Defining static constexpr values in an enum class

I have a class that I regularly use for an enum wrapper, however this requires a cpp file. Can anyone show me how to use constexpr

The header file is as follows:

// Extend-able in the future
class CDUVariable {
public:
    enum Value {
        baud, firmware, type, serial
    };
    static const CDUVariable Baud, Firmware, Type, Serial;

    /// Comparison operator (used for strict weak ordering).
    bool operator<(const CDUVariable& rhs) const {
        return mValue < rhs.mValue;
    }

    /// Integral operator cast for switch statements (cast to named enum).
    operator const Value() const {
        return mValue;
    }

    /// Serialized version of the enum.
    std::string getStringVal() const {
        return mStringVal;
    }
    static const std::set<CDUVariable>& getValues() {
        static std::set<CDUVariable> gValues;
        if (gValues.empty()) {
            gValues.insert(Baud);
            gValues.insert(Firmware);
            gValues.insert(Type);
            gValues.insert(Serial);
        }
        return gValues;
    }
    static CDUVariable valueOf(const std::string& rStringVal) {
        for (const auto& next : getValues()) {
            if (next.getStringVal() == rStringVal) {
                return next;
            }
        }
        throw std::invalid_argument(
            "Illegal Argument: " + rStringVal);
    }
private:
    CDUVariable(const Value& rValue, const std::string& rStringVal)
        : mValue(rValue)
        , mStringVal(rStringVal)
    {}
    Value mValue;
    std::string mStringVal;
};

and the cpp file is:

const CDUVariable CDUVariable::Baud(baud, "0");
const CDUVariable CDUVariable::Firmware(firmware, "1");
const CDUVariable CDUVariable::Type(type, "2");
const CDUVariable CDUVariable::Serial(serial, "3");

I would like to know if it is possible to use the new constexpr syntax to initialize everything in the header file. I am having trouble with the syntax.

I tried prefixing modifying the header as follows:

// Extend-able in the future
class CDUVariable {
public:
    constexpr enum Value {
        baud, firmware, type, serial
    };
    constexpr static CDUVariable Baud(baud, "0") Firmware(firmware, "1"), Type(type, "2"), Serial(serial, "3");

But this ended up giving me a bunch of errors. I would like to know how to migrate to the new constexpr syntax and the plus side of this would be that I could ship a header only library containing these enum type classes.

2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(107): error C2061: syntax error: identifier 'baud'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(107): error C3646: 'Firmware': unknown override specifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(107): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(107): error C2059: syntax error: '('
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(147): error C2143: syntax error: missing ')' before ';'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(147): error C2098: unexpected token after data member 'mValue'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(147): error C2059: syntax error: ')'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(111): error C2065: 'mValue': undeclared identifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(111): error C2039: 'mValue': is not a member of 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(102): note: see declaration of 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(116): error C2065: 'mValue': undeclared identifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(126): error C3867: 'CDUVariable::Baud': non-standard syntax; use '&' to create a pointer to member
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(127): error C2065: 'Firmware': undeclared identifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(128): error C2065: 'Type': undeclared identifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(129): error C2065: 'Serial': undeclared identifier
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(146): error C2614: 'CDUVariable': illegal member initialization: 'mValue' is not a base or member
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(165): error C2350: 'CDUVariable::Baud' is not a static member
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(165): note: see declaration of 'CDUVariable::Baud'
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(165): error C2248: 'CDUVariable::CDUVariable': cannot access private member declared in class 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(143): note: see declaration of 'CDUVariable::CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(102): note: see declaration of 'CDUVariable'
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(166): error C2039: 'Firmware': is not a member of 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(102): note: see declaration of 'CDUVariable'
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(166): error C2065: 'firmware': undeclared identifier
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(167): error C2039: 'Type': is not a member of 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(102): note: see declaration of 'CDUVariable'
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(167): error C2065: 'type': undeclared identifier
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(168): error C2039: 'Serial': is not a member of 'CDUVariable'
2>c:\users\johnc\main\app739\include\vcdu\vcduconstants.h(102): note: see declaration of 'CDUVariable'
2>c:\users\johnc\main\app739\src\vcdu\vcduconstants.cpp(168): error C2065: 'serial': undeclared identifier
2>Generating Code...