boost::serialization 用基类指针转存派生类（错误多多，一波三折）

boost::serialization 也支持c++的多态，这样我们就可以通过使用基类的指针来转存派生类，
我们接着上一篇（ 

boost::serialization(2)序列化基类

）的例子来看：

基类和派生类的代码如下：

class student_info{public:student_info() {}virtual ~student_info() {}student_info(const std::string& sn, const std::string& snm, const std::string& sg): name_(sn), number_(snm), grade_(sg){}virtual void print_info() const{std::cout << name_ << " " << number_ << " " << grade_ << std::endl;}private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){ar & BOOST_SERIALIZATION_NVP(name_);ar & BOOST_SERIALIZATION_NVP(number_);ar & BOOST_SERIALIZATION_NVP(grade_);}private:std::string name_;std::string number_;std::string grade_;};class middle_student : public student_info{public:middle_student() {}virtual ~middle_student() {}middle_student(const std::string& sn, const std::string& snm, const std::string& sg, int age): student_info(sn, snm, sg), age_(age){}virtual void print_info(){student_info::print_info();std::cout << age_ << std::endl;}private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){ar & boost::serialization::base_object<student_info>(*this);ar & BOOST_SERIALIZATION_NVP(age_);}private:int age_;};

在派生类中使用了基类的基类的序列化: ar & boost::serialization::base_object<student_info>(*this);
下面我们来看怎么使用基类的指针转存派生类：
save的代码：

void save(){std::ofstream ofs("t7.xml");boost::archive::xml_oarchive oa(ofs);student_info* sdinfo = new middle_student("wyp", "0099", "1", 15);//#1oa << BOOST_SERIALIZATION_NVP(sdinfo);//#2std::cout << "xxxx" << std::endl;delete sdinfo;}

#1:用一个基类的指针指向了一个用new申请的派生类的指针，很简单，都知道这就是c++的多态。
#2：这个代码和以前的一样，还是用一个宏来包装指针。

load的代码：

void load(){std::ifstream ifs("t7.xml");boost::archive::xml_iarchive ia(ifs);student_info* sdinfo = NULL;//#1ia >> BOOST_SERIALIZATION_NVP(sdinfo);//#2middle_student* mds = dynamic_cast<middle_student*>(sdinfo);//#3mds->print_info();}

#1:基类的指针
#2：load的时候也需要宏来包装
#3：这个大家都熟悉

测试代码：

void fun(){save();load();}

编译运行！。。。。。。。
结果抛出异常：boost::archive::archive_exception at memory location 0x0017eb30...
google了一下，下面链接给出了一个解决方法
http://stackoverflow.com/questions/1332602/how-to-serialize-derived-template-classes-with-boost-serialize
大概分为3个步骤：
步骤1:BOOST_SERIALIZATION_ASSUME_ABSTRACT(className)，用这个宏来告诉boost className是一个抽象类
步骤2：在save操作中注册派生类：oa.template register_type<middle_student>(NULL)，
       一定要在oa << BOOST_SERIALIZATION_NVP(sdinfo)之前注册。
步骤3：在load操作中注册派生了：ia.template register_type<middle_student>(NULL)
       一定要在ia >> BOOST_SERIALIZATION_NVP(sdinfo)之前注册。
修改后的代码如下：

void save(){std::ofstream ofs("t7.xml");boost::archive::xml_oarchive oa(ofs);oa.template register_type<middle_student>(NULL);student_info* sdinfo = new middle_student("wyp", "0099", "1", 15);oa << BOOST_SERIALIZATION_NVP(sdinfo);delete sdinfo;}void load(){std::ifstream ifs("t7.xml");boost::archive::xml_iarchive ia(ifs);ia.template register_type<middle_student>(NULL);student_info* sdinfo = NULL;ia >> BOOST_SERIALIZATION_NVP(sdinfo);middle_student* mds = dynamic_cast<middle_student*>(sdinfo);mds->print_info();}

好这下应改没有异常了吧。

结果编译就出错了！！！

错误在这个函数里面

 // Anything not an attribute and not a name-value pair is an    // error and should be trapped here.    template<class T>    void save_override(T & t, BOOST_PFTO int)    {        // If your program fails to compile here, its most likely due to        // not specifying an nvp wrapper around the variable to        // be serialized.        BOOST_MPL_ASSERT((serialization::is_wrapper< T >));        this->detail_common_oarchive::save_override(t, 0);    }

看注释就知道了，序列化时存在有些数据没有包装，就是没有用那个宏。

就仔细看一下序列化的代码发现这段代码中有一个没用宏来包装：

private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){ar & boost::serialization::base_object<student_info>(*this);//here!!!!!!!!!!!ar & BOOST_SERIALIZATION_NVP(age_);}

在调用基类的序列化时没用宏包装

修改如下：

private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){//ar & boost::serialization::base_object<student_info>(*this);ar & BOOST_SERIALIZATION_BASE_OBJECT_NVP(student_info);ar & BOOST_SERIALIZATION_NVP(age_);}

编译运行ok！

运行结果如下 t7.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!DOCTYPE boost_serialization>
<boost_serialization signature="serialization::archive" version="10">
<sdinfo class_id="0" tracking_level="1" version="0" object_id="_0">
<student_info class_id="1" tracking_level="1" version="0" object_id="_1">
<name_>wyp</name_>
<number_>0099</number_>
<grade_>1</grade_>
</student_info>
<age_>15</age_>
</sdinfo>
</boost_serialization>

完整代码如下

#include <fstream>#include <iostream>#include <algorithm>#include <boost/archive/xml_iarchive.hpp>#include <boost/archive/xml_oarchive.hpp>#include <boost/serialization/base_object.hpp>class student_info{public:student_info() {}virtual ~student_info() {}student_info(const std::string& sn, const std::string& snm, const std::string& sg): name_(sn), number_(snm), grade_(sg){}virtual void print_info() const{std::cout << name_ << " " << number_ << " " << grade_ << std::endl;}private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){ar & BOOST_SERIALIZATION_NVP(name_);ar & BOOST_SERIALIZATION_NVP(number_);ar & BOOST_SERIALIZATION_NVP(grade_);}private:std::string name_;std::string number_;std::string grade_;};BOOST_SERIALIZATION_ASSUME_ABSTRACT(student_info)class middle_student : public student_info{public:middle_student() {}virtual ~middle_student() {}middle_student(const std::string& sn, const std::string& snm, const std::string& sg, int age): student_info(sn, snm, sg), age_(age){}virtual void print_info(){student_info::print_info();std::cout << age_ << std::endl;}private:friend class boost::serialization::access;template<typename Archive>void serialize(Archive& ar, const unsigned int version){ar & BOOST_SERIALIZATION_BASE_OBJECT_NVP(student_info);ar & BOOST_SERIALIZATION_NVP(age_);}private:int age_;};void save(){std::ofstream ofs("t7.xml");boost::archive::xml_oarchive oa(ofs);oa.template register_type<middle_student>(NULL);student_info* sdinfo = new middle_student("wyp", "0099", "1", 15);oa << BOOST_SERIALIZATION_NVP(sdinfo);delete sdinfo;}void load(){std::ifstream ifs("t7.xml");boost::archive::xml_iarchive ia(ifs);ia.template register_type<middle_student>(NULL);student_info* sdinfo = NULL;ia >> BOOST_SERIALIZATION_NVP(sdinfo);middle_student* mds = dynamic_cast<middle_student*>(sdinfo);mds->print_info();}void fun(){save();load();}