Initialising constexpr - " illegal initialization of 'constexpr' entity with a non-constant expression"

Question

I have two enum class types: Type and SocketType. The following code won't compile and fails with the message mentioned in the question, in VC++ 2017:

static constexpr std::map<Type,SocketType> PacketTypeMap =
    {
        {Type::JUSTJOINED,      SocketType::TCP},
        {Type::CHAT_MESSAGE,    SocketType::TCP},
        {Type::REQUEST_WORLD,   SocketType::TCP},
        {Type::DATA_WORLD,      SocketType::TCP},
        {Type::DATA_PLAYER,     SocketType::UDP},
        {Type::RESPAWN_PLAYER,  SocketType::TCP}
    };

Been trying some variations and nothing works, but I'm sure I'm just missing something simple with the syntax.

Answer 1

std::map is not compatible with constexpr. There exists an experimental(?) library called frozen, which provides a constexpr-compatible frozen::map (besides frozen::unordered_map, frozen::string, and others).

However, most probably you just want to pick a simpler solution (e.g., a switch statement in a constexpr function).