Lookup table with constexpr; constexpr array and std::initializer_list
I have a constexpr function that's too slow to use a run time, for my purposes I want to use a lookup table, and ideally I want to use the same function at run time and compile time.
I came up with this
template<template<size_t>class f, size_t... values>
struct lookup_table{
static constexpr auto lookup(size_t i){
auto my_table = {f<values>::value...};
return *(my_table.begin() + i);
}
};
template<size_t n>
class some_function{
// this is a terrible way to calculate an exponential
static constexpr auto slow_exponential(size_t x){
double y = 1 + ((double)x / 1000000.0);
double retval = 1;
for (int i = 0; i < 1000000; i++)
retval *= y;
return retval;
}
public:
static constexpr double value = slow_exponential(n);
};
int main(int, char**){
using my_table = lookup_table<some_function, 0,1,2,3,4,5,6,7,8,9>;
// test for constexprness
constexpr int x = my_table::lookup(7);
using X = std::integral_constant<int, x>;
std::cout << "enter n" << std::endl;
int n;
std::cin >> n;
std::cout << "exp(" << n << ") = " << my_table::lookup(n) << std::endl;
std::cout << "exp(" << n << ") = " << std::exp(n) << std::endl;
return 0;
}
This compiles and works as expected with Clang 3.5 but I'm not 100% sure that it's actually valid (it doesn't feel constexpr). Have I strayed into undefined behaviour somehow?
