C++: What is std::optional in C++17?

1#include <optional>
2
3std::optional<int> findValue(int key) {
4    if (key == 42) return 100;
5    return std::nullopt;
6}
7
8auto result = findValue(42);
9
10if (result.has_value()) {
11    std::cout << *result;  // 100
12}
13
14// Value or default
15int val = result.value_or(0);  // 100
16
17// Emplace
18std::optional<std::string> opt;
19opt.emplace("hello");
20
21// Reset
22opt.reset();  // No value