C++: What is std::expected and error handling patterns?

1#include <expected>
2
3// Error types
4enum class Error { NotFound, InvalidInput, NetworkError };
5
6std::expected<int, Error> divide(int a, int b) {
7    if (b == 0) return std::unexpected(Error::InvalidInput);
8    return a / b;
9}
10
11// Chain operations
12auto result = divide(10, 2)
13    .transform([](int val) { return val * 10; })
14    .and_then([](int val) -> std::expected<int, Error> {
15        if (val > 50) return std::unexpected(Error::NetworkError);
16        return val;
17    });
18
19// Match
20if (result) {
21    std::cout << "Value: " << *result;
22} else {
23    switch (result.error()) {
24        case Error::NotFound:     std::cout << "Not found"; break;
25        case Error::InvalidInput:  std::cout << "Invalid"; break;
26        case Error::NetworkError:  std::cout << "Network"; break;
27    }
28}
29
30// Value or default
31int val = divide(10, 0).value_or(-1);  // -1
32
33// Error transformation
34auto result2 = divide(10, 0)
35    .transform_error([](Error e) {
36        return "Error: " + std::to_string(static_cast<int>(e));
37    });