val val - 1 month ago 26
C++ Question

Constexpr lookup table with VS2015

I was looking around how to make a lookup table and found this simple and elegant answer.

I didnt want to necro the thread, so I thought I make a new question. When trying to compile that answer in VS2015 i get the following errors:

template<int N>
struct Table
{
constexpr Table() : values()
{
// C3538 auto must always deduce the same type
// C3250 i declaration is not allowed in constexpr body
// C3249 illegal statement for sub-expression 'constexpr' function
for (auto i = 0; i < N; ++i)
{
values[i][0] = i;
values[i][1] = i * i * i;
}
}
int values[N][2];
};

int main(){
// defctor does not produce a constant value
constexpr auto a = Table<1000>();
return 0;
}


I tried to replace the C style array with an
std::array
, thought that might help with the iteration process, and i checked the code on some online compiler, there it works, but not in VS.

What's the problem, and how could i replicate the solution without template bloating the code?

Answer

As indicated in the comments above, MSVC 2015 doesn't support C++14 constexpr, but it does support lots of other C++14 features, so you can do this:

#include <cstddef>
#include <utility>

template<int N> struct Table
{
   constexpr Table() : Table(std::make_index_sequence<N>{}) { }
   int values[N][2];
private:
   template<std::size_t... Is> constexpr Table(std::index_sequence<Is...>)
      : values{{Is, Is * Is * Is}...} { }
};

int main()
{
   constexpr auto a = Table<100>{};
   static_assert(a.values[77][0] == 77 && a.values[77][1] == 77 * 77 * 77, "oops");
}

IntelliSense still complains about the initialization of a, but that's a bug. MSVC accepts the code (and so do Clang and GCC in C++14 mode).

Note that using a large value for N (Table<1000>) as in the original code will generate a warning:

warning C4503: 'Table<1000>::Table': decorated name length exceeded, name was truncated

As far as I can tell from reading the docs, the warning can safely be ignored in this context.