Implementing IXmlWriter Part 8: Supporting WriteStartAttribute() and WriteEndAttribute()

This is part 8 of my Implementing IXmlWriter post series.

Today I will add support for the functions WriteStartAttribute() and WriteEndAttribute() to last time’s IXmlWriter.

These functions are (obviously) used to denote the start and end of an attribute; the attribute value is written using WriteString() (this usage is analogous to WriteStartElement() and WriteEndElement()). Because WriteString() must now be aware of whether it is writing an attribute value or element content, I must keep track of the state the IXmlWriter is in — a change that affects nearly every function.

The current state of the IXmlWriter is very important. Not only does it determine what type of encoding WriteString() must use, it also determines what operations are valid at what time. For example, it doesn’t make sense for a user to call WriteAttributeString() unless an element is still opened. In other words, the function call sequence WriteStartElement(), WriteString(), WriteAttributeString() is clearly illegal. To this end, I define a number of different states (see enum WriteState below), and most functions check this state to determine whether or not they should allow the action. Generating errors for illegal operations given the current IXmlWriter state is unimplemented but left as a TODO comment. Note my extensive use of the switch statement; I even use it for consistency’s sake when it may be a tiny bit less code to use if/else (e.g. WriteEndAttribute()). Also note that I use the switch statement’s fall-through feature to allow processing to continue as if the IXmlWriter is in a different state; these occurrences are clearly indicated with a // FALL THROUGH comment.

Here’s the test case:

StringXmlWriter xmlWriter;

xmlWriter.WriteStartElement("root");
  xmlWriter.WriteStartElement("element");
    xmlWriter.WriteStartAttribute("att");
      xmlWriter.WriteString("\"&");
    xmlWriter.WriteEndAttribute();
  xmlWriter.WriteEndElement();
xmlWriter.WriteEndElement();

std::string strXML = xmlWriter.GetXmlString();
// strXML should be <root><element att="&quot;&amp;"/></root>

Here’s the new header file:

// StringXmlWriter.h

class StringXmlWriter
{
private:
    enum WriteState
    {
        WriteState_Attribute, // An attribute value is being written
        WriteState_Content, // Element content is being written
        WriteState_Element, // An element start tag has been written (and is unclosed)
        WriteState_Start, // No Write() methods have been called
    };

    WriteState m_writeState;
    std::stack<std::string> m_openedElements;
    std::string m_xmlStr;

public:
    StringXmlWriter();

    std::string GetXmlString() const;
    void WriteAttributeString(const std::string& localName,
                              const std::string& text);
    void WriteElementString(const std::string& localName,
                            const std::string& text);
    void WriteEndAttribute();
    void WriteEndElement();
    void WriteStartAttribute(const std::string& localName);
    void WriteStartElement(const std::string& localName);
    void WriteString(const std::string& text);

private:
    // Disable copy construction and assignment
    StringXmlWriter(const StringXmlWriter&);
    StringXmlWriter& operator=(const StringXmlWriter&);
};

Here’s the new implementation file:

// StringXmlWriter.cpp

#include "StringXmlWriter.h"

#define ARRAYSIZE(x) ( sizeof(x) / sizeof(x[0]) )

struct CharTranslation
{
    char OriginalChar;
    const char* ReplacementString;
};

static const CharTranslation AttributeValueTranslations[] =
{
    { '"', "&quot;" },
    { '&', "&amp;" },
};

static const CharTranslation CharDataTranslations[] =
{
    { '&', "&amp;" },
    { '<', "&lt;" },
    { '>', "&gt;" },
};

struct OriginalCharEquals :
    public std::binary_function<CharTranslation, char, bool>
{
    bool operator() (const CharTranslation& translation, char ch) const
    {
        return (translation.OriginalChar == ch);
    }
};

static std::string TranslateString(const std::string& originalStr,
                                   const CharTranslation* translations,
                                   int numTranslations)
{
    // Actually one past end, needed for proper std::find_if semantics
    const CharTranslation* endTranslations = translations + numTranslations;

    std::string translatedStr;

    for (std::string::const_iterator stringIter = originalStr.begin();
         stringIter != originalStr.end();
         ++stringIter)
    {
        char ch = *stringIter;

        const CharTranslation* translation = std::find_if
            (
            translations,
            endTranslations,
            std::bind2nd(OriginalCharEquals(), ch)
            );
        if (translation != endTranslations)
        {
            translatedStr += translation->ReplacementString;
        }
        else
        {
            translatedStr += ch;
        }
    }

    return translatedStr;
}

StringXmlWriter::StringXmlWriter() :
    m_writeState(WriteState_Start)
{
}

std::string StringXmlWriter::GetXmlString() const
{
    return m_xmlStr;
}

void StringXmlWriter::WriteAttributeString(const std::string& localName,
                                           const std::string& text)
{
    WriteStartAttribute(localName);
    WriteString(text);
    WriteEndAttribute();
}

void StringXmlWriter::WriteElementString(const std::string& localName,
                                         const std::string& text)
{
    WriteStartElement(localName);
    WriteString(text);
    WriteEndElement();
}

void StringXmlWriter::WriteEndAttribute()
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        m_xmlStr += '"';
        m_writeState = WriteState_Element;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteEndElement()
{
    switch (m_writeState)
    {
    case WriteState_Content:
        {
            m_xmlStr += "</";
            m_xmlStr += m_openedElements.top();
            m_xmlStr += '>';
            m_openedElements.pop();
            m_writeState = WriteState_Content;
            break;
        }
    case WriteState_Element:
        {
            m_xmlStr += "/>";
            m_openedElements.pop();
            m_writeState = WriteState_Content;
            break;
        }
    default:

        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteStartAttribute(const std::string& localName)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        m_xmlStr += ' ';
        m_xmlStr += localName;
        m_xmlStr += "=\"";
        m_writeState = WriteState_Attribute;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteStartElement(const std::string& localName)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        // An element is currently open.  Close the element so we can open
        // a new one.
        m_xmlStr += '>';
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Start:
        m_openedElements.push(localName);
        m_xmlStr += '<';
        m_xmlStr += localName;
        m_writeState = WriteState_Element;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteString(const std::string& text)
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        m_xmlStr += TranslateString
            (
            text,
            AttributeValueTranslations,
            ARRAYSIZE(AttributeValueTranslations)
            );
        break;
    case WriteState_Element:
        // An element is currently open.  Close the element so we can start
        // writing the element content.
        m_xmlStr += '>';
        m_writeState = WriteState_Content;
        // FALL THROUGH
    case WriteState_Content:
        m_xmlStr += TranslateString
            (
            text,
            CharDataTranslations,
            ARRAYSIZE(CharDataTranslations)
            );
        break;
    default:
        // TODO: Generate error
        break;
    }
}

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