QLoggingCategory Class

The QLoggingCategory class represents a category, or 'area' in the logging infrastructure. More...

This class was introduced in Qt 5.2.

• List of all members, including inherited members

Note: All functions in this class are thread-safe.

Public Types

Public Functions

Static Public Members

Macros

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.

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}");

Member Type Documentation

typedef QLoggingCategory::CategoryFilter

This is a typedef for a pointer to a function with the following signature:

void myCategoryFilter(QLoggingCategory *);

A function with this signature can be installed with installFilter().

Member Function Documentation

QLoggingCategory::QLoggingCategory(const char *category, QtMsgType enableForLevel)

Constructs a QLoggingCategory object with the provided category name, and enables all messages with types more severe or equal than enableForLevel.

If category is 0, the category name is changed to "default".

Note: category must be kept valid during the lifetime of this object.

This function was introduced in Qt 5.4.

QLoggingCategory::QLoggingCategory(const char *category)

Constructs a QLoggingCategory object with the provided category name. All message types for this category are enabled by default.

If category is 0, the category name is changed to "default".

Note: category must be kept valid during the lifetime of this object.

QLoggingCategory::~QLoggingCategory()

Destroys a QLoggingCategory object.

const char *QLoggingCategory::categoryName() const

Returns the name of the category.

[static] QLoggingCategory *QLoggingCategory::defaultCategory()

Returns a pointer to the global category "default" that is used, for example, by qDebug(), qInfo(), qWarning(), qCritical(), or qFatal().

Note: The pointer returned may be null during destruction of static objects. Also, don't delete this pointer, as ownership of the category isn't transferred.

[static] QLoggingCategory::CategoryFilter QLoggingCategory::installFilter(QLoggingCategory::CategoryFilter filter)

Installs a function filter that is used to determine which categories and message types should be enabled. Returns a pointer to the previous installed filter.

Every QLoggingCategory object created is passed to the filter, and the filter is free to change the respective category configuration with setEnabled().

When you define your filter, note that it can be called from different threads; but never concurrently. This filter cannot call any static functions from QLoggingCategory.

Example:

QLoggingCategory::CategoryFilter oldCategoryFilter;

void myCategoryFilter(QLoggingCategory *category)
{
    // configure driver.usb category here, otherwise forward to to default filter.
    if (qstrcmp(category->categoryName(), "driver.usb") == 0)
        category->setEnabled(QtDebugMsg, true);
    else
        oldCategoryFilter(category);
}

Alternatively, you can configure the default filter via setFilterRules().

bool QLoggingCategory::isCriticalEnabled() const

Returns true if critical messages should be shown for this category; false otherwise.

Note: The qCCritical() macro already does this check before executing any code. However, calling this method may be useful to avoid the expensive generation of data for debug output only.

bool QLoggingCategory::isDebugEnabled() const

Returns true if debug messages should be shown for this category; false otherwise.

Note: The qCDebug() macro already does this check before running any code. However, calling this method may be useful to avoid the expensive generation of data for debug output only.

bool QLoggingCategory::isEnabled(QtMsgType msgtype) const

Returns true if a message of type msgtype for the category should be shown; false otherwise.

bool QLoggingCategory::isInfoEnabled() const

Returns true if informational messages should be shown for this category; false otherwise.

Note: The qCInfo() macro already does this check before executing any code. However, calling this method may be useful to avoid the expensive generation of data for debug output only.

This function was introduced in Qt 5.5.

bool QLoggingCategory::isWarningEnabled() const

Returns true if warning messages should be shown for this category; false otherwise.

Note: The qCWarning() macro already does this check before executing any code. However, calling this method may be useful to avoid the expensive generation of data for debug output only.

void QLoggingCategory::setEnabled(QtMsgType type, bool enable)

Changes the message type type for the category to enable.

This method is meant for use only from inside a filter installed with installFilter(). For an overview on how to configure categories globally, see Configuring Categories.

Note: QtFatalMsg cannot be changed; it will always remain true.

See also isEnabled().

[static] void QLoggingCategory::setFilterRules(const QString &rules)

Configures which categories and message types should be enabled through a set of rules.

Example:

    QLoggingCategory::setFilterRules(QStringLiteral("driver.usb.debug=true"));

Note: The rules might be ignored if a custom category filter is installed with installFilter(), or if the user has defined the QT_LOGGING_CONF or the QT_LOGGING_RULES environment variable.

QLoggingCategory &QLoggingCategory::operator()()

Returns the object itself. This allows for both: a QLoggingCategory variable, and a factory method that returns a QLoggingCategory, to be used in qCDebug(), qCWarning(), or qCCritical() macros.

const QLoggingCategory &QLoggingCategory::operator()() const

Returns the object itself. This allows for both: a QLoggingCategory variable, and a factory method that returns a QLoggingCategory, to be used in qCDebug(), qCWarning(), or qCCritical() macros.

    QLoggingCategory category("driver.usb");
    qCCritical(category, "a critical message logged into category %s", category.categoryName());

    QLoggingCategory category("driver.usb");
    qCCritical(category) << "a critical message";

    QLoggingCategory category("driver.usb");
    qCDebug(category, "a debug message logged into category %s", category.categoryName());

    QLoggingCategory category("driver.usb");
    qCDebug(category) << "a debug message";

    QLoggingCategory category("driver.usb");
    qCInfo(category, "an informational message logged into category %s", category.categoryName());

    QLoggingCategory category("driver.usb");
    qCInfo(category) << "an informational message";

    QLoggingCategory category("driver.usb");
    qCWarning(category, "a warning message logged into category %s", category.categoryName());

    QLoggingCategory category("driver.usb");
    qCWarning(category) << "a warning message";