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