on Mon Jan 09 2012, Ábel Sinkovics <abel-AT-elte.hu> wrote:
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; }
----
Yeah, what you can't do is: 1. generate that type based on the /value/ of a function argument (yes, you can deduce the length of the string, and thus that type, from the /type/ of a string literal argument). 2. Generate any types or choose template specializations based on the actual characters used in the string literal. I don't see how you're going to generate anything other than a table-based regexp recognizer if you can't do that. The fundamental limitation with constexpr functions is that the function you're calling must be valid even if its arguments are /not/ compile-time constants. Combine that with the lack of raw user-defined string literals, and, well... we're cooked.
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.
If you can demonstrate something that works, I'll be really excited. -- Dave Abrahams BoostPro Computing http://www.boostpro.com