I'm trying to convert a C++ literal string into an instance of the following template:
template <char ... C>
struct string_literal {
typedef string_constant type;
static constexpr const char value[sizeof...(C)] = {C...};
constexpr operator const char* (void) const {
return value;
}
};
template <char ... C>
constexpr const char string_literal<C...>::value[sizeof...(C)];
I came up with these helpers based on various sources for 'unpacking' the quoted string value into the template above.
template <unsigned N, const char (&S) [N], typename U>
struct selector;
template <unsigned N, const char (&S) [N], unsigned ...I>
struct selector<N, S, index_sequence<I...>> {
using type = string_literal<S[I]...>;
};
template <unsigned N, const char (&S) [N]>
struct unpack {
using type = typename selector<N, S, make_index_sequence<N>>::type;
};
However, when a call this I get a compiler error:
template <unsigned N>
constexpr auto make_string_literal(const char (&s) [N]) {
return unpack<N, s>{}; // Error here
}
constexpr auto literal = make_string_literal("test");
// string_literal<'t','e','s','t','\0'>
GCC 4.9+ reports: error: 'const char (& s)[1]' is not a valid template argument for type 'const char (&)[1]' because a reference variable does not have a constant address
Clang 3.7.1 reports: error: non-type template argument refers to object 's' that does not have linkage
I tried a few different approaches but the errors are mostly the same. What am I missing here?
Is this a satisfactory solution to meet your needs?
template <char ... C>
struct string_literal {
static constexpr const char value[sizeof...(C)] = {C...};
constexpr operator const char* (void) const {
return value;
}
void foo() {std::cout << value << '\n';}
};
template <char ... C> constexpr const char string_literal<C...>::value[sizeof...(C)];
template <typename CharT, CharT... Cs>
constexpr string_literal<Cs...> operator ""_create() {
return {};
}
int main() {
string_literal<'t','e','s','t'> s = "test"_create;
std::cout << s << '\n'; // test
s.foo(); // test
}
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