如何使用 boost 库反序列化和获取成员值

Question

首先，我想告诉你

1. 我是序列化技术的新手
2. 我对使用 boost 序列化并不是完全空白，但我所经历的大多数示例都非常直接（例如，只有一个类，最多一个基类和一个派生类）。

我以here 为例。在这里，代码作者通过使用类bus_schedule 使用了save 和load 函数。在此处调用的对象new_schedule 的帮助下，我遇到了获取所有成员变量的问题。我假设，这个对象将帮助我获取其他类的所有成员（也许我在这一点上完全错了）。

除此之外，我已经看到，大部分成员变量都在这里private，这意味着我无法从main 函数访问它们。这就是为什么，我设置了一些 getter 函数来返回一些成员值。例如，从bus_schedule 类中，如果我可以返回schedule 列表，那么我可以访问trip_info 结构值以及类型为bus_route 的指针。

我用过的返回schedule列表的函数是：

std::list<std::pair<trip_info, bus_route *> >  return_schedule()
     {
         return schedule;
     }

在我使用过的main函数中：

bus_schedule new_schedule;
auto returning_schedule = new_schedule.return_schedule();
for(auto i:returning_schedule)
    {
        cout<<(i.first)<<" ~~~~~~~ "<<(i.second)<<endl;
       /*with i.first I can access the struct info, i.second pointer gives me the info of
       bus_route pointer value ater dereferencing*/
    }

但在这里，我再次坚持从 i.second 指针获取单独的值。我知道，bus_route 类有一个名为stops 的列表。 Here 也返回了这个。但在我看来，每次进行这种反算都很麻烦。

我的问题是，boost 是否有任何直接解决方案可以在反序列化后访问所有成员变量？我将为所有班级设置 getter 函数，但害怕找到使用这些函数的途径。

Answer 1

这个问题实际上与序列化是分开的。这是没有序列化内容的数据结构。我对代码进行了现代化改造以使用 C++17 功能

• auto 和移动语义
• defaulted 特别会员
• [[nodiscard]], override 关键字
• 显式构造函数和emplace
• 带解构的ranged-for循环
• make_unique<> 而不是 new/delete
• 扩展聚合初始化器

总而言之，它使代码更短、更简单、更高效¹。此外，它突出显示了您想知道的一些事情，我将在下面展示。

Live On Coliru

#include <iostream>
#include <list>
#include <memory>
#include <string>

struct gps_position {
    int degrees;
    int minutes;
    float seconds;

    friend std::ostream& operator<<(std::ostream& os, const gps_position& gp) {
        return os << ' ' << gp.degrees << "º" << gp.minutes << "'" << gp.seconds << '"';
    }
};

/////////////////////////////////////////////////////////////
// One bus stop
struct bus_stop {
    gps_position latitude, longitude;

    [[nodiscard]] virtual std::string description() const = 0;
    virtual ~bus_stop() = default;
    friend std::ostream& operator<<(std::ostream& os, const bus_stop& bs) {
        return os << bs.latitude << bs.longitude << ' ' << bs.description();
    }

  protected:
    explicit bus_stop(gps_position _lat, gps_position _long)
            : latitude(_lat), longitude(_long) {}
};

/////////////////////////////////////////////////////////////
// Several kinds of bus stops
struct bus_stop_corner : bus_stop {
    std::string street1, street2;
    [[nodiscard]] std::string description() const override {
        return street1 + " and " + street2;
    }

    explicit bus_stop_corner(gps_position _lat, gps_position _long, std::string _s1, std::string _s2)
        : bus_stop(_lat, _long), street1(std::move(_s1)), street2(std::move(_s2)) {}
};

struct bus_stop_destination : bus_stop {
    std::string name;
    [[nodiscard]] std::string description() const override { return name; }

    bus_stop_destination(gps_position _lat, gps_position _long, std::string _name)
        : bus_stop(_lat, _long), name(std::move(_name)) {}
};

struct bus_route {
    using bus_stop_pointer = bus_stop*;
    std::list<bus_stop_pointer> stops;

    void append(bus_stop* _bs) { stops.insert(stops.end(), _bs); }
    friend std::ostream& operator<<(std::ostream& os, const bus_route& br) {
        for (auto& stop : br.stops) {
            os << '\n' << std::hex << "0x" << stop << std::dec << ' ' << *stop;
        }
        return os;
    }
};

/////////////////////////////////////////////////////////////
// a bus schedule is a collection of routes each with a starting time
struct bus_schedule {
    struct trip_info { int hour, minute; std::string driver; };

    void append(const std::string& _d, int _h, int _m, bus_route* _br) {
        schedule.emplace(schedule.end(), trip_info{_h, _m, _d}, _br);
    }

  private:
    friend std::ostream& operator<<(std::ostream& os, const bus_schedule& bs) {
        for (auto const& [k,v] : bs.schedule) { os << k << *v; }
        return os;
    }
    friend std::ostream& operator<<(std::ostream& os, const bus_schedule::trip_info& ti) {
        return os << '\n' << ti.hour << ':' << ti.minute << ' ' << ti.driver << ' ';
    }
    std::list<std::pair<trip_info, bus_route*>> schedule;
};

int main() {
    // fill in the data
    // make a few stops
    auto bs0 = std::make_unique<bus_stop_corner>(
        gps_position{ 34, 135, 52.560F }, gps_position{ 134, 22, 78.30F },
        "24th Street", "10th Avenue");
    auto bs1 = std::make_unique<bus_stop_corner>(
        gps_position{ 35, 137, 23.456F }, gps_position{ 133, 35, 54.12F },
        "State street", "Cathedral Vista Lane");
    auto bs2 = std::make_unique<bus_stop_destination>(
        gps_position{ 35, 136, 15.456F }, gps_position{ 133, 32, 15.300F },
        "White House");
    auto bs3 = std::make_unique<bus_stop_destination>(
        gps_position{ 35, 134, 48.789F }, gps_position{ 133, 32, 16.230F },
        "Lincoln Memorial");

    // make the schedule
    bus_schedule original_schedule;
    bus_route route0;

    bus_route route1;

    {
        // make a route
        route0.append(bs0.get());
        route0.append(bs1.get());
        route0.append(bs2.get());

        // add trips to schedule
        original_schedule.append("bob",   6,  24, &route0);
        original_schedule.append("bob",   9,  57, &route0);
        original_schedule.append("alice", 11, 2,  &route0);
    }

    {
        // make aother routes
        route1.append(bs3.get());
        route1.append(bs2.get());
        route1.append(bs1.get());

        // add trips to schedule
        original_schedule.append("ted",   7,  17, &route1);
        original_schedule.append("ted",   9,  38, &route1);
        original_schedule.append("alice", 11, 47, &route1);
    }

    // display the complete schedule
    std::cout << "schedule" << original_schedule;
}

打印：

schedule
6:24 bob 
 34º135'52.56" 134º22'78.3" 24th Street and 10th Avenue
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane
 35º136'15.456" 133º32'15.3" White House
9:57 bob 
 34º135'52.56" 134º22'78.3" 24th Street and 10th Avenue
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane
 35º136'15.456" 133º32'15.3" White House
11:2 alice 
 34º135'52.56" 134º22'78.3" 24th Street and 10th Avenue
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane
 35º136'15.456" 133º32'15.3" White House
7:17 ted 
 35º134'48.789" 133º32'16.23" Lincoln Memorial
 35º136'15.456" 133º32'15.3" White House
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane
9:38 ted 
 35º134'48.789" 133º32'16.23" Lincoln Memorial
 35º136'15.456" 133º32'15.3" White House
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane
11:47 alice 
 35º134'48.789" 133º32'16.23" Lincoln Memorial
 35º136'15.456" 133º32'15.3" White House
 35º137'23.456" 133º35'54.12" State street and Cathedral Vista Lane

结论

为了方便地使用 map 数据结构，您可以使用循环 as the example has it：

std::list<
    std::pair<bus_schedule::trip_info, bus_route*>>::const_iterator it;
for (it = bs.schedule.begin(); it != bs.schedule.end(); it++) {
    os << it->first << *(it->second);
}

但是，在 C++17 中，您的编写方式与以下内容完全相同：

for (auto const& [info,route] : bs.schedule) { os << info << *route; }

您可以看到[k, v] 使用结构化绑定来提取值类型的“第一”和“第二”部分。如果您的编译器中没有 C++17 功能，您始终可以手动执行相同操作：

for (auto const& pair : bs.schedule) {
    trip_info const& info = pair.first;
    bus_route const& route = *pair.second;
    os << info << route;
}

四处逛逛private 成员

我认为这种数据结构不是典型的或“好的”惯用 C++。使用原始指针会导致内存管理错误（例如，反序列化路由的每一站都会泄露）。

添加到这个“装置”getter/setter 风险putting lipstick on a pig²。

我会选择公开您需要公开的内容。这是一个从 main 打印详细信息的示例，而不会重载 operator<< 运算符：

// display the complete schedule
std::cout << "schedule";
for (auto const& [info,route] : original_schedule.schedule) {
    std::cout << '\n' << info.hour << ':' << info.minute << ' ' << info.driver << ' ';
    for (auto& stop : route->stops)
        std::cout << '\n' << stop->latitude << stop->longitude << ' ' << stop->description();
}

打印和以前一样。

完整列表

Live On Coliru

#include <iostream>
#include <list>
#include <memory>
#include <string>

struct gps_position {
    int degrees;
    int minutes;
    float seconds;

    friend std::ostream& operator<<(std::ostream& os, const gps_position& gp) {
        return os << ' ' << gp.degrees << "º" << gp.minutes << "'" << gp.seconds << '"';
    }
};

/////////////////////////////////////////////////////////////
// One bus stop
struct bus_stop {
    gps_position latitude, longitude;

    [[nodiscard]] virtual std::string description() const = 0;
    virtual ~bus_stop() = default;

    explicit bus_stop(gps_position _lat, gps_position _long)
            : latitude(_lat), longitude(_long) {}
};

/////////////////////////////////////////////////////////////
// Several kinds of bus stops
struct bus_stop_corner : bus_stop {
    std::string street1, street2;
    [[nodiscard]] std::string description() const override {
        return street1 + " and " + street2;
    }

    explicit bus_stop_corner(gps_position _lat, gps_position _long, std::string _s1, std::string _s2)
        : bus_stop(_lat, _long), street1(std::move(_s1)), street2(std::move(_s2)) {}
};

struct bus_stop_destination : bus_stop {
    std::string name;
    [[nodiscard]] std::string description() const override { return name; }

    bus_stop_destination(gps_position _lat, gps_position _long, std::string _name)
        : bus_stop(_lat, _long), name(std::move(_name)) {}
};

struct bus_route {
    std::list<bus_stop*> stops;
    void append(bus_stop* _bs) { stops.push_back(_bs); }
};

/////////////////////////////////////////////////////////////
// a bus schedule is a collection of routes each with a starting time
struct bus_schedule {
    struct trip_info { int hour, minute; std::string driver; };

    void append(const std::string& _d, int _h, int _m, bus_route* _br) {
        schedule.emplace(schedule.end(), trip_info{_h, _m, _d}, _br);
    }

    std::list<std::pair<trip_info, bus_route*>> schedule;
};

int main() {
    // fill in the data
    // make a few stops
    auto bs0 = std::make_unique<bus_stop_corner>(
        gps_position{ 34, 135, 52.560F }, gps_position{ 134, 22, 78.30F },
        "24th Street", "10th Avenue");
    auto bs1 = std::make_unique<bus_stop_corner>(
        gps_position{ 35, 137, 23.456F }, gps_position{ 133, 35, 54.12F },
        "State street", "Cathedral Vista Lane");
    auto bs2 = std::make_unique<bus_stop_destination>(
        gps_position{ 35, 136, 15.456F }, gps_position{ 133, 32, 15.300F },
        "White House");
    auto bs3 = std::make_unique<bus_stop_destination>(
        gps_position{ 35, 134, 48.789F }, gps_position{ 133, 32, 16.230F },
        "Lincoln Memorial");

    // make the schedule
    bus_schedule original_schedule;
    bus_route route0;

    bus_route route1;

    {
        // make a route
        route0.append(bs0.get());
        route0.append(bs1.get());
        route0.append(bs2.get());

        // add trips to schedule
        original_schedule.append("bob",   6,  24, &route0);
        original_schedule.append("bob",   9,  57, &route0);
        original_schedule.append("alice", 11, 2,  &route0);
    }

    {
        // make aother routes
        route1.append(bs3.get());
        route1.append(bs2.get());
        route1.append(bs1.get());

        // add trips to schedule
        original_schedule.append("ted",   7,  17, &route1);
        original_schedule.append("ted",   9,  38, &route1);
        original_schedule.append("alice", 11, 47, &route1);
    }

    // display the complete schedule
    std::cout << "schedule";
    for (auto const& [info,route] : original_schedule.schedule) {
        std::cout << '\n' << info.hour << ':' << info.minute << ' ' << info.driver << ' ';
        for (auto& stop : route->stops)
            std::cout << '\n' << stop->latitude << stop->longitude << ' ' << stop->description();
    }
}

请注意，即使removing all blank lines and comments, and using the same formatting，代码也比示例短 100 行。

---

¹ 请注意，代码显然根本不是客观上“高效”的，因为它执行各种动态分配和运行时多态性

² 这种反模式对于具有传统 OOP 背景的人来说很常见，请参阅 pseudo/quasi-classes (PDF)