Implementing IXmlWriter Part 10: Supporting WriteComment()

Implementing IXmlWriter Part 10: Supporting WriteComment()

This is part 10 of my Implementing IXmlWriter post series.

Today I will add support for the function WriteComment() to last time’s IXmlWriter.

Quoting from Section 2.5: Comments of the XML 1.0 spec:

Comments MAY appear anywhere in a document outside other markup; in addition, they MAY appear within the document type declaration at places allowed by the grammar.

Considering this, we should allow writing comments in virtually every WriteState that the IXmlWriter can be in. In fact, some quick thought confirms that we should allow it for every WriteState but WriteState_Attribute, as a comment cannot be legally represented between the quotation marks which delimit an attribute value. With this in mind, here’s the test case I wrote:

StringXmlWriter xmlWriter;
xmlWriter.WriteComment("comment1");
xmlWriter.WriteStartDocument();
  xmlWriter.WriteComment("comment2");
  xmlWriter.WriteStartElement("root");
    xmlWriter.WriteComment("comment3");
    xmlWriter.WriteString("value");
xmlWriter.WriteEndDocument();

std::string strXML = xmlWriter.GetXmlString();
// strXML should be:
// <!--comment1--><?xml version="1.0"?><!--comment2--><root><!--comment3-->value</root>

Before I provide the implementation of writing comments (which is, in fact, trivial), I wanted to mention that in the course of writing and testing this feature I noticed a bug from last time’s IXmlWriter. Consider the above test case with the WriteComment() lines removed:

StringXmlWriter xmlWriter;
xmlWriter.WriteStartDocument();
  xmlWriter.WriteStartElement("root");
    xmlWriter.WriteString("value");
xmlWriter.WriteEndDocument();

std::string strXML = xmlWriter.GetXmlString();
// strXML should be <?xml version="1.0"?><root>value</root>

If you run this test case against the previous version of IXmlWriter, it outputs the clearly invalid string <?xml version="1.0"?><root>value. This is because WriteEndElement() didn’t properly anticipate the WriteState of WriteState_Content. Therefore, in addition to adding WriteComment() support, I also fixed this bug.

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_Prolog, // The prolog is being written
        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 WriteComment(const std::string& text);
    void WriteElementString(const std::string& localName,
                            const std::string& text);
    void WriteEndAttribute();
    void WriteEndDocument();
    void WriteEndElement();
    void WriteStartAttribute(const std::string& localName);
    void WriteStartDocument();
    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::WriteComment(const std::string& text)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        // An element is currently open. Close the element so we can open
        // a new one.
        m_xmlStr += '>';
        m_writeState = WriteState_Content;
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Prolog:
    case WriteState_Start:
        m_xmlStr += "<!--";
        m_xmlStr += text;
        m_xmlStr += "–>";
        break;
    default:
        // It doesn't make sense to allow writing comments when writing an
        // attribute value.
        // TODO: Generate error
        break;
    }
}

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::WriteEndDocument()
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        WriteEndAttribute();
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Element:
        while (!m_openedElements.empty())
        {
            WriteEndElement();
        }
        break;
    case WriteState_Start:
    case WriteState_Prolog:
        // DO NOTHING
        break;
    default:
        // TODO: Generate error
        break;
    }

    m_writeState = WriteState_Start;
}

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::WriteStartDocument()
{
    switch (m_writeState)
    {
    case WriteState_Start:
        m_xmlStr += "<?xml version=\"1.0\"?>";
        m_writeState = WriteState_Prolog;
        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_Prolog:
    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;
    }
}