std::optional, std::variant and std::string_view

#include <iostream>
#include <optional>
#include <string>
#include <vector>

std::optional<int> findRoll(const std::vector<std::string>& names, const std::string& who) {
    for (std::size_t i = 0; i < names.size(); ++i) {
        if (names[i] == who) return static_cast<int>(i) + 1;
    }
    return std::nullopt;                    // "no value"
}

int main() {
    std::vector<std::string> cls = {"Asha", "Ravi", "Meena"};
    if (auto roll = findRoll(cls, "Ravi")) {   // true if it has a value
        std::cout << "Roll " << *roll << '\n';   // Roll 2
    }
    auto missing = findRoll(cls, "Om");
    std::cout << missing.value_or(-1) << '\n';   // -1
    std::cout << missing.has_value() << '\n';    // 0
}

#include <iostream>
#include <string>
#include <variant>

using PaymentResult = std::variant<std::string, int>;   // a transaction id OR an error code

PaymentResult pay(int amount) {
    if (amount <= 0) return 400;                          // error code
    return std::string("TXN") + std::to_string(amount);   // transaction id
}

int main() {
    auto r = pay(500);
    if (std::holds_alternative<std::string>(r)) {
        std::cout << "Paid: " << std::get<std::string>(r) << '\n';   // Paid: TXN500
    }
    std::visit([](const auto& v) { std::cout << "Result: " << v << '\n'; }, pay(0));   // Result: 400
}

#include <string_view>

bool isUpiId(std::string_view s) {                 // accepts std::string, literals, substrings
    auto at = s.find('@');
    return at != std::string_view::npos && at > 0 && at + 1 < s.size();
}

std::string id = "asha@okbank";
isUpiId(id);              // no copy
isUpiId("ravi@upi");      // no std::string created
std::string_view domain = std::string_view(id).substr(5);   // "okbank", no allocation