QCborStreamWriter Class

    writer.startMap(4);    // 4 elements in the map

    writer.append(QLatin1String("label"));
    writer.append(QLatin1String("journald"));

    writer.append(QLatin1String("autoDetect"));
    writer.append(false);

    writer.append(QLatin1String("condition"));
    writer.append(QLatin1String("libs.journald"));

    writer.append(QLatin1String("output"));
    writer.startArray(1);
    writer.append(QLatin1String("privateFeature"));
    writer.endArray();

    writer.endMap();

Member Function Documentation

QCborStreamWriter::QCborStreamWriter(QByteArray *data)

Creates a QCborStreamWriter object that will append the stream to data. All streaming is done immediately to the byte array, without the need for flushing any buffers.

The following example writes a number to a byte array then returns it.

    QByteArray encodedNumber(qint64 value)
    {
        QByteArray ba;
        QCborStreamWriter writer(&ba);
        writer.append(value);
        return ba;
    }

QCborStreamWriter does not take ownership of data.

QCborStreamWriter::QCborStreamWriter(QIODevice *device)

Creates a QCborStreamWriter object that will write the stream to device. The device must be opened before the first append() call is made. This constructor can be used with any class that derives from QIODevice, such as QFile, QProcess or QTcpSocket.

QCborStreamWriter has no buffering, so every append() call will result in one or more calls to the device's write() method.

The following example writes an empty map to a file:

    QFile f("output", QIODevice::WriteOnly);
    QCborStreamWriter writer(&f);
    writer.startMap(0);
    writer.endMap();

QCborStreamWriter does not take ownership of device.

See also device() and setDevice().

QCborStreamWriter::~QCborStreamWriter()

Destroys this QCborStreamWriter object and frees any resources associated.

QCborStreamWriter does not perform error checking to see if all required items were written to the stream prior to the object being destroyed. It is the programmer's responsibility to ensure that it was done.

void QCborStreamWriter::append(quint64 u)

This is an overloaded function.

Appends the 64-bit unsigned value u to the CBOR stream, creating a CBOR Unsigned Integer value. In the following example, we write the values 0, 2³² and UINT64_MAX:

    writer.append(0U);
    writer.append(Q_UINT64_C(4294967296));
    writer.append(std::numeric_limits<quint64>::max());

See also QCborStreamReader::isUnsignedInteger() and QCborStreamReader::toUnsignedInteger().

void QCborStreamWriter::append(qint64 i)

This is an overloaded function.

Appends the 64-bit signed value i to the CBOR stream. This will create either a CBOR Unsigned Integer or CBOR NegativeInteger value based on the sign of the parameter. In the following example, we write the values 0, -1, 2³² and INT64_MAX:

    writer.append(0);
    writer.append(-1);
    writer.append(Q_INT64_C(4294967296));
    writer.append(std::numeric_limits<qint64>::max());

See also QCborStreamReader::isInteger() and QCborStreamReader::toInteger().

void QCborStreamWriter::append(QCborNegativeInteger n)

This is an overloaded function.

Appends the 64-bit negative value n to the CBOR stream. QCborNegativeInteger is a 64-bit enum that holds the absolute value of the negative number we want to write. If n is zero, the value written will be equivalent to 2⁶⁴ (that is, -18,446,744,073,709,551,616).

In the following example, we write the values -1, -2³² and INT64_MIN:

    writer.append(QCborNegativeInteger(1));
    writer.append(QCborNegativeInteger(Q_INT64_C(4294967296)));
    writer.append(QCborNegativeInteger(-quint64(std::numeric_limits<qint64>::min())));

Note how this function can be used to encode numbers that cannot fit a standard computer's 64-bit signed integer like qint64. That is, if n is larger than std::numeric_limits<qint64>::max() or is 0, this will represent a negative number smaller than std::numeric_limits<qint64>::min().

See also QCborStreamReader::isNegativeInteger() and QCborStreamReader::toNegativeInteger().

void QCborStreamWriter::append(const QByteArray &ba)

This is an overloaded function.

Appends the byte array ba to the stream, creating a CBOR Byte String value. QCborStreamWriter will attempt to write the entire string in one chunk.

The following example will load and append the contents of a file to the stream:

    void writeFile(QCborStreamWriter &writer, const QString &fileName)
    {
        QFile f(fileName);
        if (f.open(QIODevice::ReadOnly))
            writer.append(f.readAll());
    }

As the example shows, unlike JSON, CBOR requires no escaping for binary content.

See also appendByteString(), QCborStreamReader::isByteArray(), and QCborStreamReader::readByteArray().

void QCborStreamWriter::append(QLatin1String str)

This is an overloaded function.

Appends the text string str to the stream, creating a CBOR Text String value. QCborStreamWriter will attempt to write the entire string in one chunk.

The following example appends a simple string to the stream:

    writer.append(QLatin1String("Hello, World"));

Performance note: CBOR requires that all Text Strings be encoded in UTF-8, so this function will iterate over the characters in the string to determine whether the contents are US-ASCII or not. If the string is found to contain characters outside of US-ASCII, it will allocate memory and convert to UTF-8. If this check is unnecessary, use appendTextString() instead.

See also QCborStreamReader::isString() and QCborStreamReader::readString().

void QCborStreamWriter::append(QStringView str)

This is an overloaded function.

Appends the text string str to the stream, creating a CBOR Text String value. QCborStreamWriter will attempt to write the entire string in one chunk.

The following example writes an arbitrary QString to the stream:

    void writeString(QCborStreamWriter &writer, const QString &str)
    {
        writer.append(str);
    }

See also QCborStreamReader::isString() and QCborStreamReader::readString().

void QCborStreamWriter::append(QCborTag tag)

This is an overloaded function.

Appends the CBOR tag tag to the stream, creating a CBOR Tag value. All tags must be followed by another type which they provide meaning for.

In the following example, we append a CBOR Tag 36 (Regular Expression) and a QRegularExpression's pattern to the stream:

    void writeRxPattern(QCborStreamWriter &writer, const QRegularExpression &rx)
    {
        writer.append(QCborTag(36));
        writer.append(rx.pattern());
    }

See also QCborStreamReader::isTag() and QCborStreamReader::toTag().

void QCborStreamWriter::append(QCborKnownTags tag)

This is an overloaded function.

Appends the CBOR tag tag to the stream, creating a CBOR Tag value. All tags must be followed by another type which they provide meaning for.

In the following example, we append a CBOR Tag 1 (Unix time_t) and an integer representing the current time to the stream, obtained using the time() function:

    void writeCurrentTime(QCborStreamWriter &writer)
    {
        writer.append(QCborKnownTags::UnixTime_t);
        writer.append(time(nullptr));
    }

See also QCborStreamReader::isTag() and QCborStreamReader::toTag().

void QCborStreamWriter::append(QCborSimpleType st)

This is an overloaded function.

Appends the CBOR simple type st to the stream, creating a CBOR Simple Type value. In the following example, we write the simple type for Null as well as for type 32, which Qt has no support for.

   writer.append(QCborSimpleType::Null);
   writer.append(QCborSimpleType(32));

See also QCborStreamReader::isSimpleType() and QCborStreamReader::toSimpleType().

void QCborStreamWriter::append(std::nullptr_t)

This is an overloaded function.

Appends a CBOR Null value to the stream. This function is equivalent to (and implemented as): The parameter is ignored.

    writer.append(QCborSimpleType::Null);

See also appendNull(), append(QCborSimpleType), and QCborStreamReader::isNull().

void QCborStreamWriter::append(qfloat16 f)

This is an overloaded function.

Appends the floating point number f to the stream, creating a CBOR 16-bit Half-Precision Floating Point value. The following code can be used to convert a C++ float to qfloat16 if there's no loss of precision and append it, or instead append the float.

    void writeFloat(QCborStreamWriter &writer, float f)
    {
        qfloat16 f16 = f;
        if (qIsNaN(f) || f16 == f)
            writer.append(f16);
        else
            writer.append(f);
    }

See also QCborStreamReader::isFloat16() and QCborStreamReader::toFloat16().

void QCborStreamWriter::append(float f)

This is an overloaded function.

Appends the floating point number f to the stream, creating a CBOR 32-bit Single-Precision Floating Point value. The following code can be used to convert a C++ double to float if there's no loss of precision and append it, or instead append the double.

    void writeFloat(QCborStreamWriter &writer, double d)
    {
        float f = d;
        if (qIsNaN(d) || d == f)
            writer.append(f);
        else
            writer.append(d);
    }

See also QCborStreamReader::isFloat() and QCborStreamReader::toFloat().

void QCborStreamWriter::append(double d)

This is an overloaded function.

Appends the floating point number d to the stream, creating a CBOR 64-bit Double-Precision Floating Point value. QCborStreamWriter always appends the number as-is, performing no check for whether the number is the canonical form for NaN, an infinite, whether it is denormal or if it could be written with a shorter format.

The following code performs all those checks, except for the denormal one, which is expected to be taken into account by the system FPU or floating point emulation directly.

    void writeDouble(QCborStreamWriter &writer, double d)
    {
        float f;
        if (qIsNaN(d)) {
            writer.append(qfloat16(qQNaN()));
        } else if (qIsInf(d)) {
            writer.append(d < 0 ? -qInf() : qInf());
        } else if ((f = d) == d) {
            qfloat16 f16 = f;
            if (f16 == f)
                writer.append(f16);
            else
                writer.append(f);
        } else {
            writer.append(d);
        }
    }

Determining if a double can be converted to an integral with no loss of precision is left as an exercise to the reader.

See also QCborStreamReader::isDouble() and QCborStreamReader::toDouble().

void QCborStreamWriter::append(bool b)

This is an overloaded function.

Appends the boolean value b to the stream, creating either a CBOR False value or a CBOR True value. This function is equivalent to (and implemented as):

    writer.append(b ? QCborSimpleType::True : QCborSimpleType::False);

See also appendNull(), appendUndefined(), QCborStreamReader::isBool(), and QCborStreamReader::toBool().

void QCborStreamWriter::append(const char *str, qsizetype size = -1)

This is an overloaded function.

Appends size bytes of text starting from str to the stream, creating a CBOR Text String value. QCborStreamWriter will attempt to write the entire string in one chunk. If size is -1, this function will write strlen(\a str) bytes.

The string pointed to by str is expected to be properly encoded UTF-8. QCborStreamWriter performs no validation that this is the case.

Unlike the QLatin1String overload of append(), this function is not limited to 2 GB. However, note that neither QCborStreamReader nor QCborValue support reading CBOR streams with text strings larger than 2 GB.

See also append(QLatin1String), append(QStringView), QCborStreamReader::isString(), and QCborStreamReader::readString().

void QCborStreamWriter::appendByteString(const char *data, qsizetype len)

Appends len bytes of data starting from data to the stream, creating a CBOR Byte String value. QCborStreamWriter will attempt to write the entire string in one chunk.

Unlike the QByteArray overload of append(), this function is not limited by QByteArray's size limits. However, note that neither QCborStreamReader::readByteArray() nor QCborValue support reading CBOR streams with byte arrays larger than 2 GB.

See also append(), appendTextString(), QCborStreamReader::isByteArray(), and QCborStreamReader::readByteArray().

void QCborStreamWriter::appendNull()

Appends a CBOR Null value to the stream. This function is equivalent to (and implemented as):

    writer.append(QCborSimpleType::Null);

See also append(std::nullptr_t), append(QCborSimpleType), and QCborStreamReader::isNull().

void QCborStreamWriter::appendTextString(const char *utf8, qsizetype len)

Appends len bytes of text starting from utf8 to the stream, creating a CBOR Text String value. QCborStreamWriter will attempt to write the entire string in one chunk.

The string pointed to by utf8 is expected to be properly encoded UTF-8. QCborStreamWriter performs no validation that this is the case.

Unlike the QLatin1String overload of append(), this function is not limited to 2 GB. However, note that neither QCborStreamReader::readString() nor QCborValue support reading CBOR streams with text strings larger than 2 GB.

See also append(QLatin1String), append(QStringView), QCborStreamReader::isString(), and QCborStreamReader::readString().

void QCborStreamWriter::appendUndefined()

Appends a CBOR Undefined value to the stream. This function is equivalent to (and implemented as):

    writer.append(QCborSimpleType::Undefined);

See also append(QCborSimpleType) and QCborStreamReader::isUndefined().

QIODevice *QCborStreamWriter::device() const

Returns the QIODevice that this QCborStreamWriter object is writing to. The device must have previously been set with either the constructor or with setDevice().

If this object was created by writing to a QByteArray, this function will return an internal instance of QBuffer, which is owned by QCborStreamWriter.

See also setDevice().

bool QCborStreamWriter::endArray()

Terminates the array started by either overload of startArray() and returns true if the correct number of elements was added to the array. This function must be called for every startArray() used.

A return of false indicates error in the application and an unrecoverable error in this stream. QCborStreamWriter also writes a warning using qWarning() if that happens.

Calling this function when the current container is not an array is also an error, though QCborStreamWriter cannot currently detect this condition.

See also startArray(), startArray(quint64), and endMap().

bool QCborStreamWriter::endMap()

Terminates the map started by either overload of startMap() and returns true if the correct number of elements was added to the array. This function must be called for every startMap() used.

A return of false indicates error in the application and an unrecoverable error in this stream. QCborStreamWriter also writes a warning using qWarning() if that happens.

Calling this function when the current container is not a map is also an error, though QCborStreamWriter cannot currently detect this condition.

See also startMap(), startMap(quint64), and endArray().

void QCborStreamWriter::setDevice(QIODevice *device)

Replaces the device or byte array that this QCborStreamWriter object is writing to with device.

See also device().

void QCborStreamWriter::startArray()

Starts a CBOR Array with indeterminate length in the CBOR stream. Each startArray() call must be paired with one endArray() call and the current CBOR element extends until the end of the array.

The array created by this function has no explicit length. Instead, its length is implied by the elements contained in it. Note, however, that use of indeterminate-length arrays is not compliant with canonical CBOR encoding.

The following example appends elements from the linked list of strings passed as input:

    void appendList(QCborStreamWriter &writer, const QLinkedList<QString> &list)
    {
        writer.startArray();
        for (const QString &s : list)
            writer.append(s);
        writer.endArray();
    }

See also startArray(quint64), endArray(), startMap(), QCborStreamReader::isArray(), and QCborStreamReader::isLengthKnown().

void QCborStreamWriter::startArray(quint64 count)

This is an overloaded function.

Starts a CBOR Array with explicit length of count items in the CBOR stream. Each startArray call must be paired with one endArray() call and the current CBOR element extends until the end of the array.

The array created by this function has an explicit length and therefore exactly count items must be added to the CBOR stream. Adding fewer or more items will result in failure during endArray() and the CBOR stream will be corrupt. However, explicit-length arrays are required by canonical CBOR encoding.

The following example appends all strings found in the QStringList passed as input:

    void appendList(QCborStreamWriter &writer, const QStringList &list)
    {
        writer.startArray(list.size());
        for (const QString &s : list)
            writer.append(s);
        writer.endArray();
    }

Size limitations: The parameter to this function is quint64, which would seem to allow up to 2⁶⁴-1 elements in the array. However, both QCborStreamWriter and QCborStreamReader are currently limited to 2³²-2 items on 32-bit systems and 2⁶⁴-2 items on 64-bit ones. Also note that QCborArray is currently limited to 2²⁷ elements in any platform.

See also startArray(), endArray(), startMap(), QCborStreamReader::isArray(), and QCborStreamReader::isLengthKnown().

void QCborStreamWriter::startMap()

Starts a CBOR Map with indeterminate length in the CBOR stream. Each startMap() call must be paired with one endMap() call and the current CBOR element extends until the end of the map.

The map created by this function has no explicit length. Instead, its length is implied by the elements contained in it. Note, however, that use of indeterminate-length maps is not compliant with canonical CBOR encoding (canonical encoding also requires keys to be unique and in sorted order).

The following example appends elements from the linked list of int and string pairs passed as input:

    void appendMap(QCborStreamWriter &writer, const QLinkedList<QPair<int, QString>> &list)
    {
        writer.startMap();
        for (const auto pair : list) {
            writer.append(pair.first)
            writer.append(pair.second);
        }
        writer.endMap();
    }

See also startMap(quint64), endMap(), startArray(), QCborStreamReader::isMap(), and QCborStreamReader::isLengthKnown().

void QCborStreamWriter::startMap(quint64 count)

This is an overloaded function.

Starts a CBOR Map with explicit length of count items in the CBOR stream. Each startMap call must be paired with one endMap() call and the current CBOR element extends until the end of the map.

The map created by this function has an explicit length and therefore exactly count pairs of items must be added to the CBOR stream. Adding fewer or more items will result in failure during endMap() and the CBOR stream will be corrupt. However, explicit-length map are required by canonical CBOR encoding.

The following example appends all strings found in the QMap passed as input:

    void appendMap(QCborStreamWriter &writer, const QMap<int, QString> &map)
    {
        writer.startMap(map.size());
        for (auto it = map.begin(); it != map.end(); ++it) {
            writer.append(it.key());
            writer.append(it.value());
        }
        writer.endMap();
    }

Size limitations: The parameter to this function is quint64, which would seem to allow up to 2⁶⁴-1 pairs in the map. However, both QCborStreamWriter and QCborStreamReader are currently limited to 2³¹-1 items on 32-bit systems and 2⁶³-1 items on 64-bit ones. Also note that QCborMap is currently limited to 2²⁶ elements in any platform.

See also startMap(), endMap(), startArray(), QCborStreamReader::isMap(), and QCborStreamReader::isLengthKnown().