Hi Dave,
constexpr is somewhat crippled: although a constexpr function executes at compile-time, it has to obey the same rules as any other function. For example, its return type can depend on the type, but never on the *value*, of its arguments, and once you're inside the function, the contents of the argument are not treated as compile-time constants. So, I think constexpr actually doesn't promise it. A constexpr can still be used to access the characters of a string at compile-time. Having access to each character we can build an MPL list of characters. Here is an example of what I'm thinking of:
---- using namespace boost::mpl; template <int N> constexpr char nth(const char s[N], int n) { return n >= N ? 0 : s[n]; } #define S "cool" typedef push_back< push_back< push_back< push_back< push_back< push_back< string<>, char_<nth<sizeof(S)>(S, 0)>
::type, char_<nth<sizeof(S)>(S, 1)> ::type, char_<nth<sizeof(S)>(S, 2)> ::type, char_<nth<sizeof(S)>(S, 3)> ::type, char_<nth<sizeof(S)>(S, 4)> ::type, char_<nth<sizeof(S)>(S, 5)> ::type str;
int main() { std::cout << c_str<str>::type::value << std::endl; } ---- The code getting the characters one by one and building the MPL list (or string in the above example) can be generated by a macros - I know it is not that nice and the length of the string will be tricky, but we'll have something. The above code snippet compiles with gcc 4.6. Regards, Ábel