How to call the base constructor inside the derived constructor body [duplicate]

I'm trying to write an error that derives from runtime_error.

Here's the definition:

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

    read_error(Code code);
    const char* what() const throw();
    Code getCode() const throw();
private:
    Code code;
};

And I'm trying to call the constructor like so:

read_error::read_error(Code code): code(code) {
    std::string _what;
    switch (code) {
        case Code::TIME_OUT:
            _what = "Read exception: Reading timed out";
        case Code::DESERIALIZE:
            _what = "Read exception: Something went wrong while deserializing the object";
        case Code::CHECKSUM:
            _what = "Read exception: Mismatched checksum";
        case Code::NO_START_BYTE:
            _what = "Read exception: Malformed message, no start byte at front";
        case Code::NO_END_BYTE:
            _what = "Read exception: Malformed message, no end byte at back";
        runtime_error(_what);
    }
}

But that doesn't work. It seems that I have to use the initializer like

read_error::read_error(Code code): code(code), runtime_error(/*something*/) {}

But I can't put a switch statement in there. How do I work around this? I know that I have to initialize it, but I can't edit the what_arg inside the runtime_error after the initialization. How do I initialize it using that switch statement?

(I know that there's something still wrong with the enum class definition, I'm working on that too)