QCborStreamWriter Class

The QCborStreamWriter class is a simple CBOR encoder operating on a one-way stream. More...

Header: #include <QCborStreamWriter>
qmake: QT += core
Since: Qt 5.12

This class was introduced in Qt 5.12.

Note: All functions in this class are reentrant.

Public Functions

QCborStreamWriter(const QCborStreamWriter &)
QCborStreamWriter(QByteArray *data)
QCborStreamWriter(QIODevice *device)
QCborStreamWriter &operator=(const QCborStreamWriter &)
~QCborStreamWriter()
void append(quint64 u)
void append(qint64 i)
void append(QCborNegativeInteger n)
void append(const QByteArray &ba)
void append(QLatin1String str)
void append(QStringView str)
void append(QCborTag tag)
void append(QCborKnownTags tag)
void append(QCborSimpleType st)
void append(std::nullptr_t)
void append(qfloat16 f)
void append(float f)
void append(double d)
void append(bool b)
void append(const char *str, qsizetype size = -1)
void appendByteString(const char *data, qsizetype len)
void appendNull()
void appendTextString(const char *utf8, qsizetype len)
void appendUndefined()
QIODevice *device() const
bool endArray()
bool endMap()
void setDevice(QIODevice *device)
void startArray()
void startArray(quint64 count)
void startMap()
void startMap(quint64 count)

Detailed Description

This class can be used to quickly encode a stream of CBOR content directly to either a QByteArray or QIODevice. CBOR is the Concise Binary Object Representation, a very compact form of binary data encoding that is compatible with JSON. It was created by the IETF Constrained RESTful Environments (CoRE) WG, which has used it in many new RFCs. It is meant to be used alongside the CoAP protocol.

QCborStreamWriter provides a StAX-like API, similar to that of QXmlStreamWriter. It is rather low-level and requires a bit of knowledge of CBOR encoding. For a simpler API, see QCborValue and especially the encoding function QCborValue::toCbor().

The typical use of QCborStreamWriter is to create the object on the target QByteArray or QIODevice, then call one of the append() overloads with the desired type to be encoded. To create arrays and maps, QCborStreamWriter provides startArray() and startMap() overloads, which must be terminated by the corresponding endArray() and endMap() functions.

The following example encodes the equivalent of this JSON content:

{ "label": "journald", "autoDetect": false, "condition": "libs.journald", "output": [ "privateFeature" ] }

   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();

CBOR support

QCborStreamWriter supports all CBOR features required to create canonical and strict streams. It implements almost all of the features specified in RFC 7049.

The following table lists the CBOR features that QCborStreamWriter supports.

FeatureSupport
Unsigned numbersYes (full range)
Negative numbersYes (full range)
Byte stringsYes
Text stringsYes
Chunked stringsNo
TagsYes (arbitrary)
BooleansYes
NullYes
UndefinedYes
Arbitrary simple valuesYes
Half-precision float (16-bit)Yes
Single-precision float (32-bit)Yes
Double-precision float (64-bit)Yes
Infinities and NaN floating pointYes
Determinate-length arrays and mapsYes
Indeterminate-length arrays and mapsYes
Map key types other than strings and integersYes (arbitrary)

Canonical CBOR encoding

Canonical CBOR encoding is defined by Section 3.9 of RFC 7049. Canonical encoding is not a requirement for Qt's CBOR decoding functionality, but it may be required for some protocols. In particular, protocols that require the ability to reproduce the same stream identically may require this.

In order to be considered "canonical", a CBOR stream must meet the following requirements:

  • Integers must be as small as possible. QCborStreamWriter always does this (no user action is required and it is not possible to write overlong integers).
  • Array, map and string lengths must be as short as possible. As above, QCborStreamWriter automatically does this.
  • Arrays, maps and strings must use explicit length. QCborStreamWriter always does this for strings; for arrays and maps, be sure to call startArray() and startMap() overloads with explicit length.
  • Keys in every map must be sorted in ascending order. QCborStreamWriter offers no help in this item: the developer must ensure that before calling append() for the map pairs.
  • Floating point values should be as small as possible. QCborStreamWriter will not convert floating point values; it is up to the developer to perform this check prior to calling append() (see those functions' examples).

Strict CBOR mode

Strict mode is defined by Section 3.10 of RFC 7049. As for Canonical encoding above, QCborStreamWriter makes it possible to create strict CBOR streams, but does not require them or validate that the output is so.

  • Keys in a map must be unique. QCborStreamWriter performs no validation of map keys.
  • Tags may be required to be paired only with the correct types, according to their specification. QCborStreamWriter performs no validation of tag usage.
  • Text Strings must be properly-encoded UTF-8. QCborStreamWriter always writes proper UTF-8 for strings added with append(), but performs no validation for strings added with appendTextString().

Invalid CBOR stream

It is also possible to misuse QCborStreamWriter and produce invalid CBOR streams that will fail to be decoded by a receiver. The following actions will produce invalid streams:

See also QCborStreamReader, QCborValue, and QXmlStreamWriter.

Member Function Documentation

QCborStreamWriter::QCborStreamWriter(const QCborStreamWriter &)

Default constructs an instance of QCborStreamWriter.

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::operator=(const QCborStreamWriter &)

Copy-assignment operator.

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));

Note: Using Simple Types for which there is no specification can lead to validation errors by the remote receiver. In addition, simple type values 24 through 31 (inclusive) are reserved and must not be used.

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().

© 2018 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.