p5.cc

#include <string>
#include <array>
#include <iostream>

// compile-time string from
// http://blog.biicode.com/template-metaprogramming-cpp-ii/
template <char... C> struct String {
  static constexpr std::array<char, sizeof...(C)+1> data = {{C..., '\0'}};
  constexpr operator const char *() const { return data.data(); }
};

template <char... C>
constexpr std::array<char, sizeof...(C)+1> String<C...>::data;

constexpr char digit(int n) { return '0' + n; }

template <int want> struct Builder {
  template <int d, int p, int sum, int sign, typename Ans, typename Acc>
  struct BuilderImpl;

  template <int d, int p, int sum, int sign, char... Ans, char... Acc>
  struct BuilderImpl<d, p, sum, sign, String<Ans...>, String<Acc...>> {
    using result = typename BuilderImpl<
        d + 1, p * 10 + d, sum, sign, String<Ans..., digit(d)>,
        typename BuilderImpl<
            d + 1, 0, sum + sign *(p * 10 + d), -1,
            String<Ans..., digit(d), '-'>,
            typename BuilderImpl<d + 1, 0, sum + sign *(p * 10 + d), 1,
                                 String<Ans..., digit(d), '+'>,
                                 String<Acc...>>::result>::result>::result;
  };

  template <int sum, typename Ans, typename Acc> struct Check;

  template <int sum, char... Ans, char... Acc>
  struct Check<sum, String<Ans...>, String<Acc...>> {
    using result = String<Acc...>;
  };

  template <char... Ans, char... Acc>
  struct Check<want, String<Ans...>, String<Acc...>> {
    using result = String<Acc..., Ans..., '\n'>;
  };

  template <int p, int sum, int sign, char... Ans, char... Acc>
  struct BuilderImpl<9, p, sum, sign, String<Ans...>, String<Acc...>> {
    using result = typename Check<sum + sign *(p * 10 + 9), String<Ans..., '9'>,
                                  String<Acc...>>::result;
  };

  using result = typename BuilderImpl<1, 0, 0, 1, String<>, String<>>::result;
};

int main() { std::cout << Builder<100>::result(); }