class BadLengthException{ public: int n; BadLengthException(int e){ n=e; } int what() { return n; } };

If showing error in integer or const something stuffs, just remove the 'const' keyword after as : class BadLengthException : public exception { private: int exceptionname; public: explicit BadLengthException(const int &exceptionexample) { exceptionname = exceptionexample;}

int what()    // removing 'const' after 'what()' works , not any other
{       
    return exceptionname;
}

I cannot see the proper solution where we inherit all methods. So, here it is:

class BadLengthException : public exception {
private:
    string message;
public:
    BadLengthException(const int& msg) : message(to_string(msg)) {}
    const char* what() const noexcept override{
        return message.c_str();
    }
};

To be syntactically coherent with error messages, we need to return the error as a C-style string, not an integer.

what defines if something is invalid???

try c++14 class BadLengthException : public exception{ private: int length; public: BadLengthException(int n){ length=n; } int what(){ return length; } };