QXmlStreamReader Class | Qt Core 6.4.3

- Qt 6.4
- Qt Core
- C++ Classes
- QXmlStreamReader
- Qt 6.4.3 Reference Documentation

# QXmlStreamReader Class

The QXmlStreamReader class provides a fast parser for reading well-formed XML via a simple streaming API. More...

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

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

enum | Error { NoError, CustomError, NotWellFormedError, PrematureEndOfDocumentError, UnexpectedElementError }
enum | ReadElementTextBehaviour { ErrorOnUnexpectedElement, IncludeChildElements, SkipChildElements }
enum | TokenType { NoToken, Invalid, StartDocument, EndDocument, StartElement, …, ProcessingInstruction }

## Properties

- namespaceProcessing : bool

## Public Functions

| QXmlStreamReader(const char *data)
| QXmlStreamReader(const QString &data)
| QXmlStreamReader(const QByteArray &data)
| QXmlStreamReader(QIODevice *device)
| QXmlStreamReader()
| ~QXmlStreamReader()
void | addData(const QByteArray &data)
void | addData(const QString &data)
void | addData(const char *data)
void | addExtraNamespaceDeclaration(const QXmlStreamNamespaceDeclaration &extraNamespaceDeclaration)
void | addExtraNamespaceDeclarations(const QXmlStreamNamespaceDeclarations &extraNamespaceDeclarations)
bool | atEnd() const
QXmlStreamAttributes | attributes() const
qint64 | characterOffset() const
void | clear()
qint64 | columnNumber() const
QIODevice * | device() const
QStringView | documentEncoding() const
QStringView | documentVersion() const
QStringView | dtdName() const
QStringView | dtdPublicId() const
QStringView | dtdSystemId() const
QXmlStreamEntityDeclarations | entityDeclarations() const
int | entityExpansionLimit() const
QXmlStreamEntityResolver * | entityResolver() const
Error | error() const
QString | errorString() const
bool | hasError() const
bool | isCDATA() const
bool | isCharacters() const
bool | isComment() const
bool | isDTD() const
bool | isEndDocument() const
bool | isEndElement() const
bool | isEntityReference() const
bool | isProcessingInstruction() const
bool | isStandaloneDocument() const
bool | isStartDocument() const
bool | isStartElement() const
bool | isWhitespace() const
qint64 | lineNumber() const
QStringView | name() const
QXmlStreamNamespaceDeclarations | namespaceDeclarations() const
bool | namespaceProcessing() const
QStringView | namespaceUri() const
QXmlStreamNotationDeclarations | notationDeclarations() const
QStringView | prefix() const
QStringView | processingInstructionData() const
QStringView | processingInstructionTarget() const
QStringView | qualifiedName() const
void | raiseError(const QString &message = QString())
QString | readElementText(ReadElementTextBehaviour behaviour = ErrorOnUnexpectedElement)
TokenType | readNext()
bool | readNextStartElement()
void | setDevice(QIODevice *device)
void | setEntityExpansionLimit(int limit)
void | setEntityResolver(QXmlStreamEntityResolver *resolver)
void | setNamespaceProcessing(bool)
void | skipCurrentElement()
QStringView | text() const
QString | tokenString() const
TokenType | tokenType() const

## Detailed Description

QXmlStreamReader provides a simple streaming API to parse well-formed XML. It is an alternative to first loading the complete XML into a DOM tree (see QDomDocument). QXmlStreamReader reads data either from a QIODevice (see setDevice()), or from a raw QByteArray (see addData()).

Qt provides QXmlStreamWriter for writing XML.

The basic concept of a stream reader is to report an XML document as a stream of tokens, similar to SAX. The main difference between QXmlStreamReader and SAX is how these XML tokens are reported. With SAX, the application must provide handlers (callback functions) that receive so-called XML events from the parser at the parser's convenience. With QXmlStreamReader, the application code itself drives the loop and pulls tokens from the reader, one after another, as it needs them. This is done by calling readNext(), where the reader reads from the input stream until it completes the next token, at which point it returns the tokenType(). A set of convenient functions including isStartElement() and text() can then be used to examine the token to obtain information about what has been read. The big advantage of this pulling approach is the possibility to build recursive descent parsers with it, meaning you can split your XML parsing code easily into different methods or classes. This makes it easy to keep track of the application's own state when parsing XML.

A typical loop with QXmlStreamReader looks like this:

       QXmlStreamReader xml;
       ...
       while (!xml.atEnd()) {
             xml.readNext();
             ... // do processing
       }
       if (xml.hasError()) {
             ... // do error handling
       }

QXmlStreamReader is a well-formed XML 1.0 parser that does not include external parsed entities. As long as no error occurs, the application code can thus be assured, that

- the data provided by the stream reader satisfies the W3C's criteria for well-formed XML,
- tokens are provided in a valid order.

Unless QXmlStreamReader raises an error, it guarantees the following:

- All tags are nested and closed properly.
- References to internal entities have been replaced with the correct replacement text.
- Attributes have been normalized or added according to the internal subset of the DTD.
- Tokens of type StartDocument happen before all others, aside from comments and processing instructions.
- At most one DOCTYPE element (a token of type DTD) is present.
- If present, the DOCTYPE appears before all other elements, aside from StartDocument, comments and processing instructions.

In particular, once any token of type StartElement, EndElement, Characters, EntityReference or EndDocument is seen, no tokens of type StartDocument or DTD will be seen. If one is present in the input stream, out of order, an error is raised.

Note: The token types Comment and ProcessingInstruction may appear anywhere in the stream.

If an error occurs while parsing, atEnd() and hasError() return true, and error() returns the error that occurred. The functions errorString(), lineNumber(), columnNumber(), and characterOffset() are for constructing an appropriate error or warning message. To simplify application code, QXmlStreamReader contains a raiseError() mechanism that lets you raise custom errors that trigger the same error handling described.

The QXmlStream Bookmarks Example illustrates how to use the recursive descent technique to read an XML bookmark file (XBEL) with a stream reader.

### Namespaces

QXmlStream understands and resolves XML namespaces. E.g. in case of a StartElement, namespaceUri() returns the namespace the element is in, and name() returns the element's local name. The combination of namespaceUri and name uniquely identifies an element. If a namespace prefix was not declared in the XML entities parsed by the reader, the namespaceUri is empty.

If you parse XML data that does not utilize namespaces according to the XML specification or doesn't use namespaces at all, you can use the element's qualifiedName() instead. A qualified name is the element's prefix() followed by colon followed by the element's local name() - exactly like the element appears in the raw XML data. Since the mapping namespaceUri to prefix is neither unique nor universal, qualifiedName() should be avoided for namespace-compliant XML data.

In order to parse standalone documents that do use undeclared namespace prefixes, you can turn off namespace processing completely with the namespaceProcessing property.

### Incremental Parsing

QXmlStreamReader is an incremental parser. It can handle the case where the document can't be parsed all at once because it arrives in chunks (e.g. from multiple files, or over a network connection). When the reader runs out of data before the complete document has been parsed, it reports a PrematureEndOfDocumentError. When more data arrives, either because of a call to addData() or because more data is available through the network device(), the reader recovers from the PrematureEndOfDocumentError error and continues parsing the new data with the next call to readNext().

For example, if your application reads data from the network using a network access manager, you would issue a network request to the manager and receive a network reply in return. Since a QNetworkReply is a QIODevice, you connect its readyRead() signal to a custom slot, e.g. `slotReadyRead()` in the code snippet shown in the discussion for QNetworkAccessManager. In this slot, you read all available data with readAll() and pass it to the XML stream reader using addData(). Then you call your custom parsing function that reads the XML events from the reader.

### Performance and Memory Consumption

QXmlStreamReader is memory-conservative by design, since it doesn't store the entire XML document tree in memory, but only the current token at the time it is reported. In addition, QXmlStreamReader avoids the many small string allocations that it normally takes to map an XML document to a convenient and Qt-ish API. It does this by reporting all string data as QStringView rather than real QString objects. Calling toString() on any of those objects returns an equivalent real QString object.

## Member Type Documentation

### enum QXmlStreamReader::Error

This enum specifies different error cases

Constant | Value | Description
--- | --- | ---
`QXmlStreamReader::NoError` | `0` | No error has occurred.
`QXmlStreamReader::CustomError` | `2` | A custom error has been raised with raiseError()
`QXmlStreamReader::NotWellFormedError` | `3` | The parser internally raised an error due to the read XML not being well-formed.
`QXmlStreamReader::PrematureEndOfDocumentError` | `4` | The input stream ended before a well-formed XML document was parsed. Recovery from this error is possible if more XML arrives in the stream, either by calling addData() or by waiting for it to arrive on the device().
`QXmlStreamReader::UnexpectedElementError` | `1` | The parser encountered an element or token that was different to those it expected.

### enum QXmlStreamReader::ReadElementTextBehaviour

This enum specifies the different behaviours of readElementText().

Constant | Value | Description
--- | --- | ---
`QXmlStreamReader::ErrorOnUnexpectedElement` | `0` | Raise an UnexpectedElementError and return what was read so far when a child element is encountered.
`QXmlStreamReader::IncludeChildElements` | `1` | Recursively include the text from child elements.
`QXmlStreamReader::SkipChildElements` | `2` | Skip child elements.

### enum QXmlStreamReader::TokenType

This enum specifies the type of token the reader just read.

Constant | Value | Description
--- | --- | ---
`QXmlStreamReader::NoToken` | `0` | The reader has not yet read anything.
`QXmlStreamReader::Invalid` | `1` | An error has occurred, reported in error() and errorString().
`QXmlStreamReader::StartDocument` | `2` | The reader reports the XML version number in documentVersion(), and the encoding as specified in the XML document in documentEncoding(). If the document is declared standalone, isStandaloneDocument() returns `true`; otherwise it returns `false`.
`QXmlStreamReader::EndDocument` | `3` | The reader reports the end of the document.
`QXmlStreamReader::StartElement` | `4` | The reader reports the start of an element with namespaceUri() and name(). Empty elements are also reported as StartElement, followed directly by EndElement. The convenience function readElementText() can be called to concatenate all content until the corresponding EndElement. Attributes are reported in attributes(), namespace declarations in namespaceDeclarations().
`QXmlStreamReader::EndElement` | `5` | The reader reports the end of an element with namespaceUri() and name().
`QXmlStreamReader::Characters` | `6` | The reader reports characters in text(). If the characters are all white-space, isWhitespace() returns `true`. If the characters stem from a CDATA section, isCDATA() returns `true`.
`QXmlStreamReader::Comment` | `7` | The reader reports a comment in text().
`QXmlStreamReader::DTD` | `8` | The reader reports a DTD in text(), notation declarations in notationDeclarations(), and entity declarations in entityDeclarations(). Details of the DTD declaration are reported in in dtdName(), dtdPublicId(), and dtdSystemId().
`QXmlStreamReader::EntityReference` | `9` | The reader reports an entity reference that could not be resolved. The name of the reference is reported in name(), the replacement text in text().
`QXmlStreamReader::ProcessingInstruction` | `10` | The reader reports a processing instruction in processingInstructionTarget() and processingInstructionData().

## Property Documentation

### namespaceProcessing : bool

This property holds the namespace-processing flag of the stream reader.

This property controls whether or not the stream reader processes namespaces. If enabled, the reader processes namespaces, otherwise it does not.

By default, namespace-processing is enabled.

Access functions:

bool | namespaceProcessing() const
void | setNamespaceProcessing(bool)

## Member Function Documentation

### QXmlStreamReader::QXmlStreamReader(const char *data)

Creates a new stream reader that reads from data.

See also addData(), clear(), and setDevice().

### QXmlStreamReader::QXmlStreamReader(const QString &data)

Creates a new stream reader that reads from data.

See also addData(), clear(), and setDevice().

### QXmlStreamReader::QXmlStreamReader(const QByteArray &data)

Creates a new stream reader that reads from data.

See also addData(), clear(), and setDevice().

### QXmlStreamReader::QXmlStreamReader(QIODevice *device)

Creates a new stream reader that reads from device.

See also setDevice() and clear().

### QXmlStreamReader::QXmlStreamReader()

Constructs a stream reader.

See also setDevice() and addData().

### QXmlStreamReader::~QXmlStreamReader()

Destructs the reader.

### void QXmlStreamReader::addData(const QByteArray &data)

Adds more data for the reader to read. This function does nothing if the reader has a device().

See also readNext() and clear().

### void QXmlStreamReader::addData(const QString &data)

Adds more data for the reader to read. This function does nothing if the reader has a device().

See also readNext() and clear().

### void QXmlStreamReader::addData(const char *data)

Adds more data for the reader to read. This function does nothing if the reader has a device().

See also readNext() and clear().

### void QXmlStreamReader::addExtraNamespaceDeclaration(const QXmlStreamNamespaceDeclaration &extraNamespaceDeclaration)

Adds an extraNamespaceDeclaration. The declaration will be valid for children of the current element, or - should the function be called before any elements are read - for the entire XML document.

See also namespaceDeclarations(), addExtraNamespaceDeclarations(), and setNamespaceProcessing().

### void QXmlStreamReader::addExtraNamespaceDeclarations(const QXmlStreamNamespaceDeclarations &extraNamespaceDeclarations)

Adds a vector of declarations specified by extraNamespaceDeclarations.

See also namespaceDeclarations() and addExtraNamespaceDeclaration().

### bool QXmlStreamReader::atEnd() const

Returns `true` if the reader has read until the end of the XML document, or if an error() has occurred and reading has been aborted. Otherwise, it returns `false`.

When atEnd() and hasError() return true and error() returns PrematureEndOfDocumentError, it means the XML has been well-formed so far, but a complete XML document has not been parsed. The next chunk of XML can be added with addData(), if the XML is being read from a QByteArray, or by waiting for more data to arrive if the XML is being read from a QIODevice. Either way, atEnd() will return false once more data is available.

See also hasError(), error(), device(), and QIODevice::atEnd().

### QXmlStreamAttributes QXmlStreamReader::attributes() const

Returns the attributes of a StartElement.

### qint64 QXmlStreamReader::characterOffset() const

Returns the current character offset, starting with 0.

See also lineNumber() and columnNumber().

### void QXmlStreamReader::clear()

Removes any device() or data from the reader and resets its internal state to the initial state.

See also addData().

### qint64 QXmlStreamReader::columnNumber() const

Returns the current column number, starting with 0.

See also lineNumber() and characterOffset().

### QIODevice *QXmlStreamReader::device() const

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

See also setDevice().

### QStringView QXmlStreamReader::documentEncoding() const

If the tokenType() is StartDocument, this function returns the encoding string as specified in the XML declaration. Otherwise an empty string is returned.

### QStringView QXmlStreamReader::documentVersion() const

If the tokenType() is StartDocument, this function returns the version string as specified in the XML declaration. Otherwise an empty string is returned.

### QStringView QXmlStreamReader::dtdName() const

If the tokenType() is DTD, this function returns the DTD's name. Otherwise an empty string is returned.

### QStringView QXmlStreamReader::dtdPublicId() const

If the tokenType() is DTD, this function returns the DTD's public identifier. Otherwise an empty string is returned.

### QStringView QXmlStreamReader::dtdSystemId() const

If the tokenType() is DTD, this function returns the DTD's system identifier. Otherwise an empty string is returned.

### QXmlStreamEntityDeclarations QXmlStreamReader::entityDeclarations() const

If the tokenType() is DTD, this function returns the DTD's unparsed (external) entity declarations. Otherwise an empty vector is returned.

The QXmlStreamEntityDeclarations class is defined to be a QList of QXmlStreamEntityDeclaration.

### `[since 5.15]` int QXmlStreamReader::entityExpansionLimit() const

Returns the maximum amount of characters a single entity is allowed to expand into. If a single entity expands past the given limit, the document is not considered well formed.

This function was introduced in Qt 5.15.

See also setEntityExpansionLimit.

### QXmlStreamEntityResolver *QXmlStreamReader::entityResolver() const

Returns the entity resolver, or `nullptr` if there is no entity resolver.

See also setEntityResolver().

### Error QXmlStreamReader::error() const

Returns the type of the current error, or NoError if no error occurred.

See also errorString() and raiseError().

### QString QXmlStreamReader::errorString() const

Returns the error message that was set with raiseError().

See also error(), lineNumber(), columnNumber(), and characterOffset().

### bool QXmlStreamReader::hasError() const

Returns `true` if an error has occurred, otherwise `false`.

See also errorString() and error().

### bool QXmlStreamReader::isCDATA() const

Returns `true` if the reader reports characters that stem from a CDATA section; otherwise returns `false`.

See also isCharacters() and text().

### bool QXmlStreamReader::isCharacters() const

Returns `true` if tokenType() equals Characters; otherwise returns `false`.

See also isWhitespace() and isCDATA().

### bool QXmlStreamReader::isComment() const

Returns `true` if tokenType() equals Comment; otherwise returns `false`.

### bool QXmlStreamReader::isDTD() const

Returns `true` if tokenType() equals DTD; otherwise returns `false`.

### bool QXmlStreamReader::isEndDocument() const

Returns `true` if tokenType() equals EndDocument; otherwise returns `false`.

### bool QXmlStreamReader::isEndElement() const

Returns `true` if tokenType() equals EndElement; otherwise returns `false`.

### bool QXmlStreamReader::isEntityReference() const

Returns `true` if tokenType() equals EntityReference; otherwise returns `false`.

### bool QXmlStreamReader::isProcessingInstruction() const

Returns `true` if tokenType() equals ProcessingInstruction; otherwise returns `false`.

### bool QXmlStreamReader::isStandaloneDocument() const

Returns `true` if this document has been declared standalone in the XML declaration; otherwise returns `false`.

If no XML declaration has been parsed, this function returns `false`.

### bool QXmlStreamReader::isStartDocument() const

Returns `true` if tokenType() equals StartDocument; otherwise returns `false`.

### bool QXmlStreamReader::isStartElement() const

Returns `true` if tokenType() equals StartElement; otherwise returns `false`.

### bool QXmlStreamReader::isWhitespace() const

Returns `true` if the reader reports characters that only consist of white-space; otherwise returns `false`.

See also isCharacters() and text().

### qint64 QXmlStreamReader::lineNumber() const

Returns the current line number, starting with 1.

See also columnNumber() and characterOffset().

### QStringView QXmlStreamReader::name() const

Returns the local name of a StartElement, EndElement, or an EntityReference.

See also namespaceUri() and qualifiedName().

### QXmlStreamNamespaceDeclarations QXmlStreamReader::namespaceDeclarations() const

If the tokenType() is StartElement, this function returns the element's namespace declarations. Otherwise an empty vector is returned.

The QXmlStreamNamespaceDeclarations class is defined to be a QList of QXmlStreamNamespaceDeclaration.

See also addExtraNamespaceDeclaration() and addExtraNamespaceDeclarations().

### QStringView QXmlStreamReader::namespaceUri() const

Returns the namespaceUri of a StartElement or EndElement.

See also name() and qualifiedName().

### QXmlStreamNotationDeclarations QXmlStreamReader::notationDeclarations() const

If the tokenType() is DTD, this function returns the DTD's notation declarations. Otherwise an empty vector is returned.

The QXmlStreamNotationDeclarations class is defined to be a QList of QXmlStreamNotationDeclaration.

### QStringView QXmlStreamReader::prefix() const

Returns the prefix of a StartElement or EndElement.

See also name() and qualifiedName().

### QStringView QXmlStreamReader::processingInstructionData() const

Returns the data of a ProcessingInstruction.

### QStringView QXmlStreamReader::processingInstructionTarget() const

Returns the target of a ProcessingInstruction.

### QStringView QXmlStreamReader::qualifiedName() const

Returns the qualified name of a StartElement or EndElement;

A qualified name is the raw name of an element in the XML data. It consists of the namespace prefix, followed by colon, followed by the element's local name. Since the namespace prefix is not unique (the same prefix can point to different namespaces and different prefixes can point to the same namespace), you shouldn't use qualifiedName(), but the resolved namespaceUri() and the attribute's local name().

See also name(), prefix(), and namespaceUri().

### void QXmlStreamReader::raiseError(const QString &message = QString())

Raises a custom error with an optional error message.

See also error() and errorString().

### QString QXmlStreamReader::readElementText(ReadElementTextBehaviour behaviour = ErrorOnUnexpectedElement)

Convenience function to be called in case a StartElement was read. Reads until the corresponding EndElement and returns all text in-between. In case of no error, the current token (see tokenType()) after having called this function is EndElement.

The function concatenates text() when it reads either Characters or EntityReference tokens, but skips ProcessingInstruction and Comment. If the current token is not StartElement, an empty string is returned.

The behaviour defines what happens in case anything else is read before reaching EndElement. The function can include the text from child elements (useful for example for HTML), ignore child elements, or raise an UnexpectedElementError and return what was read so far (default).

### TokenType QXmlStreamReader::readNext()

Reads the next token and returns its type.

With one exception, once an error() is reported by readNext(), further reading of the XML stream is not possible. Then atEnd() returns `true`, hasError() returns `true`, and this function returns QXmlStreamReader::Invalid.

The exception is when error() returns PrematureEndOfDocumentError. This error is reported when the end of an otherwise well-formed chunk of XML is reached, but the chunk doesn't represent a complete XML document. In that case, parsing can be resumed by calling addData() to add the next chunk of XML, when the stream is being read from a QByteArray, or by waiting for more data to arrive when the stream is being read from a device().

See also tokenType() and tokenString().

### bool QXmlStreamReader::readNextStartElement()

Reads until the next start element within the current element. Returns `true` when a start element was reached. When the end element was reached, or when an error occurred, false is returned.

The current element is the element matching the most recently parsed start element of which a matching end element has not yet been reached. When the parser has reached the end element, the current element becomes the parent element.

This is a convenience function for when you're only concerned with parsing XML elements. The QXmlStream Bookmarks Example makes extensive use of this function.

See also readNext().

### void QXmlStreamReader::setDevice(QIODevice *device)

Sets the current device to device. Setting the device resets the stream to its initial state.

See also device() and clear().

### `[since 5.15]` void QXmlStreamReader::setEntityExpansionLimit(int limit)

Sets the maximum amount of characters a single entity is allowed to expand into to limit. If a single entity expands past the given limit, the document is not considered well formed.

The limit is there to prevent DoS attacks when loading unknown XML documents where recursive entity expansion could otherwise exhaust all available memory.

The default value for this property is 4096 characters.

This function was introduced in Qt 5.15.

See also entityExpansionLimit.

### void QXmlStreamReader::setEntityResolver(QXmlStreamEntityResolver *resolver)

Makes resolver the new entityResolver().

The stream reader does not take ownership of the resolver. It's the callers responsibility to ensure that the resolver is valid during the entire life-time of the stream reader object, or until another resolver or `nullptr` is set.

See also entityResolver().

### void QXmlStreamReader::skipCurrentElement()

Reads until the end of the current element, skipping any child nodes. This function is useful for skipping unknown elements.

The current element is the element matching the most recently parsed start element of which a matching end element has not yet been reached. When the parser has reached the end element, the current element becomes the parent element.

### QStringView QXmlStreamReader::text() const

Returns the text of Characters, Comment, DTD, or EntityReference.

### QString QXmlStreamReader::tokenString() const

Returns the reader's current token as string.

See also tokenType().

### TokenType QXmlStreamReader::tokenType() const

Returns the type of the current token.

The current token can also be queried with the convenience functions isStartDocument(), isEndDocument(), isStartElement(), isEndElement(), isCharacters(), isComment(), isDTD(), isEntityReference(), and isProcessingInstruction().

See also tokenString().