Re: [boost] compile time parser generator

Subject: Re: [boost] compile time parser generator
From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2012-01-09 13:09:32

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