Enum class definition seperate from declaration. Why does the rest of the code not find the definition? [closed]

I'm not allowed to make any definition in the header file, only declarations. So I have the following .h file:

class read_error final: public std::runtime_error {
public:
    enum class Code: uint8_t;

    read_error(Code code);
    Code getCode() const throw();
private:
    Code code;

    static std::string whatFromCode(Code code);
};

And in the .cpp file I want to do this:

enum class read_error::Code: uint8_t {
    TIME_OUT,
    DESERIALIZE,
    CHECKSUM,
    START_BYTE,
    END_BYTE,
    COMMAND
};

And I mean, that part does compile, but when I try to throw this error like read_error(read_error::Code::CHECKSUM);

I get the the following: error: ‘CHECKSUM’ is not a member of ‘amp::exception::read_error::Code’

I put both the cpp and the h into my makefile but it doesn't seem to find the definition. Is there any other way than defining the enum class in the .h file?