- Qt 5.15
- Qt XML Patterns
- C++ Classes
- QAbstractMessageHandler
- Qt 5.15.19 Reference Documentation

# QAbstractMessageHandler Class

The QAbstractMessageHandler class provides a callback interface for handling messages. More...

Header: | #include <QAbstractMessageHandler>
qmake: | QT += xmlpatterns
Since: | Qt 4.4

This class was introduced in Qt 4.4.

- List of all members, including inherited members

## Public Functions

| QAbstractMessageHandler(QObject *parent = nullptr)
virtual | ~QAbstractMessageHandler()
void | message(QtMsgType type, const QString &description, const QUrl &identifier = QUrl(), const QSourceLocation &sourceLocation = QSourceLocation())

## Protected Functions

virtual void | handleMessage(QtMsgType type, const QString &description, const QUrl &identifier = QUrl(), const QSourceLocation &sourceLocation = QSourceLocation()) = 0

## Detailed Description

QAbstractMessageHandler is an abstract base class that provides a callback interface for handling messages. For example, class QXmlQuery parses and runs an XQuery. When it detects a compile or runtime error, it generates an appropriate error message, but rather than output the message itself, it passes the message to the message() function of its QAbstractMessageHandler. See QXmlQuery::setMessageHandler().

You create a message handler by subclassing QAbstractMessageHandler and implementing handleMessage(). You then pass a pointer to an instance of your subclass to any classes that must generate messages. The messages are sent to the message handler via the message() function, which forwards them to your handleMessge().

A single instance of QAbstractMessageHandler can be called on to handle messages from multiple sources. Hence, the content of a message, which is the description parameter passed to message() and handleMessage(), must be interpreted in light of the context that required the message to be sent. That context is specified by the identifier and sourceLocation parameters to message() handleMessage().

## Member Function Documentation

### QAbstractMessageHandler::QAbstractMessageHandler(QObject *parent = nullptr)

Constructs a QAbstractMessageHandler. The parent is passed to the QObject base class constructor.

### `[virtual]` QAbstractMessageHandler::~QAbstractMessageHandler()

Destructs this QAbstractMessageHandler.

### `[pure virtual protected]` void QAbstractMessageHandler::handleMessage(QtMsgType type, const QString &description, const QUrl &identifier = QUrl(), const QSourceLocation &sourceLocation = QSourceLocation())

This function must be implemented by the sub-class. message() will call this function, passing in its parameters, type, description, identifier and sourceLocation unmodified.

This function can potentially be called from multiple threads. It's the reimplementation's responsibility to ensure thread safety.

### void QAbstractMessageHandler::message(QtMsgType type, const QString &description, const QUrl &identifier = QUrl(), const QSourceLocation &sourceLocation = QSourceLocation())

Sends a message to this message handler. type is the kind of message being sent. description is the message content. The identifier is a URI that identifies the message and is the key to interpreting the other arguments.

Typically, this class is used for reporting errors, as is the case for QXmlQuery, which uses a QAbstractMessageHandler to report compile and runtime XQuery errors. Hence, using a QUrl as the message identifier is was inspired by the explanation of error handling in the XQuery language. Because the identifier is composed of a namespace URI and a local part, identifiers with the same local part are unique. The caller is responsible for ensuring that identifier is either a valid QUrl or a default constructed QUrl.

sourceLocation identifies a location in a resource (i.e., file or document) where the need for reporting a message was detected.

This function unconditionally calls handleMessage(), passing all its parameters unmodified.

See also http://www.w3.org/TR/xquery/#errors.