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# QXmlQuery Class Reference
[QtXmlPatterns module]

The QXmlQuery class performs XQueries on XML data, or on non-XML data modeled to look like XML. More...

     #include <QXmlQuery>

Note: All the functions in this class are reentrant.

This class was introduced in Qt 4.4.

- List of all members, including inherited members

### Public Functions

- QXmlQuery ()
- QXmlQuery ( const QXmlQuery & other )
- QXmlQuery ( const QXmlNamePool & np )
- ~QXmlQuery ()
- void bindVariable ( const QXmlName & name, const QXmlItem & value )
- void bindVariable ( const QXmlName & name, QIODevice * device )
- void bindVariable ( const QString & localName, const QXmlItem & value )
- void bindVariable ( const QString & localName, QIODevice * device )
- void evaluateTo ( QXmlResultItems * result ) const
- bool evaluateTo ( QAbstractXmlReceiver * callback ) const
- bool evaluateTo ( QStringList * target ) const
- bool isValid () const
- QAbstractMessageHandler * messageHandler () const
- QXmlNamePool namePool () const
- void setFocus ( const QXmlItem & item )
- void setMessageHandler ( QAbstractMessageHandler * aMessageHandler )
- void setQuery ( QIODevice * sourceCode, const QUrl & documentURI = QUrl() )
- void setQuery ( const QUrl & queryURI, const QUrl & baseURI = QUrl() )
- void setQuery ( const QString & sourceCode, const QUrl & documentURI = QUrl() )
- void setUriResolver ( const QAbstractUriResolver * resolver )
- const QAbstractUriResolver * uriResolver () const
- QXmlQuery & operator= ( const QXmlQuery & other )

## Detailed Description

The QXmlQuery class performs XQueries on XML data, or on non-XML data modeled to look like XML.

The QXmlQuery class compiles and executes queries written in the XQuery language. QXmlQuery is typically used to query XML data, but it can also query non-XML data that has been modeled to look like XML.

Using QXmlQuery to query XML data, as in the snippet below, is simple because it can use the built-in XML data model as its delegate to the underlying query engine for traversing the data. The built-in data model is specified in XQuery 1.0 and XPath 2.0 Data Model.

     QXmlQuery query;
     query.setQuery("doc('index.html')/html/body/p[1]");

     QXmlSerializer serializer(query, myOutputDevice);
     query.evaluateTo(&serializer);

The example uses QXmlQuery to match the first paragraph of an XML document and then output the result to a device as XML.

Using QXmlQuery to query non-XML data requires writing a subclass of QAbstractXmlNodeModel to use as a replacement for the built-in XML data model. The custom data model will be able to traverse the non-XML data as required by the QAbstractXmlNodeModel interface. An instance of this custom data model then becomes the delegate used by the query engine to traverse the non-XML data. For an example of how to use QXmlQuery to query non-XML data, see the documentation for QAbstractXmlNodeModel.

### Running Queries

To run a query set up with QXmlQuery, call one of the evaluation functions.

- evaluateTo(QAbstractXmlReceiver *) is called with a pointer to an XML receiver, which receives the query results as a sequence of callbacks. The receiver callback class is like the callback class used for translating the output of a SAX parser. QXmlSerializer, for example, is a receiver callback class for translating the sequence of callbacks for output as unformatted XML text.

- evaluateTo(QXmlResultItems *) is called with a pointer to an iterator for an empty sequence of query result items. The Java-like iterator allows the query results to be accessed sequentially.

- evaluateTo(QStringList *) is like evaluateTo(QXmlResultItems *), but the query must evaluate to a sequence of strings.

### Binding A Query To A Starting Node

When a query is run on XML data, as in the snippet above, the `doc()` function returns the node in the built-in data model where the query evaluation will begin. But when a query is run on a custom node model containing non-XML data, one of the bindVariable() functions must be called to bind a variable name to a starting node in the custom model. A $variable reference is used in the XQuery text to access the starting node in the custom model. It is not necessary to declare the variable name external in the query. See the example in the documentation for QAbstractXmlNodeModel.

### Reentrancy and Thread-Safety

QXmlQuery is reentrant but not thread-safe. It is safe to use the QxmlQuery copy constructor to create a copy of a query and run the same query multiple times. Behind the scenes, QXmlQuery will reuse resources such as opened files and compiled queries to the extent possible. But it is not safe to use the same instance of QXmlQuery in multiple threads.

### Error Handling

Errors can occur during query evaluation. Examples include type errors and file loading errors. When an error occurs:

- The error message is sent to the messageHandler().
- QXmlResultItems::hasError() will return `true`, or evaluateTo() will return `false`;
- The results of the evaluation are undefined.

### Resource Management

When a query runs, it parses documents, allocating internal data structures to hold them, and it may load other resources over the network. It reuses these allocated resources when possible, to avoid having to reload and reparse them.

When setQuery() is called, the query text is compiled into an internal data structure and optimized. The optimized form can then be reused for multiple evaluations of the query. Â Since the compile-and-optimize process can be expensive, repeating it for the same query should be avoided by using a separate instance of QXmlQuery for each query text.

Once a document has been parsed, its internal representation is maintained in the QXmlQuery instance and shared among multiple QXmlQuery instances.

An instance of QCoreApplication must exist before QXmlQuery can be used.

## Member Function Documentation

### QXmlQuery::QXmlQuery ()

Constructs an empty query. A default constructed query cannot be evaluated until setQuery() has been called.

### QXmlQuery::QXmlQuery ( const QXmlQuery & other )

Constructs a QXmlQuery that is a copy of other. The new instance will share resources with the existing query to the extent possible.

### QXmlQuery::QXmlQuery ( const QXmlNamePool & np )

Constructs a query that will use np as its name pool. The query cannot be evaluated until setQuery() has been called.

### QXmlQuery::~QXmlQuery ()

Destroys this QXmlQuery.

### void QXmlQuery::bindVariable ( const QXmlName & name, const QXmlItem & value )

Binds the variable name to the value so that $name can be used from within the query to refer to the value.

name must not be null. name.isNull() must return false. If name has already been bound by a previous bindVariable() call, its previous binding will be overriden.

If value is null so that value.isNull() returns true, and name already has a binding, the effect is to remove the existing binding for name.

To bind a value of type QString or QUrl, wrap the value in a QVariant such that QXmlItem's QVariant constructor is called.

All strings processed by the query must be valid XQuery strings, which means they must contain only XML 1.0 characters. However, this requirement is not checked. If the query processes an invalid string, the behavior is undefined.

See also QVariant::isValid(), How QVariant maps to XQuery's Data Model, and QXmlItem::isNull().

### void QXmlQuery::bindVariable ( const QXmlName & name, QIODevice * device )

This is an overloaded member function, provided for convenience.

Binds the variable name to the device so that $name can be used from within the query to refer to the device. The QIODevice device is exposed to the query as a URI of type `xs:anyURI`, which can be passed to the `fn:doc()` function to be read. E.g., this function can be used to pass an XML document in memory to `fn:doc`.

         QByteArray myDocument;
         QBuffer buffer(&myDocument); // This is a QIODevice.
         buffer.open(QIODevice::ReadOnly);
         QXmlQuery query;
         query.bindVariable("myDocument", &buffer);
         query.setQuery("declare variable $myDocument external; doc($myDocument)");

The caller must ensure that device has been opened with at least QIODevice::ReadOnly prior to this binding. Otherwise, behavior is undefined.

If the query will access an XML document contained in a QString, use a QBuffer as shown in the following snippet. Suppose myQString contains `<document>content</document>`

         QBuffer device;
         device.setData(myQString.toUtf8());
         device.open(QIODevice::ReadOnly));

         QXmlQuery query;
         query.setQuery("declare variable $inputDocument external;"
                         "doc($inputDocument)/query[theDocument]");
         query.bindVariable("inputDocument", &device);

name must not be null. name.isNull() must return false. If name has already been bound, its previous binding will be overriden. The URI that name evaluates to is arbitrary and may change.

If the type of the variable binding changes (e.g., if a previous binding by the same name was a QVariant, or if there was no previous binding), isValid() will return `false`, and recompilation of the query text is required. To recompile the query, call setQuery(). For this reason, bindVariable() should be called before setQuery(), if possible.

Note: device must not be deleted while this QXmlQuery exists.

### void QXmlQuery::bindVariable ( const QString & localName, const QXmlItem & value )

This is an overloaded member function, provided for convenience.

This function constructs a QXmlName from localName using the query's namespace. The function then behaves as the overloaded function. It is equivalent to the following snippet.

         QXmlNamePool namePool(query.namePool());
         query.bindVariable(QXmlName(namePool, localName), value);

### void QXmlQuery::bindVariable ( const QString & localName, QIODevice * device )

This is an overloaded member function, provided for convenience.

If localName is a valid NCName, this function is equivalent to the following snippet.

         QXmlNamePool namePool(query.namePool());
         query.bindVariable(QXmlName(namePool, localName), device);

A QXmlName is constructed from localName, and is passed to the appropriate overload along with device.

See also QXmlName::isNCName().

### void QXmlQuery::evaluateTo ( QXmlResultItems * result ) const

Starts the evaluation and makes it available in result. If result is null, the behavior is undefined. The evaluation takes place incrementally (lazy evaluation), as the caller uses QXmlResultItems::next() to get the enxt result.

See also QXmlResultItems::next().

### bool QXmlQuery::evaluateTo ( QAbstractXmlReceiver * callback ) const

This is an overloaded member function, provided for convenience.

Evaluates this query and sends the result as a sequence of callbacks to the receiver callback. QXmlQuery does not take ownership of callback.

If an error occurs during the evaluation, error messages are sent to messageHandler() and false is returned.

If this query is invalid, `false` is returned and the behavior is undefined. If callback is null, behavior is undefined.

See also QAbstractXmlReceiver and isValid().

### bool QXmlQuery::evaluateTo ( QStringList * target ) const

This is an overloaded member function, provided for convenience.

Attempts to evaluate the query and returns the results in the target string list.

If the query is valid and the evaluation succeeds, true is returned. Otherwise, false is returned and the contents of target are undefined.

The query must evaluate to a sequence of `xs:string` values. If the query does not evaluate to a sequence of strings, the values can often be converted by adding a call to `string()` at the end of the XQuery.

If target is null, the behavior is undefined.

### bool QXmlQuery::isValid () const

Returns true if this query is valid. Examples of invalid queries are ones that contain syntax errors or that have not had setQuery() called for them yet.

### QAbstractMessageHandler * QXmlQuery::messageHandler () const

Returns the message handler that handles compile and runtime messages for this QXmlQuery.

See also setMessageHandler().

### QXmlNamePool QXmlQuery::namePool () const

Returns the name pool used by this QXmlQuery for constructing names. There is no setter for the name pool, because mixing name pools causes errors due to name confusion.

### void QXmlQuery::setFocus ( const QXmlItem & item )

Sets the focus to item. The focus is the set of items that the context item expression and path expressions navigate from. For example, in the expression p/span, the element that p evaluates to is the focus for the following expression, span.

The focus can be accessed using the context item expression, i.e., dot (".").

By default, the focus is not set but is undefined. It will therefore result in a dynamic error, `XPDY0002`, if the query is evaluated without a focus. In fact, the focus must be set before the query is set with setQuery().

### void QXmlQuery::setMessageHandler ( QAbstractMessageHandler * aMessageHandler )

Changes the message handler for this QXmlQuery to aMessageHandler. The query sends all compile and runtime messages to this message handler. QXmlQuery does not take ownership of aMessageHandler.

Normally, the default message handler is sufficient. It writes compile and runtime messages to stderr. The default message handler includes color codes if stderr can render colors.

Note that changing the message handler after the query has been compiled has no effect, i.e. the query uses the same message handler at runtime that it uses at compile time.

When QXmlQuery calls QAbstractMessageHandler::message(), the arguments are as follows:

message() argument | Semantics
--- | ---
QtMsgType type | Only QtWarningMsg and QtFatalMsg are used. The former identifies a compile or runtime warning, while the latter identifies a dynamic or static error.
const QString & description | An XHTML document which is the actual message. It is translated into the current language.
const QUrl &identifier | Identifies the error with a URI, where the fragment is the error code, and the rest of the URI is the error namespace.
const QSourceLocation & sourceLocation | Identifies where the error occurred.

See also messageHandler().

### void QXmlQuery::setQuery ( QIODevice * sourceCode, const QUrl & documentURI = QUrl() )

Sets this QXmlQuery to an XQuery read from the sourceCode device. The device must have been opened with at least QIODevice::ReadOnly.

documentURI represents the query obtained from the sourceCode device. It is the base URI of the static context, as defined in the XQuery language. It is used internally to resolve relative URIs that appear in the query, and for message reporting. documentURI can be empty. If it is empty, the application file path is used. If it is not empty, it may be either relative or absolute. If it is relative, it is resolved itself against the application file path before it is used. If documentURI is neither a valid URI nor empty, the result is undefined.

If the query contains a static error (e.g. syntax error), an error message is sent to the messageHandler(), and isValid() will return false.

Variables must be bound before setQuery() is called.

The encoding of the XQuery in sourceCode is detected internally using the rules for setting and detecting encoding of XQuery files, which are explained in the XQuery language.

See also isValid().

### void QXmlQuery::setQuery ( const QUrl & queryURI, const QUrl & baseURI = QUrl() )

This is an overloaded member function, provided for convenience.

Sets this QXmlQuery to the XQuery read from the queryURI. Use isValid() after calling this function. If an error occurred reading queryURI, e.g., the query does not exist, cannot be read, or is invalid, isValid() will return false.

The supported URI schemes are the same as those in the XQuery function `fn:doc`, except that queryURI can be the object of a variable binding.

baseURI is the Base URI of the static context, as defined in the XQuery language. It is used internally to resolve relative URIs that appear in the query, and for message reporting. If baseURI is empty, queryURI is used. Otherwise, baseURI is used, and it is resolved against the application file path if it is relative.

If queryURI is empty or invalid, or if baseURI is invalid, the behavior of this function is undefined.

### void QXmlQuery::setQuery ( const QString & sourceCode, const QUrl & documentURI = QUrl() )

This is an overloaded member function, provided for convenience.

The behavior and requirements of this function are the same as for setQuery(QIODevice*, const QUrl&), after the XQuery has been read from the IO device into a string. Because sourceCode is already a Unicode string, detection of its encoding is unnecessary.

### void QXmlQuery::setUriResolver ( const QAbstractUriResolver * resolver )

Sets the URI resolver to resolver. QXmlQuery does not take ownership of resolver.

See also uriResolver().

### const QAbstractUriResolver * QXmlQuery::uriResolver () const

Returns the query's URI resolver. If no URI resolver has been set, QtXmlPatterns will use the URIs in queries as they are.

The URI resolver provides a level of abstraction, or polymorphic URIs. A resolver can rewrite logical URIs to physical ones, or it can translate obsolete or invalid URIs to valid ones.

QtXmlPatterns calls the URI resolver for all URIs it encounters, except for namespaces. Specifically, all builtin functions that deal with URIs (`fn:doc()`, and `fn:doc-available()`).

In the case of `fn:doc()`, the absolute URI is the base URI in the static context (which most likely is the location of the query). Rather than use the URI the user specified, the return value of QAbstractUriResolver::resolve() will be used.

When QtXmlPatterns calls QAbstractUriResolver::resolve() the absolute URI is the URI mandated by the XQuery language, and the relative URI is the URI specified by the user.

See also setUriResolver().

### QXmlQuery & QXmlQuery::operator= ( const QXmlQuery & other )

Assigns other to this QXmlQuery instance.