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QXmlStreamWriter Class

The QXmlStreamWriter class provides an XML writer with a simple streaming API. More...

Header: #include <QXmlStreamWriter>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core

Note: All functions in this class are reentrant.

Properties

Public Functions

QXmlStreamWriter()
QXmlStreamWriter(QIODevice *device)
QXmlStreamWriter(QByteArray *array)
QXmlStreamWriter(QString *string)
~QXmlStreamWriter()
bool autoFormatting() const
int autoFormattingIndent() const
QIODevice *device() const
bool hasError() const
void setAutoFormatting(bool enable)
void setAutoFormattingIndent(int spacesOrTabs)
void setDevice(QIODevice *device)
void writeAttribute(QAnyStringView namespaceUri, QAnyStringView name, QAnyStringView value)
void writeAttribute(QAnyStringView qualifiedName, QAnyStringView value)
void writeAttribute(const QXmlStreamAttribute &attribute)
void writeAttributes(const QXmlStreamAttributes &attributes)
void writeCDATA(QAnyStringView text)
void writeCharacters(QAnyStringView text)
void writeComment(QAnyStringView text)
void writeCurrentToken(const QXmlStreamReader &reader)
void writeDTD(QAnyStringView dtd)
void writeDefaultNamespace(QAnyStringView namespaceUri)
void writeEmptyElement(QAnyStringView namespaceUri, QAnyStringView name)
void writeEmptyElement(QAnyStringView qualifiedName)
void writeEndDocument()
void writeEndElement()
void writeEntityReference(QAnyStringView name)
void writeNamespace(QAnyStringView namespaceUri, QAnyStringView prefix = {})
void writeProcessingInstruction(QAnyStringView target, QAnyStringView data = {})
void writeStartDocument(QAnyStringView version)
void writeStartDocument()
void writeStartDocument(QAnyStringView version, bool standalone)
void writeStartElement(QAnyStringView namespaceUri, QAnyStringView name)
void writeStartElement(QAnyStringView qualifiedName)
void writeTextElement(QAnyStringView namespaceUri, QAnyStringView name, QAnyStringView text)
void writeTextElement(QAnyStringView qualifiedName, QAnyStringView text)

Detailed Description

QXmlStreamWriter is the counterpart to QXmlStreamReader for writing XML. Like its related class, it operates on a QIODevice specified with setDevice(). The API is simple and straightforward: for every XML token or event you want to write, the writer provides a specialized function.

You start a document with writeStartDocument() and end it with writeEndDocument(). This will implicitly close all remaining open tags.

Element tags are opened with writeStartElement() followed by writeAttribute() or writeAttributes(), element content, and then writeEndElement(). A shorter form writeEmptyElement() can be used to write empty elements, followed by writeAttributes().

Element content consists of either characters, entity references or nested elements. It is written with writeCharacters(), which also takes care of escaping all forbidden characters and character sequences, writeEntityReference(), or subsequent calls to writeStartElement(). A convenience method writeTextElement() can be used for writing terminal elements that contain nothing but text.

The following abridged code snippet shows the basic use of the class to write formatted XML with indentation:

    QXmlStreamWriter stream(&output);
    stream.setAutoFormatting(true);
    stream.writeStartDocument();
    ...
    stream.writeStartElement("bookmark");
    stream.writeAttribute("href", "http://qt-project.org/");
    stream.writeTextElement("title", "Qt Project");
    stream.writeEndElement(); // bookmark
    ...
    stream.writeEndDocument();

QXmlStreamWriter takes care of prefixing namespaces, all you have to do is specify the namespaceUri when writing elements or attributes. If you must conform to certain prefixes, you can force the writer to use them by declaring the namespaces manually with either writeNamespace() or writeDefaultNamespace(). Alternatively, you can bypass the stream writer's namespace support and use overloaded methods that take a qualified name instead. The namespace http://www.w3.org/XML/1998/namespace is implicit and mapped to the prefix xml.

The stream writer can automatically format the generated XML data by adding line-breaks and indentation to empty sections between elements, making the XML data more readable for humans and easier to work with for most source code management systems. The feature can be turned on with the autoFormatting property, and customized with the autoFormattingIndent property.

Other functions are writeCDATA(), writeComment(), writeProcessingInstruction(), and writeDTD(). Chaining of XML streams is supported with writeCurrentToken().

QXmlStreamWriter always encodes XML in UTF-8.

If an error occurs while writing to the underlying device, hasError() starts returning true and subsequent writes are ignored.

The QXmlStream Bookmarks Example illustrates how to use a stream writer to write an XML bookmark file (XBEL) that was previously read in by a QXmlStreamReader.

Property Documentation

autoFormatting : bool

This property holds the auto-formatting flag of the stream writer.

This property controls whether or not the stream writer automatically formats the generated XML data. If enabled, the writer automatically adds line-breaks and indentation to empty sections between elements (ignorable whitespace). The main purpose of auto-formatting is to split the data into several lines, and to increase readability for a human reader. The indentation depth can be controlled through the autoFormattingIndent property.

By default, auto-formatting is disabled.

Access functions:

bool autoFormatting() const
void setAutoFormatting(bool enable)

autoFormattingIndent : int

This property holds the number of spaces or tabs used for indentation when auto-formatting is enabled. Positive numbers indicate spaces, negative numbers tabs.

The default indentation is 4.

Access functions:

int autoFormattingIndent() const
void setAutoFormattingIndent(int spacesOrTabs)

See also autoFormatting.

Member Function Documentation

QXmlStreamWriter::QXmlStreamWriter()

Constructs a stream writer.

See also setDevice().

[explicit] QXmlStreamWriter::QXmlStreamWriter(QIODevice *device)

Constructs a stream writer that writes into device;

[explicit] QXmlStreamWriter::QXmlStreamWriter(QByteArray *array)

Constructs a stream writer that writes into array. This is the same as creating an xml writer that operates on a QBuffer device which in turn operates on array.

[explicit] QXmlStreamWriter::QXmlStreamWriter(QString *string)

Constructs a stream writer that writes into string.

[noexcept] QXmlStreamWriter::~QXmlStreamWriter()

Destructor.

bool QXmlStreamWriter::autoFormatting() const

Returns true if auto formatting is enabled, otherwise false.

Note: Getter function for property autoFormatting.

See also setAutoFormatting().

QIODevice *QXmlStreamWriter::device() const

Returns the current device associated with the QXmlStreamWriter, or nullptr if no device has been assigned.

See also setDevice().

bool QXmlStreamWriter::hasError() const

Returns true if writing failed.

This can happen if the stream failed to write to the underlying device or if the data to be written contained invalid characters.

The error status is never reset. Writes happening after the error occurred may be ignored, even if the error condition is cleared.

void QXmlStreamWriter::setAutoFormatting(bool enable)

Enables auto formatting if enable is true, otherwise disables it.

The default value is false.

Note: Setter function for property autoFormatting.

See also autoFormatting().

void QXmlStreamWriter::setDevice(QIODevice *device)

Sets the current device to device. If you want the stream to write into a QByteArray, you can create a QBuffer device.

See also device().

void QXmlStreamWriter::writeAttribute(QAnyStringView namespaceUri, QAnyStringView name, QAnyStringView value)

Writes an attribute with name and value, prefixed for the specified namespaceUri. If the namespace has not been declared yet, QXmlStreamWriter will generate a namespace declaration for it.

This function can only be called after writeStartElement() before any content is written, or after writeEmptyElement().

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeAttribute(QAnyStringView qualifiedName, QAnyStringView value)

This is an overloaded function.

Writes an attribute with qualifiedName and value.

This function can only be called after writeStartElement() before any content is written, or after writeEmptyElement().

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeAttribute(const QXmlStreamAttribute &attribute)

This is an overloaded function.

Writes the attribute.

This function can only be called after writeStartElement() before any content is written, or after writeEmptyElement().

void QXmlStreamWriter::writeAttributes(const QXmlStreamAttributes &attributes)

Writes the attribute vector attributes. If a namespace referenced in an attribute not been declared yet, QXmlStreamWriter will generate a namespace declaration for it.

This function can only be called after writeStartElement() before any content is written, or after writeEmptyElement().

See also writeAttribute() and writeNamespace().

void QXmlStreamWriter::writeCDATA(QAnyStringView text)

Writes text as CDATA section. If text contains the forbidden character sequence "]]>", it is split into different CDATA sections.

This function mainly exists for completeness. Normally you should not need use it, because writeCharacters() automatically escapes all non-content characters.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeCharacters(QAnyStringView text)

Writes text. The characters "<", "&", and """ are escaped as entity references "&lt;", "&amp;, and "&quot;". To avoid the forbidden sequence "]]>", ">" is also escaped as "&gt;".

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeEntityReference().

void QXmlStreamWriter::writeComment(QAnyStringView text)

Writes text as XML comment, where text must not contain the forbidden sequence -- or end with -. Note that XML does not provide any way to escape - in a comment.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeCurrentToken(const QXmlStreamReader &reader)

Writes the current state of the reader. All possible valid states are supported.

The purpose of this function is to support chained processing of XML data.

See also QXmlStreamReader::tokenType().

void QXmlStreamWriter::writeDTD(QAnyStringView dtd)

Writes a DTD section. The dtd represents the entire doctypedecl production from the XML 1.0 specification.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeDefaultNamespace(QAnyStringView namespaceUri)

Writes a default namespace declaration for namespaceUri.

If writeStartElement() or writeEmptyElement() was called, the declaration applies to the current element; otherwise it applies to the next child element.

Note that the namespaces http://www.w3.org/XML/1998/namespace (bound to xmlns) and http://www.w3.org/2000/xmlns/ (bound to xml) by definition cannot be declared as default.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeEmptyElement(QAnyStringView namespaceUri, QAnyStringView name)

Writes an empty element with name, prefixed for the specified namespaceUri. If the namespace has not been declared, QXmlStreamWriter will generate a namespace declaration for it. Subsequent calls to writeAttribute() will add attributes to this element.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeNamespace().

void QXmlStreamWriter::writeEmptyElement(QAnyStringView qualifiedName)

This is an overloaded function.

Writes an empty element with qualified name qualifiedName. Subsequent calls to writeAttribute() will add attributes to this element.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeEndDocument()

Closes all remaining open start elements and writes a newline.

See also writeStartDocument().

void QXmlStreamWriter::writeEndElement()

Closes the previous start element.

See also writeStartElement().

void QXmlStreamWriter::writeEntityReference(QAnyStringView name)

Writes the entity reference name to the stream, as "&name;".

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeNamespace(QAnyStringView namespaceUri, QAnyStringView prefix = {})

Writes a namespace declaration for namespaceUri with prefix. If prefix is empty, QXmlStreamWriter assigns a unique prefix consisting of the letter 'n' followed by a number.

If writeStartElement() or writeEmptyElement() was called, the declaration applies to the current element; otherwise it applies to the next child element.

Note that the prefix xml is both predefined and reserved for http://www.w3.org/XML/1998/namespace, which in turn cannot be bound to any other prefix. The prefix xmlns and its URI http://www.w3.org/2000/xmlns/ are used for the namespace mechanism itself and thus completely forbidden in declarations.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeProcessingInstruction(QAnyStringView target, QAnyStringView data = {})

Writes an XML processing instruction with target and data, where data must not contain the sequence "?>".

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeStartDocument(QAnyStringView version)

Writes a document start with the XML version number version.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeEndDocument().

void QXmlStreamWriter::writeStartDocument()

This is an overloaded function.

Writes a document start with XML version number "1.0".

See also writeEndDocument().

void QXmlStreamWriter::writeStartDocument(QAnyStringView version, bool standalone)

Writes a document start with the XML version number version and a standalone attribute standalone.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeEndDocument().

void QXmlStreamWriter::writeStartElement(QAnyStringView namespaceUri, QAnyStringView name)

Writes a start element with name, prefixed for the specified namespaceUri. If the namespace has not been declared yet, QXmlStreamWriter will generate a namespace declaration for it. Subsequent calls to writeAttribute() will add attributes to this element.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeNamespace(), writeEndElement(), and writeEmptyElement().

void QXmlStreamWriter::writeStartElement(QAnyStringView qualifiedName)

This is an overloaded function.

Writes a start element with qualifiedName. Subsequent calls to writeAttribute() will add attributes to this element.

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

See also writeEndElement() and writeEmptyElement().

void QXmlStreamWriter::writeTextElement(QAnyStringView namespaceUri, QAnyStringView name, QAnyStringView text)

Writes a text element with name, prefixed for the specified namespaceUri, and text. If the namespace has not been declared, QXmlStreamWriter will generate a namespace declaration for it.

This is a convenience function equivalent to:

        writeStartElement(namespaceUri, name);
        writeCharacters(text);
        writeEndElement();

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

void QXmlStreamWriter::writeTextElement(QAnyStringView qualifiedName, QAnyStringView text)

This is an overloaded function.

Writes a text element with qualifiedName and text.

This is a convenience function equivalent to:

        writeStartElement(qualifiedName);
        writeCharacters(text);
        writeEndElement();

Note: In Qt versions prior to 6.5, this function took QString, not QAnyStringView.

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