g++: const 丢弃限定符

why do I get a discard qualifiers error:

customExc.cpp: In member function ‘virtual const char* CustomException::what() const’:
customExc.cpp: error: passing ‘const CustomException’ as ‘this’ argument of ‘char customException::code()’ discards qualifiers

关于以下代码示例

#include <iostream>


class CustomException: public std::exception {

public:

    virtual const char* what() const throw() {
        static std::string msg;
        msg  = "Error: ";
        msg += code();  // <---------- this is the line with the compile error 
        return msg.c_str();
    }

    char code() { return 'F'; }
};

在关于类似问题之前，我已经在 SOF 上进行了搜索.