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QLoggingCategory Class

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

Header: #include <QLoggingCategory>
qmake: QT += core
Since: Qt 5.2

Public Types

typedef CategoryFilter

Public Functions

QLoggingCategory(const char * category)
QLoggingCategory(const char * category, QtMsgType enableForLevel)
~QLoggingCategory()
const char * categoryName() const
bool isCriticalEnabled() const
bool isDebugEnabled() const
bool isEnabled(QtMsgType msgtype) const
bool isWarningEnabled() const
void setEnabled(QtMsgType type, bool enable)
QLoggingCategory & operator()()
const QLoggingCategory & operator()() const

Static Public Members

QLoggingCategory * defaultCategory()
CategoryFilter installFilter(CategoryFilter filter)
void setFilterRules(const QString & rules)

Macros

Q_DECLARE_LOGGING_CATEGORY( name)
Q_LOGGING_CATEGORY( name, string)
Q_LOGGING_CATEGORY( name, string, msgType)
qCCritical( category)
qCCritical( category, const char * message, ...)
qCDebug( category)
qCDebug( category, const char * message, ...)
qCWarning( category)
qCWarning( category, const char * message, ...)

Detailed Description

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

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. Whether a message type is enabled or not can be checked with the isDebugEnabled(), isWarningEnabled(), and isCriticalEnabled() methods.

All objects are meant to be configured by a common registry (see also Configuring Categories). Different objects can also represent the same category. It is therefore not recommended to export objects across module boundaries, nor to manipulate the objects directly, nor 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")

Checking Category Configuration

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

Note: The qCDebug(), qCWarning(), 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)

will log messages of type QtWarningMsg, QtCriticalMsg, QtFatalMsg, but will ignore messages of type QtDebugMsg.

If no argument is passed, all messages will be logged.

Configuring Categories

The default configuration of categories can be overridden either by setting logging rules, or by installing a custom filter.

Logging Rules

Logging rules allow logging for categories to be enabled or disabled 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 as the first or last character (or at both positions). The optional <type> must be either debug, warning, or critical. Lines that do not 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(). Since Qt 5.3, logging rules can also be set in the QT_LOGGING_RULES environment variable, and are automatically loaded from the [Rules] section of a logging configuration file. Such 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

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

Order of evaluation:

  • QtProject/qtlogging.ini
  • setFilterRules()
  • QT_LOGGING_CONF
  • QT_LOGGING_RULES

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

Set the QT_LOGGING_DEBUG environment variable to see from where logging rules are loaded.

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)

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".

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".

This function was introduced in Qt 5.4.

QLoggingCategory::​~QLoggingCategory()

Destructs 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 e.g. by qDebug(), qWarning(), qCritical(), qFatal().

Note: The returned pointer may be null during destruction of static objects.

Note: Ownership of the category is not transferred, do not delete the returned pointer.

[static] CategoryFilter QLoggingCategory::​installFilter(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().

The filter might be called concurrently from different threads, and therefore has to be reentrant.

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);
}

An alternative way of configuring the default filter is via setFilterRules().

bool QLoggingCategory::​isCriticalEnabled() const

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

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

bool QLoggingCategory::​isDebugEnabled() const

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

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

bool QLoggingCategory::​isEnabled(QtMsgType msgtype) const

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

bool QLoggingCategory::​isWarningEnabled() const

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

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

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

Changes the message type type for the category to enable.

Note: Changes only affect the current QLoggingCategory object, and won't change the settings of other objects for the same category name. Use either setFilterRules() or installFilter() to change the configuration globally.

Note: QtFatalMsg cannot be changed. It will always return true.

See also isEnabled().

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

Configures which categories and message types should be enabled through a 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 defined QT_LOGGING_CONF or QT_LOGGING_RULES environment variable.

QLoggingCategory & QLoggingCategory::​operator()()

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

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

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

Macro Documentation

Q_DECLARE_LOGGING_CATEGORY( name)

Declares a logging category name. The macro can be used to declare a common logging category shared in different parts of the program.

This macro must be used outside of a class or method.

This function was introduced in Qt 5.2.

See also Q_LOGGING_CATEGORY().

Q_LOGGING_CATEGORY( name, string)

Defines a logging category name, and makes it configurable under the string identifier. By default, all message types are enabled.

Only one translation unit in a library or executable can define a category with a specific name.

This macro must be used outside of a class or method.

This function was introduced in Qt 5.2.

See also Q_DECLARE_LOGGING_CATEGORY().

Q_LOGGING_CATEGORY( name, string, msgType)

Defines a logging category name, and makes it configurable under the string identifier. By default, messages of QtMsgType msgType and more severe are enabled, types with a lower severity are disabled.

Only one translation unit in a library or executable can define a category with a specific name.

This macro must be used outside of a class or method. It is only defined if variadic macros are supported.

This function was introduced in Qt 5.4.

See also Q_DECLARE_LOGGING_CATEGORY().

qCCritical( category)

Returns an output stream for critical messages in the logging category category.

The macro expands to code that checks whether QLoggingCategory::isCriticalEnabled() evaluates to true. If so, the stream arguments are processed and sent to the message handler.

Example:

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

Note: Arguments are not processed if critical output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.2.

See also qCritical().

qCCritical( category, const char * message, ...)

Logs a critical message message in the logging category category. message might contain place holders that are replaced by additional arguments, similar to the C printf() function.

Example:

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

Note: Arguments might not be processed if critical output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.3.

See also qCritical().

qCDebug( category)

Returns an output stream for debug messages in the logging category category.

The macro expands to code that checks whether QLoggingCategory::isDebugEnabled() evaluates to true. If so, the stream arguments are processed and sent to the message handler.

Example:

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

Note: Arguments are not processed if debug output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.2.

See also qDebug().

qCDebug( category, const char * message, ...)

Logs a debug message message in the logging category category. message might contain place holders that are replaced by additional arguments, similar to the C printf() function.

Example:

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

Note: Arguments might not be processed if debug output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.3.

See also qDebug().

qCWarning( category)

Returns an output stream for warning messages in the logging category category.

The macro expands to code that checks whether QLoggingCategory::isWarningEnabled() evaluates to true. If so, the stream arguments are processed and sent to the message handler.

Example:

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

Note: Arguments are not processed if warning output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.2.

See also qWarning().

qCWarning( category, const char * message, ...)

Logs a warning message message in the logging category category. message might contain place holders that are replaced by additional arguments, similar to the C printf() function.

Example:

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

Note: Arguments might not be processed if warning output for the category is not enabled, so do not rely on any side effects.

This function was introduced in Qt 5.3.

See also qWarning().

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