Re: [boost] compile time parser generator

Subject: Re: [boost] compile time parser generator
From: Ábel Sinkovics (abel_at_[hidden])
Date: 2012-01-09 01:31:36

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