From: Dave Abrahams
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.
The return value of the constexr is a compile time constant. We ought to be able to use it as a template parameter. Doesn't Abel's example work? I tried this little test: #include <stdio.h> template <int N> constexpr char nth(const char s[N], int n) { return n >= N ? 0 : s[n]; } #define S "cool" template <char T> void foo() { printf("not cool\n"); } template <> void foo<'c'>() { printf( "cool\n"); } template <> void foo<'o'>() { printf( "way cool\n"); } template <> void foo<'l'>() { printf( "way way cool\n"); } int main() { foo<nth<sizeof(S)>(S, 0)>(); foo<nth<sizeof(S)>(S, 1)>(); foo<nth<sizeof(S)>(S, 2)>(); foo<nth<sizeof(S)>(S, 3)>(); return 0; } With compile line: gcc/4.6.2/bin/g++ -std=c++0x And got the following output: cool way cool way cool way way cool It looks like you can specialize a template based on the individual characters within a string literal using constexpr quite simply. From here we ought to be able to perform arbitrary TMP on the contents of string literals. I would think that an ebnf as a string literal in a template parameter (with a macro call around it) ought to be able to generate a function that matches that grammar as an extreme example. Regards, Luke