初始化constexpr-“用非常量表达式非法初始化'constexpr'实体”

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

Migrating the answer from the comments section into the answer section.

There are no constexpr maps. It uses dynamic allocation, which is not possible with constexpr. Get rid of constexpr, or use a different container for compile-type map.

Answer 2

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 (eg, a switch statement in a constexpr function).