QLoggingCategory Class

Detailed Description

QLoggingCategory represents a certain logging category - identified by a string - at runtime. A category can be configured to enable or disable logging of messages per message type.

To check whether a message type is enabled or not, use one of these methods: isDebugEnabled(), isInfoEnabled(), isWarningEnabled(), and isCriticalEnabled().

All objects are meant to be configured by a common registry, as described in Configuring Categories. Different objects can also represent the same category. Therefore, it's not recommended to export objects across module boundaries, to manipulate the objects directly, or to inherit from QLoggingCategory.

Creating Category Objects

The Q_DECLARE_LOGGING_CATEGORY() and Q_LOGGING_CATEGORY() macros conveniently declare and create QLoggingCategory objects:

// in a header
Q_DECLARE_LOGGING_CATEGORY(driverUsb)

// in one source file
Q_LOGGING_CATEGORY(driverUsb, "driver.usb")

Category names are free text; to configure categories using Logging Rules, their names should follow this convention:

• Use letters and numbers only.
• Use dots to further structure categories into common areas.
• Avoid the category names: debug, info, warning, and critical.
• Category names with the qt prefix are solely reserved for Qt modules.

QLoggingCategory objects that are implicitly defined by Q_LOGGING_CATEGORY() are created on first use, in a thread-safe manner.

Checking Category Configuration

QLoggingCategory provides isDebugEnabled(), isInfoEnabled(), isWarningEnabled(), isCriticalEnabled(), as well as isEnabled() to check whether messages for the given message type should be logged.

The qCDebug(), qCWarning(), and qCCritical() macros prevent arguments from being evaluated if the respective message types are not enabled for the category, so explicit checking is not needed:

    // usbEntries() will only be called if driverUsb category is enabled
    qCDebug(driverUsb) << "devices: " << usbEntries();

Default Category Configuration

Both the QLoggingCategory constructor and the Q_LOGGING_CATEGORY() macro accept an optional QtMsgType argument, which disables all message types with a lower severity. That is, a category declared with

Q_LOGGING_CATEGORY(driverUsbEvents, "driver.usb.events", QtWarningMsg)

logs messages of type QtWarningMsg, QtCriticalMsg, QtFatalMsg, but ignores messages of type QtDebugMsg and QtInfoMsg.

If no argument is passed, all messages are logged. Only Qt internal categories which start with qt are handled differently: For these, only messages of type QtInfoMsg, QtWarningMsg, and QtCriticalMsg are logged by default.

Configuring Categories

You can override the default configuration for categories either by setting logging rules, or by installing a custom filter.

Logging Rules

Logging rules let you enable or disable logging for categories in a flexible way. Rules are specified in text, where every line must have the format:

    <category>[.<type>] = true|false

<category> is the name of the category, potentially with * as a wildcard symbol for the first or last character; or at both positions. The optional <type> must be debug, info, warning, or critical. Lines that don't fit this scheme are ignored.

Rules are evaluated in text order, from first to last. That is, if two rules apply to a category/type, the rule that comes later is applied.

Rules can be set via setFilterRules():

    QLoggingCategory::setFilterRules("*.debug=false\n"
                                     "driver.usb.debug=true");

Logging rules are automatically loaded from the [Rules] section in a logging configuration file. These configuration files are looked up in the QtProject configuration directory, or explicitly set in a QT_LOGGING_CONF environment variable:

    [Rules]
    *.debug=false
    driver.usb.debug=true

Logging rules can also be specified in a QT_LOGGING_RULES environment variable; multiple rules can also be separated by semicolons:

    QT_LOGGING_RULES="*.debug=false;driver.usb.debug=true"

Rules set by setFilterRules() take precedence over rules specified in the QtProject configuration directory. In turn, these rules can be overwritten by those from the configuration file specified by QT_LOGGING_CONF, and those set by QT_LOGGING_RULES.

The order of evaluation is as follows:

1. [QLibraryInfo::DataPath]/qtlogging.ini
2. QtProject/qtlogging.ini
3. setFilterRules()
4. QT_LOGGING_CONF
5. QT_LOGGING_RULES

The QtProject/qtlogging.ini file is looked up in all directories returned by QStandardPaths::GenericConfigLocation.

Set the QT_LOGGING_DEBUG environment variable to find out where your logging rules are loaded from.

Installing a Custom Filter

As a lower-level alternative to the text rules, you can also implement a custom filter via installFilter(). All filter rules are ignored in this case.

Printing the Category

Use the %{category} placeholder to print the category in the default message handler:

    qSetMessagePattern("%{category} %{message}");