Constexpr generation of character lookup tables

constexpr-char-lookup-table.cpp

// generate character-based static lookup tables
// that can be used for character classification,
// e.g. like in cctype functions (isdigit, isalpha ...)

struct CharLookupTable
{
  bool data[256];

  template <unsigned int N>
  constexpr CharLookupTable(const char (&str)[N])
    : data{}
  {
    for (unsigned int i = 0; i < (N - 1); ++i)
      data[static_cast<unsigned char>(str[i])] = true;
  }
};

constexpr CharLookupTable digits{"0123456789"};
constexpr CharLookupTable hexdigits{"0123456789abcdefABCDEF"};

bool isdigit(char c)
{
  return digits.data[static_cast<unsigned char>(c)];
}

bool ishexdigit(char c)
{
  return hexdigits.data[static_cast<unsigned char>(c)];
}