constexpr_interned_str.cpp

////////////////////////////////////////////////////////////////////////////////
// Writing a string comparison that would consistently work as a compile time
// function, even in older compilers (think GCC 4.8) is not that easy...
////////////////////////////////////////////////////////////////////////////////

template<std::size_t N>
constexpr bool fixed_equal(char const * a, char const * b) {
    return *a == *b && fixed_equal<N-1>(a + 1, b + 1);
}

template<>
constexpr bool fixed_equal<0>(char const * a, char const * b) {
    return true;
}

template<std::size_t N, std::size_t M>
constexpr bool strings_equal(const char(&a)[N], const char(&b)[M]) {
    return N == M && fixed_equal<N>(a, b);
}

////////////////////////////////////////////////////////////////////////////////
// This is the important part
////////////////////////////////////////////////////////////////////////////////

int strToSymbol(const char * a);

// List all builtin symbols here
#define FORALL_BUILTIN_SYMBOLS(_) \
_(A) \
_(B)

// NOTE: BuiltinSymbols::fistFreeId can be used to get the lowest ID
// which can be assigned to a string interned at run time.
#define DEFINE_SYMBOL(s) s,
enum class BuiltinSymbols : int {
    FORALL_BUILTIN_SYMBOLS(DEFINE_SYMBOL)
    firstFreeId 
};
#undef DEFINE_SYMBOL

template<std::size_t N>
constexpr int _(const char (&a)[N]) {
#define DEFINE_CASE(s) strings_equal(a, #s) ? static_cast<int>(BuiltinSymbols::s) : 
    return FORALL_BUILTIN_SYMBOLS(DEFINE_CASE) (strToSymbol(a));
#undef DEFINE_CASE
}

int main() {
    return _("A");
}