Catching by Value Slices the Object

1#include <iostream>
2#include <exception>
3 
4class AppError : public std::exception {
5public:
6    const char* what() const noexcept override { return "generic app error"; }
7};
8 
9class ConfigError : public AppError {
10public:
11    const char* what() const noexcept override { return "config file missing"; }
12};
13 
14int main() {
15    try {
16        throw ConfigError();
17    } catch (AppError e) {
18        std::cout << "by value: " << e.what() << "\n";
19    }
20    try {
21        throw ConfigError();
22    } catch (const AppError& e) {
23        std::cout << "by reference: " << e.what() << "\n";
24    }
25    return 0;
26}