c++ - Concat two `const char` string literals

Question

Is it possible to concat two string literals using a constexpr? Or put differently, can one eliminate macros in code like:

#define nl(str) str "
"

int main()
{
  std::cout <<
      nl("usage: foo")
      nl("print a message")
      ;

  return 0;
}

Update: There is nothing wrong with using " ", however I would like to know whether one can use constexpr to replace those type of macros.

Answer 1

A little bit of constexpr, sprinkled with some TMP and a topping of indices gives me this:

#include <array>

template<unsigned... Is> struct seq{};
template<unsigned N, unsigned... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};
template<unsigned... Is>
struct gen_seq<0, Is...> : seq<Is...>{};

template<unsigned N1, unsigned... I1, unsigned N2, unsigned... I2>
constexpr std::array<char const, N1+N2-1> concat(char const (&a1)[N1], char const (&a2)[N2], seq<I1...>, seq<I2...>){
  return {{ a1[I1]..., a2[I2]... }};
}

template<unsigned N1, unsigned N2>
constexpr std::array<char const, N1+N2-1> concat(char const (&a1)[N1], char const (&a2)[N2]){
  return concat(a1, a2, gen_seq<N1-1>{}, gen_seq<N2>{});
}

Live example.

I'd flesh this out some more, but I have to get going and wanted to drop it off before that. You should be able to work from that.