QLoggingCategory Class

// in a header
Q_DECLARE_LOGGING_CATEGORY(driverUsb)

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

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

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

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

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

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

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

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

void myCategoryFilter(QLoggingCategory *);

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);
}

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

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. The implicitly defined QLoggingCategory object is created on first use, in a thread-safe manner.

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. The implicitly defined QLoggingCategory object is created on first use, in a thread-safe manner.

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.

Note: This function is thread-safe.

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.

Note: This function is thread-safe.

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.

Note: This function is thread-safe.

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.

Note: This function is thread-safe.

This function was introduced in Qt 5.3.

See also qDebug().

qCInfo(category)

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

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

Example:

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

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

Note: This function is thread-safe.

This function was introduced in Qt 5.5.

See also qInfo().

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

Logs an informational 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");
    qCInfo(category, "an informational 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.

Note: This function is thread-safe.

This function was introduced in Qt 5.5.

See also qInfo().

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.

Note: This function is thread-safe.

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.

Note: This function is thread-safe.

This function was introduced in Qt 5.3.

See also qWarning().