store a string in a constexpr struct

Question

Is it possible to store a string in a constexpr struct:

So far I could only come up with:

struct A
{
    constexpr A(std::string_view n): m_name(n) {}   
    constexpr auto name(){ return m_name; }

    std::string_view m_name; // This might become dangling!!
} 

which is cleary only a good idea if this class is only used like this

A a = {"Hello"};
constexpr A b = {"World"};

and not like this

auto makeA(std::string n) { return A{n}; }
A a = makeA("Hello"); // Dangling internal std::string_view

I need the constexpr to construct the struct at compile time. Is it possible to make this safer at run-time, because with std::string_view, its not.

Answer 1

You might do:

template<typename Char, Char... Cs>
struct CharSeq
{
    static constexpr const Char s[] = {Cs..., 0}; // The unique address
};

// That template uses the extension
template<typename Char, Char... Cs>
constexpr CharSeq<Char, Cs...> operator"" _cs() {
    return {};
}

See my answer from String-interning at compiletime for profiling to have MAKE_STRING macro if you cannot used the extension (Really more verbose, and hard coded limit for accepted string length).

Then

struct A
{
    template <char ... Cs> 
    constexpr A(CharSeq<char, Cs...>) : m_name(CharSeq<char, Cs...>::s) {}

    constexpr auto name(){ return m_name; }

    std::string_view m_name;
};

With only valid usages similar to:

A a = {"Hello"_cs};
constexpr A b = {"World"_cs};