Optional Values

std::optional represents a value that may or may not be present.

optional_values.cpp

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

std::optional<int> readCount(const std::string& text) {
    if (text == "none") {
        return std::nullopt;
    }
    return std::stoi(text);
}

int main() {
    std::string raw = ;

    std::optional<int> count = readCount(raw);

    if (count.has_value()) {
        std::cout << "count=" << count.value() << std::endl;
    } else {
        std::cout << "count=missing" << std::endl;
    }

    std::cout << "fallback=" << count.value_or(0) << std::endl;
    return 0;
}

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

std::optional<int> readCount(const std::string& text) {
    if (text == "none") {
        return std::nullopt;
    }
    return std::stoi(text);
}

int main() {
    std::string raw = ;

    std::optional<int> count = readCount(raw);

    if (count.has_value()) {
        std::cout << "count=" << count.value() << std::endl;
    } else {
        std::cout << "count=missing" << std::endl;
    }

    std::cout << "fallback=" << count.value_or(0) << std::endl;
    return 0;
}

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

std::optional<int> readCount(const std::string& text) {
    if (text == "none") {
        return std::nullopt;
    }
    return std::stoi(text);
}

int main() {
    std::string raw = ;

    std::optional<int> count = readCount(raw);

    if (count.has_value()) {
        std::cout << "count=" << count.value() << std::endl;
    } else {
        std::cout << "count=missing" << std::endl;
    }

    std::cout << "fallback=" << count.value_or(0) << std::endl;
    return 0;
}

optional An optional value makes the missing-value case visible instead of using a special number.