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，这样我们就能够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

）的样例来看：

基类和派生类的代码例如以下：

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);
以下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);//#1
	oa << BOOST_SERIALIZATION_NVP(sdinfo);//#2
	std::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，非常easy，都知道这就是c++的多态。
#2：这个代码和曾经的一样，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;//#1
	ia >> BOOST_SERIALIZATION_NVP(sdinfo);//#2
	middle_student* mds = dynamic_cast<middle_student*>(sdinfo);//#3
	mds->print_info();
}

#1:基类的指针
#2：load的时候也须要宏来包装
#3：这个大家都熟悉

測试代码：

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

编译执行！。

。

。。。

。。
结果抛出异常：boost::archive::archive_exception at memory location 0x0017eb30...
google了一下，以下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操作中注冊派生类：oa.template register_type<middle_student>(NULL)，
       一定要在oa << BOOST_SERIALIZATION_NVP(sdinfo)之前注冊。
步骤3：在load操作中注冊派生了：ia.template register_type<middle_student>(NULL)
       一定要在ia >> BOOST_SERIALIZATION_NVP(sdinfo)之前注冊。

改动后的代码例如以下：

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);
    }

看凝视就知道了，序列化时存在有些数据没有包装，就是没实用那个宏。

就细致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

改动例如以下：

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_);
	}

编译执行ok！

执行结果例如以下 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>

完整代码例如以下

#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();
}