c++中std::optional的使用场景_c++ optional可选值类型的应用场合

std::optional<int> find_first_even(const std::vector<int>& vec) {
    for (int val : vec) {
        if (val % 2 == 0) {
            return val;  // 找到，返回值
        }
    }
    return std::nullopt;  // 未找到，返回无值
}

if (auto result = find_first_even(nums); result.has_value()) {
    std::cout << "Found: " << *result << std::endl;
} else {
    std::cout << "No even number found." << std::endl;
}

struct ServerConfig {
    std::string host;
    int port;
    std::optional<std::string> ssl_cert_path;
    std::optional<int> timeout_seconds;
};

class UserData {
    std::string name;
    std::optional<std::string> email;
public:
    UserData(std::string n, std::optional<std::string> e)
        : name(std::move(n)), email(std::move(e)) {}
    
    void set_email(std::string em) { email = std::move(em); }
    void clear_email() { email = std::nullopt; }
};

bool try_parse_int(const std::string& s, int& out);

std::optional<int> parse_int(const std::string& s) {
    try {
        size_t pos;
        int value = std::stoi(s, &pos);
        if (pos == s.size()) return value;
        return std::nullopt;
    } catch (...) {
        return std::nullopt;
    }
}

if (auto num = parse_int("123")) {
    use(*num);
}