Being able to convert the string passed to a literal operator into an MPL sequence would be useful, since we would then be able to control code generation based on the contents of the string. Previously, I thought this to be impossible, since the arguments to a constexpr function are not regarded as constant expressions within the function's body. However, I came up with the following workaround that compiles under Clang 3.4.2 and GCC 4.8.2:
#include <cstdint>
#include <iostream>
#include <typeinfo>
struct string
{
const uintmax_t m_str[64];
const size_t m_length;
template <class... Ts>
constexpr string(const Ts... ts) :
m_str{(uintmax_t)ts...}, m_length{sizeof...(Ts)} {}
constexpr size_t size() const { return m_length; }
constexpr size_t length() const { return m_length; }
constexpr uintmax_t operator[](size_t n) const { return m_str[n]; }
};
template <uintmax_t... Ts> struct sequence {};
constexpr auto
operator"" _tag(const char* str, size_t n)
{
return n == 0 ? string{} :
n == 1 ? string{str[0]} :
n == 2 ? string{str[0], str[1]} :
n == 3 ? string{str[0], str[1], str[2]} :
n == 4 ? string{str[0], str[1], str[2], str[3]} :
n == 5 ? string{str[0], str[1], str[2], str[3], str[4]} :
string{str[0], str[1], str[2], str[3], str[4], str[5]};
}
int main()
{
static constexpr auto string = "Hello!"_tag;
using sequence = sequence<string[0], string[1], string[2], string[3], string[4], string[5]>;
std::cout << typeid(sequence{}).name() << std::endl;
}
Output:
sequence<72ul, 101ul, 108ul, 108ul, 111ul, 33ul>
Question
Does this code invoke undefined behavior, or is it legal?
