C++17 关键新特性

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

struct User {
    int id;
    std::string name;
    std::string email;
    std::string role;
};

class UserService {
public:
    void addUser(User u) {
        users_[u.id] = std::move(u);
    }

    // C++17: optional 表示可能无值
    std::optional<User> findById(int id) const {
        auto it = users_.find(id);
        if (it != users_.end())
            return it->second;
        return std::nullopt;
    }

    // 返回结构化绑定所需数据
    std::vector<User> allUsers() const {
        std::vector<User> result;
        for (const auto& [id, user] : users_)  // 结构化绑定
            result.push_back(user);
        return result;
    }

private:
    std::map<int, User> users_;
};

int main() {
    std::cout << "=== 飞翔科技用户查询系统 (C++17) ===\n\n";

    UserService svc;
    svc.addUser({1, "大翔",  "daxiang@feixiang.net",   "CTO"});
    svc.addUser({2, "白歌",  "baige@feixiang.net",     "架构师"});
    svc.addUser({3, "孔蓝",  "konglan@feixiang.net",   "产品经理"});
    svc.addUser({4, "黄俪",  "huangli@feixiang.net",   "前端开发"});

    // 1. optional 处理查询
    std::cout << "--- 可选值查询 ---\n";
    auto user = svc.findById(2);
    if (user) {
        // 结构化绑定
        auto [id, name, email, role] = *user;
        std::cout << "ID:" << id << " 姓名:" << name
                  << " 邮箱:" << email << " 角色:" << role << "\n";
    }

    auto missing = svc.findById(99);
    std::cout << "用户 99 存在: " << (missing.has_value() ? "是" : "否") << "\n\n";

    // 2. 结构化绑定遍历
    std::cout << "--- 全量用户遍历 ---\n";
    for (const auto& user : svc.allUsers()) {
        auto [id, name, email, role] = user;
        std::cout << "  [" << id << "] " << name << " <" << email << ">\n";
    }

    // 3. if constexpr 编译期分支
    std::cout << "\n--- 编译期分支 ---\n";
    auto printTypeInfo = [](const auto& val) {
        if constexpr (std::is_integral_v<decltype(val)>) {
            std::cout << "整型值: " << val << " (sizeof="
                      << sizeof(val) << ")\n";
        } else if constexpr (std::is_floating_point_v<decltype(val)>) {
            std::cout << "浮点值: " << val << " (sizeof="
                      << sizeof(val) << ")\n";
        } else {
            std::cout << "其他类型: sizeof=" << sizeof(val) << "\n";
        }
    };

    printTypeInfo(42);
    printTypeInfo(3.14);
    printTypeInfo(std::string("hello"));
}

#include <iostream>
#include <variant>
#include <any>
#include <string_view>
#include <string>
#include <vector>
#include <unordered_map>

// C++17 variant：类型安全的 union
using ConfigValue = std::variant<int, double, std::string, bool>;

class ConfigCenter {
public:
    void set(std::string_view key, ConfigValue val) {
        configs_[std::string(key)] = std::move(val);
    }

    // 使用 std::visit 多态访问 variant
    void printAll() const {
        for (const auto& [key, val] : configs_) {
            std::cout << "  " << key << " = ";
            std::visit([](const auto& v) {
                using T = std::decay_t<decltype(v)>;
                if constexpr (std::is_same_v<T, std::string>)
                    std::cout << "\"" << v << "\"";
                else if constexpr (std::is_same_v<T, bool>)
                    std::cout << (v ? "true" : "false");
                else
                    std::cout << v;
            }, val);
            std::cout << "\n";
        }
    }

private:
    std::unordered_map<std::string, ConfigValue> configs_;
};

// C++17 string_view：零拷贝字符串引用
void analyzeLog(std::string_view log) {
    // 不需要分配内存，直接引用原字符串
    if (log.find("ERROR") != std::string_view::npos) {
        std::cout << "  [告警] 错误日志: " << log.substr(0, 40) << "...\n";
    }
}

int main() {
    std::cout << "=== 飞翔科技配置中心 (C++17) ===\n\n";

    // 1. variant 配置
    ConfigCenter cc;
    cc.set("server.port",      443);
    cc.set("server.host",      std::string("0.0.0.0"));
    cc.set("cache.ttl",        3600.0);
    cc.set("debug.enabled",    false);
    cc.set("max.connections",  10000);

    std::cout << "--- 配置项 ---\n";
    cc.printAll();

    // 2. any：类型擦除
    std::cout << "\n--- std::any 运行时多态 ---\n";
    std::vector<std::any> genericList;
    genericList.push_back(42);
    genericList.push_back(std::string("飞翔科技"));
    genericList.push_back(3.14159);
    genericList.push_back(true);

    for (const auto& item : genericList) {
        if (item.type() == typeid(int))
            std::cout << "  int: " << std::any_cast<int>(item) << "\n";
        else if (item.type() == typeid(std::string))
            std::cout << "  string: " << std::any_cast<std::string>(item) << "\n";
        else if (item.type() == typeid(double))
            std::cout << "  double: " << std::any_cast<double>(item) << "\n";
        else if (item.type() == typeid(bool))
            std::cout << "  bool: " << std::any_cast<bool>(item) << "\n";
    }

    // 3. string_view 零拷贝
    std::cout << "\n--- string_view 日志分析 ---\n";
    std::string rawLog =
        "INFO: 服务启动\n"
        "ERROR: 数据库连接超时，请检查网络配置\n"
        "INFO: 健康检查通过\n"
        "WARN: 内存使用率达到 85%\n";

    // 不复制，直接分析
    analyzeLog(rawLog);

    // 对子串创建 string_view 也不分配内存
    std::string_view sv(rawLog);
    auto pos = sv.find("健康检查");
    if (pos != std::string_view::npos) {
        std::string_view sub = sv.substr(pos, 12);
        std::cout << "  子串(零拷贝): " << sub << "\n";
    }

    // 4. 折叠表达式
    std::cout << "\n--- 折叠表达式 ---\n";
    auto sumAll = [](auto... args) {
        return (... + args);  // 一元右折叠
    };
    std::cout << "sumAll(1,2,3,4,5) = " << sumAll(1, 2, 3, 4, 5) << "\n";

    auto printAll = [](auto... args) {
        ((std::cout << args << " "), ...);  // 逗号折叠
    };
    std::cout << "printAll: ";
    printAll("飞翔", "科技", 2025);
    std::cout << "\n";
}

std::string_view getView() {
    std::string s = "临时";
    return std::string_view(s);  // ❌ s 被销毁，view 悬挂
}

std::any a = 42;
auto s = std::any_cast<std::string>(a);  // ❌ 抛 std::bad_any_cast

std::variant<int, std::string> v = 42;
// 用 std::get 需知道准确类型，否则抛异常
// 用 std::visit 或 std::holds_alternative 安全