Qt中三种解析xml的方式

落日映苍穹つ 2022-10-02 11:58 256阅读 0赞

在下面的随笔中，我会根据xml的结构，给出Qt中解析这个xml的三种方式的代码。虽然，这个代码时通过调用Qt的函数实现的，但是，很多开源的C++解析xml的库，甚至很多其他语言解析xml的库，都和下面三种解析xml采用相同的原理，所以就算你不是学习qt，也可以大致参看一下代码，对三种解析方式有一种大致的感觉。

先给出xml如下：

![复制代码][Image 1]

<?xml version="1.0" encoding="utf-8"?>
    <school>
        <teacher>
            <entry name="Job">
                <age>30</age>
                <sport>soccer</sport>
            </entry>
            <entry name="Tom">
                <age>32</age>
                <sport>swimming</sport>
            </entry>
        </teacher>
        <student>
            <entry name="Lily">
                <age>20</age>
                <sport>dancing</sport>
            </entry>
            <entry name="Keith">
                <age>21</age>
                <sport>running</sport>
            </entry>
        </student>
    </school>

![复制代码][Image 1]

下面给出qt中解析xml的三种方式，通过解析xml，创建student列表和teacher列表。先给出存储的结构体和辅助函数：

![复制代码][Image 1]

#include <string>
    #include <ostream>
    
    namespace School
    {
    
    struct Teacher
    {
        std::string name;
        int age;
        std::string loveSport;
    
        Teacher(std::string name_, int age_, std::string loveSport_)
            : name(std::move(name_)), age(age_), loveSport(std::move(loveSport_))
        {
    
        }
    };
    
    struct Student
    {
        std::string name;
        int age;
        std::string loveSport;
    
        Student(std::string name_, int age_, std::string loveSport_)
            : name(std::move(name_)), age(age_), loveSport(std::move(loveSport_))
        {
    
        }
    };
    
    inline void print(std::ostream &out, const Teacher& teacher)
    {
        out << "teacher: " << teacher.name << std::endl;
        out << "\tage: " << teacher.age << std::endl;
        out << "\tfavorite sport: " << teacher.loveSport << std::endl;
    }
    
    inline void print(std::ostream& out, const Student& student)
    {
        out << "student: " << student.name << std::endl;
        out << "\tage: " << student.age << std::endl;
        out << "\tfavorite sport: " << student.loveSport << std::endl;
    }
    
    }

![复制代码][Image 1]

另外需要注意在.pro中添加

QT += xml

（1）通过QXmlStreamReader：

![复制代码][Image 1]

#include <QXmlStreamReader>
    #include "schooldefine.h"
    
    class XmlStreamReader
    {
    public:
        XmlStreamReader();
    
        bool readFile(const QString& fileName);
        void printAllMembers();
    
    private:
        void readSchoolMembers();
        void readTeacherMembers();
        void readTeacher(const QStringRef& teacherName);
        void readStudentMembers();
        void readStudent(const QStringRef& studentName);
        void skipUnknownElement();
    
        QXmlStreamReader reader;
    
        std::vector<School::Teacher> m_teachers;
        std::vector<School::Student> m_students;
    };

![复制代码][Image 1]

#include "XmlStreamReader.h"
    #include <QFile>
    #include <iostream>
    #include <QDebug>
    
    XmlStreamReader::XmlStreamReader()
    {
    
    }
    
    bool XmlStreamReader::readFile(const QString &fileName)
    {
        QFile file(fileName);
        if (!file.open(QFile::ReadOnly | QFile::Text))
        {
            std::cerr << "Error: Cannot read file " << qPrintable(fileName)
                      << ": " << qPrintable(file.errorString())
                      << std::endl;
            return false;
        }
        reader.setDevice(&file);
    
        reader.readNext();
        while (!reader.atEnd())
        {
            if (reader.isStartElement())
            {
                if (reader.name() == "school")
                {
                    readSchoolMembers();
                }
                else
                {
                    reader.raiseError(QObject::tr("Not a school file"));
                }
            }
            else
            {
                reader.readNext();
            }
        }
    
        file.close();
        if (reader.hasError())
        {
            std::cerr << "Error: Failed to parse file "
                      << qPrintable(fileName) << ": "
                      << qPrintable(reader.errorString()) << std::endl;
            return false;
        }
        else if (file.error() != QFile::NoError)
        {
            std::cerr << "Error: Cannot read file " << qPrintable(fileName)
                      << ": " << qPrintable(file.errorString())
                      << std::endl;
            return false;
        }
        return true;
    }
    
    void XmlStreamReader::printAllMembers()
    {
        std::cout << "All teachers: " << std::endl;
        for (const auto& teacher : m_teachers)
        {
            School::print(std::cout, teacher);
        }
        std::cout << "All students: " << std::endl;
        for (const auto& student : m_students)
        {
            School::print(std::cout, student);
        }
    }
    
    void XmlStreamReader::readSchoolMembers()
    {
        reader.readNext();
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                if (reader.name() == "teacher")
                {
                    readTeacherMembers();
                }
                else if (reader.name() == "student")
                {
                    readStudentMembers();
                }
                else
                {
                    skipUnknownElement();
                }
            }
            else
            {
                reader.readNext();
            }
        }
    }
    
    void XmlStreamReader::readTeacherMembers()
    {
        reader.readNext();
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                if (reader.name() == "entry")
                {
                    readTeacher(reader.attributes().value("name"));
                }
                else
                {
                    skipUnknownElement();
                }
            }
            else
            {
                reader.readNext();
            }
        }
    }
    
    void XmlStreamReader::readTeacher(const QStringRef& teacherName)
    {
        reader.readNext();
    
        int age = 0;
        std::string favoriteSport;
    
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                if (reader.name() == "age")
                {
                    age = reader.readElementText().toInt();
                }
                else if (reader.name() == "sport")
                {
                    favoriteSport = reader.readElementText().toStdString();
                }
                else
                {
                    skipUnknownElement();
                }
            }
            reader.readNext();
        }
    
        m_teachers.emplace_back(teacherName.toString().toStdString(), age, favoriteSport);
    }
    
    void XmlStreamReader::readStudentMembers()
    {
        reader.readNext();
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                if (reader.name() == "entry")
                {
                    readStudent(reader.attributes().value("name"));
                }
                else
                {
                    skipUnknownElement();
                }
            }
            else
            {
                reader.readNext();
            }
        }
    }
    
    void XmlStreamReader::readStudent(const QStringRef &studentName)
    {
        reader.readNext();
    
        int age = 0;
        std::string favoriteSport;
    
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                if (reader.name() == "age")
                {
                    age = reader.readElementText().toInt();
                }
                else if (reader.name() == "sport")
                {
                    favoriteSport = reader.readElementText().toStdString();
                }
                else
                {
                    skipUnknownElement();
                }
            }
            reader.readNext();
        }
    
        m_students.emplace_back(studentName.toString().toStdString(), age, favoriteSport);
    }
    
    void XmlStreamReader::skipUnknownElement()
    {
        reader.readNext();
        while (!reader.atEnd())
        {
            if (reader.isEndElement())
            {
                reader.readNext();
                break;
            }
    
            if (reader.isStartElement())
            {
                skipUnknownElement();
            }
            else
            {
                reader.readNext();
            }
        }
    }

![复制代码][Image 1]

（2）通过DOM方式：

![复制代码][Image 1]

#include <QString>
    #include <QDomElement>
    #include "schooldefine.h"
    
    class DomParser
    {
    public:
        DomParser();
    
        bool readFile(const QString &fileName);
        void printAllMembers();
    
    private:
        void parseSchoolMembers(const QDomElement &element);
        void parseTeacherMembers(const QDomElement &element);
        void parseStudentMembers(const QDomElement &element);
        void parseTeacher(const QDomElement &element);
        void parseStudent(const QDomElement &element);
    
        std::vector<School::Teacher> m_teachers;
        std::vector<School::Student> m_students;
    };

![复制代码][Image 1]

#include "domparser.h"
    #include <QDomDocument>
    #include <QFile>
    #include <iostream>
    
    DomParser::DomParser()
    {
    
    }
    
    bool DomParser::readFile(const QString &fileName)
    {
        QFile file(fileName);
        if (!file.open(QFile::ReadOnly | QFile::Text)) {
            std::cerr << "Error: Cannot read file " << qPrintable(fileName)
                      << ": " << qPrintable(file.errorString())
                      << std::endl;
            return false;
        }
    
        QString errorStr;
        int errorLine;
        int errorColumn;
    
        QDomDocument doc;
        if (!doc.setContent(&file, false, &errorStr, &errorLine, &errorColumn))
        {
            std::cerr << "Error: Parse error at line " << errorLine << ", "
                      << "column " << errorColumn << ": "
                      << qPrintable(errorStr) << std::endl;
            return false;
        }
    
        QDomElement root = doc.documentElement();
        if (root.tagName() != "school")
        {
            std::cerr << "Error: Not a school file" << std::endl;
            return false;
        }
    
        parseSchoolMembers(root);
        return true;
    }
    
    void DomParser::printAllMembers()
    {
        std::cout << "All teachers: " << std::endl;
        for (const auto& teacher : m_teachers)
        {
            School::print(std::cout, teacher);
        }
        std::cout << "All students: " << std::endl;
        for (const auto& student : m_students)
        {
            School::print(std::cout, student);
        }
    }
    
    
    void DomParser::parseSchoolMembers(const QDomElement &element)
    {
        QDomNode child = element.firstChild();
        while (!child.isNull())
        {
            if (child.toElement().tagName() == "teacher")
            {
                parseTeacherMembers(child.toElement());
            }
            else if (child.toElement().tagName() == "student")
            {
                parseStudentMembers(child.toElement());
            }
            child = child.nextSibling();
        }
    }
    
    void DomParser::parseTeacherMembers(const QDomElement &element)
    {
        QDomNode child = element.firstChild();
        while (!child.isNull())
        {
            if (child.toElement().tagName() == "entry")
            {
                parseTeacher(child.toElement());
            }
            child = child.nextSibling();
        }
    }
    
    void DomParser::parseStudentMembers(const QDomElement &element)
    {
        QDomNode child = element.firstChild();
        while (!child.isNull())
        {
            if (child.toElement().tagName() == "entry")
            {
                parseStudent(child.toElement());
            }
            child = child.nextSibling();
        }
    }
    
    void DomParser::parseTeacher(const QDomElement &element)
    {
        auto children = element.childNodes();
        auto firstChild = children.at(0).toElement();
        auto secondChild = children.at(1).toElement();
        int age = firstChild.text().toInt();
    
        m_teachers.emplace_back(element.attribute("name").toStdString(),
                    age, secondChild.text().toStdString());
    }
    
    void DomParser::parseStudent(const QDomElement &element)
    {
        auto children = element.childNodes();
        auto firstChild = children.at(0).toElement();
        auto secondChild = children.at(1).toElement();
        int age = firstChild.text().toInt();
    
        m_students.emplace_back(element.attribute("name").toStdString(),
                    age, secondChild.text().toStdString());
    }

![复制代码][Image 1]

3. 采用QXmlSimpleReader方式，也就是回调函数方式：

![复制代码][Image 1]

#include <QXmlDefaultHandler>
    #include "schooldefine.h"
    
    class SaxHandler : public QXmlDefaultHandler
    {
    public:
        SaxHandler();
    
        bool readFile(const QString &fileName);
        void printAllMembers();
    
    protected:
        bool startElement(const QString &namespaceURI,
                          const QString &localName,
                          const QString &qName,
                          const QXmlAttributes &atts) override;
        bool endElement(const QString &namespaceURL,
                        const QString &localName,
                        const QString &qName) override;
        bool characters(const QString &ch) override;
        bool fatalError(const QXmlParseException &exception) override;
    
    private:
        bool m_isStudent = false;
        QString  m_currentContext;
        std::vector<School::Teacher> m_teachers;
        std::vector<School::Student> m_students;
    };

![复制代码][Image 1]

#include "saxhandler.h"
    #include <iostream>
    
    SaxHandler::SaxHandler()
    {
    
    }
    
    bool SaxHandler::readFile(const QString &fileName)
    {
        QFile file(fileName);
        QXmlInputSource inputSource(&file);
        QXmlSimpleReader reader;
        reader.setContentHandler(this);
        reader.setErrorHandler(this);;
        return reader.parse(inputSource);
    }
    
    void SaxHandler::printAllMembers()
    {
        std::cout << "All teachers: " << std::endl;
        for (const auto& teacher : m_teachers)
        {
            School::print(std::cout, teacher);
        }
        std::cout << "All students: " << std::endl;
        for (const auto& student : m_students)
        {
            School::print(std::cout, student);
        }
    }
    
    bool SaxHandler::startElement(const QString &namespaceURI,
                                  const QString &localName,
                                  const QString &qName,
                                  const QXmlAttributes &atts)
    {
        if (qName == "teacher")
        {
            m_isStudent = false;
        }
        else if (qName == "student")
        {
            m_isStudent = true;
        }
        else if (qName == "entry")
        {
            if (m_isStudent)
            {
                m_students.push_back(School::Student("", 0, ""));
                m_students.back().name = atts.value("name").toStdString();
            }
            else
            {
                m_teachers.push_back(School::Teacher("", 0, ""));
                m_teachers.back().name = atts.value("name").toStdString();
            }
        }
        else if (qName == "age")
        {
            m_currentContext.clear();
        }
        else if (qName == "sport")
        {
            m_currentContext.clear();
        }
        return true;
    }
    
    bool SaxHandler::characters(const QString &ch)
    {
        m_currentContext += ch;
        return true;
    }
    
    bool SaxHandler::endElement(const QString &namespaceURL,
                                const QString &localName,
                                const QString &qName)
    {
        if (qName == "age")
        {
            if (m_isStudent)
            {
                m_students.back().age = m_currentContext.toInt();
            }
            else
            {
                m_teachers.back().age = m_currentContext.toInt();
            }
        }
        else if (qName == "sport")
        {
            if (m_isStudent)
            {
                m_students.back().loveSport = m_currentContext.toStdString();
            }
            else
            {
                m_teachers.back().loveSport = m_currentContext.toStdString();
            }
        }
        m_currentContext.clear();
        return true;
    }
    
    bool SaxHandler::fatalError(const QXmlParseException &exception)
    {
        std::cerr << "Parse error at line" << exception.lineNumber()
                  << ", " << "column " << exception.columnNumber() << ": "
                  << qPrintable(exception.message()) << std::endl;
        return false;
    }

![复制代码][Image 1]

下面简单对上述三种方式予以说明：

(1) 从代码行数来看，采用DOM和QXmlSimpleReader的方式，代码行数比较少，而QXmlStreamReader代码行数较多。

(2) 从代码逻辑分析来看，采用DOM方式最容易理解，采用QXmlStreamReader的方式稍微难理解一些，而采用QXmlSimpleReader由于使用了较多的回调，引入了大量的类数据成员，使得代码会很难理解。

(3) 从内存占用来看，DOM的方式会耗费最多的内存，因为需要一次性将所有的内容构建成树，DOM和QXmlSimpleReader对内存要求都较低。

(4) 从运行时间消耗来看，DOM的消耗，可能会稍微大一些，因为DOM正常要经历2次的遍历，一次遍历构建树，一次遍历，构建自己需要的数据。而QXmlSimpleReader和QXmlStreamReader正常只需要遍历一次。

(5) 从处理异常来看，DOM和QXmlStreamReader应该会更容易一些，因为不涉及回调函数，但是对于xml来说，很多时候主要确认内容正确与否，如果错误就退出，查看xml中的错误。当然，这个也是比较重要的项。

对于我来说，因为大多数情况下，解析的xml不是很大，而且基本只涉及加载过程中，所以使用DOM的情况比较多。如果xml比较大，或者调用比较频繁，可以考虑使用QXmlStreamReader的方式。

[Image 1]: