QDebug Class

The QDebug class provides an output stream for debugging information. More...

Header: #include <QDebug>
qmake: QT += core

Public Functions

QDebug(QIODevice *device)
QDebug(QString *string)
QDebug(QtMsgType type)
QDebug(const QDebug &other)
~QDebug()
bool autoInsertSpaces() const
QDebug &maybeQuote(char c = '"')
QDebug &maybeSpace()
QDebug &noquote()
QDebug &nospace()
QDebug &quote()
QDebug &resetFormat()
void setAutoInsertSpaces(bool b)
void setVerbosity(int verbosityLevel)
QDebug &space()
void swap(QDebug &other)
int verbosity() const
QDebug &operator<<(QChar t)
QDebug &operator<<(bool t)
QDebug &operator<<(char t)
QDebug &operator<<(signed short i)
QDebug &operator<<(unsigned short i)
QDebug &operator<<(char16_t t)
QDebug &operator<<(char32_t t)
QDebug &operator<<(signed int i)
QDebug &operator<<(unsigned int i)
QDebug &operator<<(signed long l)
QDebug &operator<<(unsigned long l)
QDebug &operator<<(qint64 i)
QDebug &operator<<(quint64 i)
QDebug &operator<<(float f)
QDebug &operator<<(double f)
QDebug &operator<<(const char *s)
QDebug &operator<<(const QString &s)
QDebug &operator<<(const QStringRef &s)
QDebug &operator<<(QStringView s)
QDebug &operator<<(QLatin1String s)
QDebug &operator<<(const QByteArray &b)
QDebug &operator<<(const void *p)
QDebug &operator=(const QDebug &other)
QDebug operator<<(QDebug stream, const QList<T> &list)
QDebug operator<<(QDebug stream, const std::list<T, Alloc> &list)
QDebug operator<<(QDebug stream, const QVector<T> &vector)
QDebug operator<<(QDebug stream, const std::vector<T, Alloc> &vector)
QDebug operator<<(QDebug stream, const QSet<T> &set)
QDebug operator<<(QDebug stream, const QMap<Key, T> &map)
QDebug operator<<(QDebug stream, const std::map<Key, T, Compare, Alloc> &map)
QDebug operator<<(QDebug stream, const std::multimap<Key, T, Compare, Alloc> &map)
QDebug operator<<(QDebug stream, const QHash<Key, T> &hash)
QDebug operator<<(QDebug stream, const QPair<T1, T2> &pair)
QDebug operator<<(QDebug stream, const QFlags<T> &flag)

Detailed Description

The QDebug class provides an output stream for debugging information.

QDebug is used whenever the developer needs to write out debugging or tracing information to a device, file, string or console.

Basic Use

In the common case, it is useful to call the qDebug() function to obtain a default QDebug object to use for writing debugging information.

    qDebug() << "Date:" << QDate::currentDate();
    qDebug() << "Types:" << QString("String") << QChar('x') << QRect(0, 10, 50, 40);
    qDebug() << "Custom coordinate type:" << coordinate;

This constructs a QDebug object using the constructor that accepts a QtMsgType value of QtDebugMsg. Similarly, the qWarning(), qCritical() and qFatal() functions also return QDebug objects for the corresponding message types.

The class also provides several constructors for other situations, including a constructor that accepts a QFile or any other QIODevice subclass that is used to write debugging information to files and other devices. The constructor that accepts a QString is used to write to a string for display or serialization.

Formatting Options

QDebug formats output so that it's easily readable. It automatically adds spaces between arguments, and adds quotes around QString, QByteArray, QChar arguments.

You can tweak these options through the space(), nospace() and quote(), noquote() methods. Furthermore, QTextStream manipulators can be piped into a QDebug stream.

QDebugStateSaver limits changes to the formatting to the current scope. resetFormat() resets the options to the default ones.

Writing Custom Types to a Stream

Many standard types can be written to QDebug objects, and Qt provides support for most Qt value types. To add support for custom types, you need to implement a streaming operator, as in the following example:

QDebug operator<<(QDebug debug, const Coordinate &c)
{
    QDebugStateSaver saver(debug);
    debug.nospace() << '(' << c.x() << ", " << c.y() << ')';

    return debug;
}

This is described in the Debugging Techniques and Creating Custom Qt Types documents.

Member Function Documentation

QDebug::QDebug(QIODevice *device)

Constructs a debug stream that writes to the given device.

QDebug::QDebug(QString *string)

Constructs a debug stream that writes to the given string.

QDebug::QDebug(QtMsgType type)

Constructs a debug stream that writes to the handler for the message type specified by type.

QDebug::QDebug(const QDebug &other)

Constructs a copy of the other debug stream.

QDebug::~QDebug()

Flushes any pending data to be written and destroys the debug stream.

bool QDebug::autoInsertSpaces() const

Returns true if this QDebug instance will automatically insert spaces between writes.

This function was introduced in Qt 5.0.

See also setAutoInsertSpaces() and QDebugStateSaver.

QDebug &QDebug::maybeQuote(char c = '"')

Writes a character c to the debug stream, depending on the current setting for automatic insertion of quotes, and returns a reference to the stream.

The default character is a double quote ".

This function was introduced in Qt 5.4.

See also quote() and noquote().

QDebug &QDebug::maybeSpace()

Writes a space character to the debug stream, depending on the current setting for automatic insertion of spaces, and returns a reference to the stream.

See also space() and nospace().

QDebug &QDebug::noquote()

Disables automatic insertion of quotation characters around QChar, QString and QByteArray contents and returns a reference to the stream.

When quoting is disabled, these types are printed without quotation characters and without escaping of non-printable characters.

This function was introduced in Qt 5.4.

See also quote() and maybeQuote().

QDebug &QDebug::nospace()

Disables automatic insertion of spaces and returns a reference to the stream.

See also space() and maybeSpace().

QDebug &QDebug::quote()

Enables automatic insertion of quotation characters around QChar, QString and QByteArray contents and returns a reference to the stream.

Quoting is enabled by default.

This function was introduced in Qt 5.4.

See also noquote() and maybeQuote().

QDebug &QDebug::resetFormat()

Resets the stream formatting options, bringing it back to its original constructed state.

This function was introduced in Qt 5.4.

See also space() and quote().

void QDebug::setAutoInsertSpaces(bool b)

Enables automatic insertion of spaces between writes if b is true; otherwise automatic insertion of spaces is disabled.

This function was introduced in Qt 5.0.

See also autoInsertSpaces() and QDebugStateSaver.

void QDebug::setVerbosity(int verbosityLevel)

Sets the verbosity of the stream to verbosityLevel.

The allowed range is from 0 to 7. The default value is 2.

This function was introduced in Qt 5.6.

See also verbosity().

QDebug &QDebug::space()

Writes a space character to the debug stream and returns a reference to the stream.

The stream remembers that automatic insertion of spaces is enabled for future writes.

See also nospace() and maybeSpace().

void QDebug::swap(QDebug &other)

Swaps this debug stream instance with other. This function is very fast and never fails.

This function was introduced in Qt 5.0.

int QDebug::verbosity() const

Returns the verbosity of the debug stream.

Streaming operators can check the value to decide whether verbose output is desired and print more information depending on the level. Higher values indicate that more information is desired.

The allowed range is from 0 to 7. The default value is 2.

This function was introduced in Qt 5.6.

See also setVerbosity().

QDebug &QDebug::operator<<(QChar t)

Writes the character, t, to the stream and returns a reference to the stream. Normally, QDebug prints control characters and non-US-ASCII characters as their C escape sequences or their Unicode value (\u1234). To print non-printable characters without transformation, enable the noquote() functionality, but note that some QDebug backends may not be 8-bit clean and may not be able to represent t.

QDebug &QDebug::operator<<(bool t)

Writes the boolean value, t, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(char t)

Writes the character, t, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(signed short i)

Writes the signed short integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(unsigned short i)

Writes then unsigned short integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(char16_t t)

Writes the UTF-16 character, t, to the stream and returns a reference to the stream.

This function was introduced in Qt 5.5.

QDebug &QDebug::operator<<(char32_t t)

Writes the UTF-32 character, t, to the stream and returns a reference to the stream.

This function was introduced in Qt 5.5.

QDebug &QDebug::operator<<(signed int i)

Writes the signed integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(unsigned int i)

Writes then unsigned integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(signed long l)

Writes the signed long integer, l, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(unsigned long l)

Writes then unsigned long integer, l, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(qint64 i)

Writes the signed 64-bit integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(quint64 i)

Writes then unsigned 64-bit integer, i, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(float f)

Writes the 32-bit floating point number, f, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(double f)

Writes the 64-bit floating point number, f, to the stream and returns a reference to the stream.

QDebug &QDebug::operator<<(const char *s)

Writes the '\0'-terminated string, s, to the stream and returns a reference to the stream. The string is never quoted nor transformed to the output, but note that some QDebug backends might not be 8-bit clean.

QDebug &QDebug::operator<<(const QString &s)

Writes the string, s, to the stream and returns a reference to the stream. Normally, QDebug prints the string inside quotes and transforms non-printable characters to their Unicode values (\u1234).

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

Output examples:

QString s;

s = "a";
qDebug().noquote() << s;    // prints: a
qDebug() << s;              // prints: "a"

s = "\"a\r\n\"";
qDebug() << s;              // prints: "\"a\r\n\""

s = "\033";                 // escape character
qDebug() << s;              // prints: "\u001B"

s = "\u00AD";               // SOFT HYPHEN
qDebug() << s;              // prints: "\u00AD"

s = "\u00E1";               // LATIN SMALL LETTER A WITH ACUTE
qDebug() << s;              // prints: "á"

s = "a\u0301";              // "a" followed by COMBINING ACUTE ACCENT
qDebug() << s;              // prints: "á";

s = "\u0430\u0301";         // CYRILLIC SMALL LETTER A followed by COMBINING ACUTE ACCENT
qDebug() << s;              // prints: "а́"

QDebug &QDebug::operator<<(const QStringRef &s)

Writes the string, s, to the stream and returns a reference to the stream. Normally, QDebug prints the string inside quotes and transforms non-printable characters to their Unicode values (\u1234).

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

See the QString overload for examples.

QDebug &QDebug::operator<<(QStringView s)

Writes the string view, s, to the stream and returns a reference to the stream. Normally, QDebug prints the string inside quotes and transforms non-printable characters to their Unicode values (\u1234).

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

See the QString overload for examples.

This function was introduced in Qt 5.10.

QDebug &QDebug::operator<<(QLatin1String s)

Writes the string, s, to the stream and returns a reference to the stream. Normally, QDebug prints the string inside quotes and transforms non-printable characters to their Unicode values (\u1234).

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

See the QString overload for examples.

QDebug &QDebug::operator<<(const QByteArray &b)

Writes the byte array, b, to the stream and returns a reference to the stream. Normally, QDebug prints the array inside quotes and transforms control or non-US-ASCII characters to their C escape sequences (\xAB). This way, the output is always 7-bit clean and the string can be copied from the output and pasted back into C++ sources, if necessary.

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

Output examples:

QByteArray ba;

ba = "a";
qDebug().noquote() << ba;    // prints: a
qDebug() << ba;              // prints: "a"

ba = "\"a\r\n\"";
qDebug() << ba;              // prints: "\"a\r\n\""

ba = "\033";                 // escape character
qDebug() << ba;              // prints: "\x1B"

ba = "\xC3\xA1";
qDebug() << ba;              // prints: "\xC3\xA1"

ba = QByteArray("a\0b", 3);
qDebug() << ba               // prints: "\a\x00""b"

Note how QDebug needed to close and reopen the string in the way C and C++ languages concatenate string literals so that the letter 'b' is not interpreted as part of the previous hexadecimal escape sequence.

QDebug &QDebug::operator<<(const void *p)

Writes a pointer, p, to the stream and returns a reference to the stream.

QDebug &QDebug::operator=(const QDebug &other)

Assigns the other debug stream to this stream and returns a reference to this stream.

Related Non-Members

QDebug operator<<(QDebug stream, const QList<T> &list)

Writes the contents of list to stream. T needs to support streaming into QDebug.

QDebug operator<<(QDebug stream, const std::list<T, Alloc> &list)

Writes the contents of list to stream. T needs to support streaming into QDebug.

This function was introduced in Qt 5.7.

QDebug operator<<(QDebug stream, const QVector<T> &vector)

Writes the contents of vector to stream. T needs to support streaming into QDebug.

QDebug operator<<(QDebug stream, const std::vector<T, Alloc> &vector)

Writes the contents of vector to stream. T needs to support streaming into QDebug.

This function was introduced in Qt 5.7.

QDebug operator<<(QDebug stream, const QSet<T> &set)

Writes the contents of set to stream. T needs to support streaming into QDebug.

QDebug operator<<(QDebug stream, const QMap<Key, T> &map)

Writes the contents of map to stream. Both Key and T need to support streaming into QDebug.

QDebug operator<<(QDebug stream, const std::map<Key, T, Compare, Alloc> &map)

Writes the contents of map to stream. Both Key and T need to support streaming into QDebug.

This function was introduced in Qt 5.7.

QDebug operator<<(QDebug stream, const std::multimap<Key, T, Compare, Alloc> &map)

Writes the contents of map to stream. Both Key and T need to support streaming into QDebug.

This function was introduced in Qt 5.7.

QDebug operator<<(QDebug stream, const QHash<Key, T> &hash)

Writes the contents of hash to stream. Both Key and T need to support streaming into QDebug.

QDebug operator<<(QDebug stream, const QPair<T1, T2> &pair)

Writes the contents of pair to stream. Both T1 and T2 need to support streaming into QDebug.

QDebug operator<<(QDebug stream, const QFlags<T> &flag)

Writes flag to stream.

This function was introduced in Qt 4.7.

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