1#include <iostream>
2#include <stdexcept>
3using namespace std;
4 
5class MyError : public runtime_error {
6public:
7    MyError() : runtime_error("base message") {}
8    const char* what() const noexcept override {
9        return "derived message";
10    }
11};
12 
13int main() {
14    try {
15        throw MyError();
16    } catch (runtime_error e) {  // catch by VALUE
17        cout << e.what() << endl;
18    }
19    return 0;
20}