QCborStreamReader

The QCborStreamReader class is a simple CBOR stream decoder, operating on either a QByteArray or QIODevice . More

Inheritance diagram of PySide2.QtCore.QCborStreamReader

New in version 5.12.

Synopsis

Functions

Detailed Description

This class can be used to decode a stream of CBOR content directly from either a QByteArray or a 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 .

QCborStreamReader provides a StAX-like API, similar to that of QXmlStreamReader . Using it requires a bit of knowledge of CBOR encoding. For a simpler API, see QCborValue and especially the decoding function fromCbor() .

Typically, one creates a QCborStreamReader by passing the source QByteArray or QIODevice as a parameter to the constructor, then pop elements off the stream if there were no errors in decoding. There are three kinds of CBOR types:

Kind

Types

Behavior

Fixed-width

Integers, Tags, Simple types, Floating point

Value is pre-parsed by QCborStreamReader , so accessor functions are const . Must call next() to advance.

Strings

Byte arrays, Text strings

Length (if known) is pre-parsed, but the string itself is not. The accessor functions are not const and may allocate memory. Once called, the accessor functions automatically advance to the next element.

Containers

Arrays, Maps

Length (if known) is pre-parsed. To access the elements, you must call enterContainer() , read all elements, then call leaveContainer() . That function advances to the next element.

So a processor function typically looks like this:

void handleStream(QCborStreamReader &reader)
{
    switch (reader.type())
    case QCborStreamReader::UnsignedInteger:
    case QCborStreamReader::NegativeInteger:
    case QCborStreamReader::SimpleType:
    case QCborStreamReader::Float16:
    case QCborStreamReader::Float:
    case QCborStreamReader::Double:
        handleFixedWidth(reader);
        reader.next();
        break;
    case QCborStreamReader::ByteArray:
    case QCborStreamReader::String:
        handleString(reader);
        break;
    case QCborStreamReader::Array:
    case QCborStreamReader::Map:
        reader.enterContainer();
        while (reader.lastError() == QCborError::NoError)
            handleStream(reader);
        if (reader.lastError() == QCborError::NoError)
            reader.leaveContainer();
    }
}

CBOR support

The following table lists the CBOR features that QCborStreamReader supports.

Feature

Support

Unsigned numbers

Yes (full range)

Negative numbers

Yes (full range)

Byte strings

Yes

Text strings

Yes

Chunked strings

Yes

Tags

Yes (arbitrary)

Booleans

Yes

Null

Yes

Undefined

Yes

Arbitrary simple values

Yes

Half-precision float (16-bit)

Yes

Single-precision float (32-bit)

Yes

Double-precision float (64-bit)

Yes

Infinities and NaN floating point

Yes

Determinate-length arrays and maps

Yes

Indeterminate-length arrays and maps

Yes

Map key types other than strings and integers

Yes (arbitrary)

Dealing with invalid or incomplete CBOR streams

QCborStreamReader is capable of detecting corrupt input on its own. The library it uses has been extensively tested against invalid input of any kind and is quite able to report errors. If any is detected, QCborStreamReader will set lastError() to a value besides NoError , indicating which situation was detected.

Most errors detected by QCborStreamReader during normal item parsing are not recoverable. The code using QCborStreamReader may opt to handle the data that was properly decoded or it can opt to discard the entire data.

The only recoverable error is EndOfFile , which indicates that more data is required in order to complete the parsing. This situation is useful when data is being read from an asynchronous source, such as a pipe ( QProcess ) or a socket ( QTcpSocket , QUdpSocket , QNetworkReply , etc.). When more data arrives, the surrounding code needs to call either addData() , if parsing from a QByteArray , or reparse() , if it is instead reading directly a the QIDOevice that now has more data available (see setDevice() ).

class QCborStreamReader

QCborStreamReader(device)

QCborStreamReader(data)

QCborStreamReader(data, len)

QCborStreamReader(data, len)

param device

QIODevice

param data

QByteArray

param len

long long

Creates a QCborStreamReader object with no source data. After construction, QCborStreamReader will report an error parsing.

You can add more data by calling addData() or by setting a different source device using setDevice() .

See also

addData() isValid()

This is an overloaded function.

Creates a QCborStreamReader object that will parse the CBOR stream found by reading from device . QCborStreamReader does not take ownership of device , so it must remain valid until this oject is destroyed.

PySide2.QtCore.QCborStreamReader.Type

This enumeration contains all possible CBOR types as decoded by QCborStreamReader . CBOR has 7 major types, plus a number of simple types carrying no value, and floating point values.

Constant

Description

QCborStreamReader.UnsignedInteger

(Major type 0) Ranges from 0 to 2 64 - 1 (18,446,744,073,709,551,616)

QCborStreamReader.NegativeInteger

(Major type 1) Ranges from -1 to -2 64 (-18,446,744,073,709,551,616)

QCborStreamReader.ByteArray

(Major type 2) Arbitrary binary data.

QCborStreamReader.ByteString

An alias to .

QCborStreamReader.String

(Major type 3) Unicode text, possibly containing NULs.

QCborStreamReader.TextString

An alias to String

QCborStreamReader.Array

(Major type 4) Array of heterogeneous items.

QCborStreamReader.Map

(Major type 5) Map/dictionary of heterogeneous items.

QCborStreamReader.Tag

(Major type 6) Numbers giving further semantic value to generic CBOR items. See QCborTag for more information.

QCborStreamReader.SimpleType

(Major type 7) Types carrying no further value. Includes booleans (true and false), null, undefined.

QCborStreamReader.Float16

IEEE 754 half-precision floating point (qfloat16).

QCborStreamReader.HalfFloat

An alias to Float16.

QCborStreamReader.Float

IEEE 754 single-precision floating point (float).

QCborStreamReader.Double

IEEE 754 double-precision floating point (double).

QCborStreamReader.Invalid

Not a valid type, either due to parsing error or due to reaching the end of an array or map.

PySide2.QtCore.QCborStreamReader.StringResultCode

This enum is returned by readString() and readByteArray() and is used to indicate what the status of the parsing is.

Constant

Description

QCborStreamReader.EndOfString

The parsing for the string is complete, with no error.

QCborStreamReader.Ok

The function returned data; there was no error.

QCborStreamReader.Error

Parsing failed with an error.

PySide2.QtCore.QCborStreamReader.addData(data)
Parameters

dataQByteArray

Adds data to the CBOR stream and reparses the current element. This function is useful if the end of the data was previously reached while processing the stream, but now more data is available.

PySide2.QtCore.QCborStreamReader.addData(data, len)
Parameters
  • data – str

  • lenlong long

PySide2.QtCore.QCborStreamReader.addData(data, len)
Parameters
  • dataquint8

  • lenlong long

PySide2.QtCore.QCborStreamReader.clear()

Clears the decoder state and resets the input source data to an empty byte array. After this function is called, QCborStreamReader will be indicating an error parsing.

Call addData() to add more data to be parsed.

See also

reset() setDevice()

PySide2.QtCore.QCborStreamReader.containerDepth()
Return type

int

Returns the number of containers that this stream has entered with enterContainer() but not yet left.

PySide2.QtCore.QCborStreamReader.currentOffset()
Return type

qint64

Returns the offset in the input stream of the item currently being decoded. The current offset is the number of decoded bytes so far only if the source data is a QByteArray or it is a QIODevice that was positioned at its beginning when decoding started.

PySide2.QtCore.QCborStreamReader.currentStringChunkSize()
Return type

long long

Returns the size of the current text or byte string chunk. If the CBOR stream contains a non-chunked string (that is, if isLengthKnown() returns true ), this function returns the size of the entire string, the same as length() .

This function is useful to pre-allocate the buffer whose pointer can be passed to readStringChunk() later.

See also

readString() readByteArray() readStringChunk()

PySide2.QtCore.QCborStreamReader.device()
Return type

QIODevice

Returns the QIODevice that was set with either setDevice() or the QCborStreamReader constructor. If this object was reading from a QByteArray , this function returns nullptr instead.

See also

setDevice()

PySide2.QtCore.QCborStreamReader.enterContainer()
Return type

bool

Enters the array or map that is the current item and prepares for iterating the elements contained in the container. Returns true if entering the container succeeded, false otherwise (usually, a parsing error). Each call to must be paired with a call to leaveContainer() .

This function may only be called if the current item is an array or a map (that is, if isArray() , isMap() or isContainer() is true). Calling it in any other condition is an error.

PySide2.QtCore.QCborStreamReader.hasNext()
Return type

bool

Returns true if there are more items to be decoded in the current container or false of we’ve reached its end. If we’re parsing the root element, returning false indicates the parsing is complete; otherwise, if the container depth is non-zero, then the outer code needs to call leaveContainer() .

PySide2.QtCore.QCborStreamReader.isArray()
Return type

bool

Returns true if the type of the current element is an array (that is, if type() returns Array ). If this function returns true, you may call enterContainer() to begin parsing that container.

When the current element is an array, you may also call isLengthKnown() to find out if the array’s size is explicit in the CBOR stream. If it is, that size can be obtained by calling length() .

The following example pre-allocates a QVariantList given the array’s size for more efficient decoding:

QVariantList populateFromCbor(QCborStreamReader &reader)
{
    QVariantList list;
    if (reader.isLengthKnown())
        list.reserve(reader.length());

    reader.enterContainer();
    while (reader.lastError() == QCborError::NoError && reader.hasNext())
        list.append(readOneElement(reader));
    if (reader.lastError() == QCborError::NoError)
        reader.leaveContainer();
}

Note

The code above does not validate that the length is a sensible value. If the input stream reports that the length is 1 billion elements, the above function will try to allocate some 16 GB or more of RAM, which can lead to a crash.

PySide2.QtCore.QCborStreamReader.isBool()
Return type

bool

Returns true if the current element is a boolean value (true or false ), false if it is anything else. If this function returns true, you may call toBool() to retrieve the value of the boolean. You may also call toSimpleType() and compare to either QCborSimpleValue::True or QCborSimpleValue::False.

PySide2.QtCore.QCborStreamReader.isByteArray()
Return type

bool

Returns true if the type of the current element is a byte array (that is, if type() returns ByteArray ). If this function returns true, you may call readByteArray() to read that data.

PySide2.QtCore.QCborStreamReader.isContainer()
Return type

bool

Returns true if the current element is a container (that is, an array or a map), false if it is anything else. If the current element is a container, the isLengthKnown() function may be used to find out if the container’s size is explicit in the stream and, if so, length() can be used to get that size.

More importantly, for a container, the enterContainer() function is available to begin iterating through the elements contained therein.

PySide2.QtCore.QCborStreamReader.isDouble()
Return type

bool

Returns true if the type of the current element is an IEEE 754 double-precision floating point (that is, if type() returns Double ). If this function returns true, you may call toDouble() to read that data.

PySide2.QtCore.QCborStreamReader.isFalse()
Return type

bool

Returns true if the current element is the false value, false if it is anything else.

PySide2.QtCore.QCborStreamReader.isFloat()
Return type

bool

Returns true if the type of the current element is an IEEE 754 single-precision floating point (that is, if type() returns Float ). If this function returns true, you may call toFloat() to read that data.

PySide2.QtCore.QCborStreamReader.isFloat16()
Return type

bool

Returns true if the type of the current element is an IEEE 754 half-precision floating point (that is, if type() returns Float16 ). If this function returns true, you may call toFloat16() to read that data.

See also

type() toFloat16() isFloat() isDouble()

PySide2.QtCore.QCborStreamReader.isInteger()
Return type

bool

Returns true if the type of the current element is either an unsigned integer or a negative one (that is, if type() returns UnsignedInteger or NegativeInteger ). If this function returns true, you may call toInteger() to read that value.

PySide2.QtCore.QCborStreamReader.isInvalid()
Return type

bool

Returns true if the current element is invalid, false otherwise. The current element may be invalid if there was a decoding error or we’ve just parsed the last element in an array or map.

Note

This function is not to be confused with isNull() . Null is a normal CBOR type that must be handled by the application.

See also

type() isValid()

PySide2.QtCore.QCborStreamReader.isLengthKnown()
Return type

bool

Returns true if the length of the current array, map, byte array or string is known (explicit in the CBOR stream), false otherwise. This function should only be called if the element is one of those.

If the length is known, it may be obtained by calling length() .

If the length of a map or an array is not known, it is implied by the number of elements present in the stream. QCborStreamReader has no API to calculate the length in that condition.

Strings and byte arrays may also have indeterminate length (that is, they may be transmitted in multiple chunks). Those cannot currently be created with QCborStreamWriter , but they could be with other encoders, so QCborStreamReader supports them.

PySide2.QtCore.QCborStreamReader.isMap()
Return type

bool

Returns true if the type of the current element is a map (that is, if type() returns Map ). If this function returns true, you may call enterContainer() to begin parsing that container.

When the current element is a map, you may also call isLengthKnown() to find out if the map’s size is explicit in the CBOR stream. If it is, that size can be obtained by calling length() .

The following example pre-allocates a QVariantMap given the map’s size for more efficient decoding:

QVariantMap populateFromCbor(QCborStreamReader &reader)
{
    QVariantMap map;
    if (reader.isLengthKnown())
        map.reserve(reader.length());

    reader.enterContainer();
    while (reader.lastError() == QCborError::NoError && reader.hasNext()) {
        QString key = readElementAsString(reader);
        map.insert(key, readOneElement(reader));
    }
    if (reader.lastError() == QCborError::NoError)
        reader.leaveContainer();
}

The example above uses a function called readElementAsString to read the map’s keys and obtain a string. That is because CBOR maps may contain any type as keys, not just strings. User code needs to either perform this conversion, reject non-string keys, or instead use a different container besides QVariantMap and QVariantHash . For example, if the map is expected to contain integer keys, which is recommended as it reduces stream size and parsing, the correct container would be \l{QMap}<int, QVariant> or \l{QHash}<int, QVariant> .

Note

The code above does not validate that the length is a sensible value. If the input stream reports that the length is 1 billion elements, the above function will try to allocate some 24 GB or more of RAM, which can lead to a crash.

PySide2.QtCore.QCborStreamReader.isNegativeInteger()
Return type

bool

Returns true if the type of the current element is a negative integer (that is if type() returns NegativeInteger ). If this function returns true, you may call toNegativeInteger() or toInteger() to read that value.

See also

type() toNegativeInteger() toInteger() isInteger() isUnsignedInteger()

PySide2.QtCore.QCborStreamReader.isNull()
Return type

bool

Returns true if the current element is the null value, false if it is anything else. Null values may be used to indicate the absence of some optional data.

Note

This function is not the opposite of isValid() . A Null value is a valid CBOR value.

PySide2.QtCore.QCborStreamReader.isSimpleType()
Return type

bool

Returns true if the type of the current element is any CBOR simple type, including a boolean value (true and false) as well as null and undefined. To find out which simple type this is, call toSimpleType() . Alternatively, to test for one specific simple type, call the overload that takes a QCborSimpleType parameter.

CBOR simple types are types that do not carry extra value. There are 255 possibilities, but there are currently only four values that have defined meaning. Code is not expected to cope with unknown simple types and may simply discard the stream as invalid if it finds an unknown one.

See also

QCborSimpleType type() isSimpleType(QCborSimpleType) toSimpleType()

PySide2.QtCore.QCborStreamReader.isSimpleType(st)
Parameters

stQCborSimpleType

Return type

bool

Returns true if the type of the current element is the simple type st , false otherwise. If this function returns true, then toSimpleType() will return st .

CBOR simple types are types that do not carry extra value. There are 255 possibilities, but there are currently only four values that have defined meaning. Code is not expected to cope with unknown simple types and may simply discard the stream as invalid if it finds an unknown one.

See also

QCborSimpleType type() isSimpleType() toSimpleType()

PySide2.QtCore.QCborStreamReader.isString()
Return type

bool

Returns true if the type of the current element is a text string (that is, if type() returns String ). If this function returns true, you may call readString() to read that data.

PySide2.QtCore.QCborStreamReader.isTag()
Return type

bool

Returns true if the type of the current element is a CBOR tag (that is, if type() returns Tag ). If this function returns true, you may call toTag() to read that data.

See also

type() toTag()

PySide2.QtCore.QCborStreamReader.isTrue()
Return type

bool

Returns true if the current element is the true value, false if it is anything else.

PySide2.QtCore.QCborStreamReader.isUndefined()
Return type

bool

Returns true if the current element is the undefined value, false if it is anything else. Undefined values may be encoded to indicate that some conversion failed or was not possible when creating the stream. QCborStreamReader never performs any replacement and this function will only return true if the stream contains an explicit undefined value.

PySide2.QtCore.QCborStreamReader.isUnsignedInteger()
Return type

bool

Returns true if the type of the current element is an unsigned integer (that is if type() returns UnsignedInteger ). If this function returns true, you may call toUnsignedInteger() or toInteger() to read that value.

PySide2.QtCore.QCborStreamReader.isValid()
Return type

bool

Returns true if the current element is valid, false otherwise. The current element may be invalid if there was a decoding error or we’ve just parsed the last element in an array or map.

Note

This function is not the opposite of isNull() . Null is a normal CBOR type that must be handled by the application.

See also

type() isInvalid()

PySide2.QtCore.QCborStreamReader.lastError()
Return type

QCborError

Returns the last error in decoding the stream, if any. If no error was encountered, this returns an NoError .

See also

isValid()

PySide2.QtCore.QCborStreamReader.leaveContainer()
Return type

bool

Leaves the array or map whose items were being processed and positions the decoder at the next item after the end of the container. Returns true if leaving the container succeeded, false otherwise (usually, a parsing error). Each call to enterContainer() must be paired with a call to .

This function may only be called if hasNext() has returned false and containerDepth() is not zero. Calling it in any other condition is an error.

PySide2.QtCore.QCborStreamReader.length()
Return type

quint64

Returns the length of the string or byte array, or the number of items in an array or the number, of item pairs in a map, if known. This function must not be called if the length is unknown (that is, if isLengthKnown() returned false). It is an error to do that and it will cause QCborStreamReader to stop parsing the input stream.

PySide2.QtCore.QCborStreamReader.next([maxRecursion=10000])
Parameters

maxRecursionint

Return type

bool

Advance the CBOR stream decoding one element. You should usually call this function when parsing fixed-width basic elements (that is, integers, simple values, tags and floating point values). But this function can be called when the current item is a string, array or map too and it will skip over that entire element, including all contained elements.

This function returns true if advancing was successful, false otherwise. It may fail if the stream is corrupt, incomplete or if the nesting level of arrays and maps exceeds maxRecursion . Calling this function when hasNext() has returned false is also an error. If this function returns false, lastError() will return the error code detailing what the failure was.

PySide2.QtCore.QCborStreamReader.parentContainerType()
Return type

Type

Returns either Array or Map , indicating whether the container that contains the current item was an array or map, respectively. If we’re currently parsing the root element, this function returns Invalid .

PySide2.QtCore.QCborStreamReader.readByteArray()
Return type

StringResult

Decodes one byte array chunk from the CBOR string and returns it. This function is used for both regular and chunked contents, so the caller must always loop around calling this function, even if isLengthKnown() has is true. The typical use of this function is as follows:

QBytearray decodeBytearray(QCborStreamReader &reader)
{
    QBytearray result;
    auto r = reader.readBytearray();
    while (r.code == QCborStreamReader::Ok) {
        result += r.data;
        r = reader.readByteArray();
    }

    if (r.code == QCborStreamReader::Error) {
        // handle error condition
        result.clear();
    }
    return result;
}

This function does not perform any type conversions, including from integers or from strings. Therefore, it may only be called if isByteArray() is true; calling it in any other condition is an error.

See also

readString() isByteArray() readStringChunk()

PySide2.QtCore.QCborStreamReader.readString()
Return type

StringResult

Decodes one string chunk from the CBOR string and returns it. This function is used for both regular and chunked string contents, so the caller must always loop around calling this function, even if isLengthKnown() has is true. The typical use of this function is as follows:

QString decodeString(QCborStreamReader &reader)
{
    QString result;
    auto r = reader.readString();
    while (r.code == QCborStreamReader::Ok) {
        result += r.data;
        r = reader.readString();
    }

    if (r.code == QCborStreamReader::Error) {
        // handle error condition
        result.clear();
    }
    return result;
}

This function does not perform any type conversions, including from integers or from byte arrays. Therefore, it may only be called if isString() returned true; calling it in any other condition is an error.

See also

readByteArray() isString() readStringChunk()

PySide2.QtCore.QCborStreamReader.reparse()

Reparses the current element. This function must be called when more data becomes available in the source QIODevice after parsing failed due to reaching the end of the input data before the end of the CBOR stream.

When reading from QByteArray(), the addData() function automatically calls this function. Calling it when the reading had not failed is a no-op.

PySide2.QtCore.QCborStreamReader.reset()

Resets the source back to the beginning and clears the decoder state. If the source data was a QByteArray , QCborStreamReader will restart from the beginning of the array.

If the source data is a QIODevice , this function will call reset() , which will seek to byte position 0. If the CBOR stream is not found at the beginning of the device (e.g., beginning of a file), then this function will likely do the wrong thing. Instead, position the QIODevice to the right offset and call setDevice() .

See also

clear() setDevice()

PySide2.QtCore.QCborStreamReader.setDevice(device)
Parameters

deviceQIODevice

Sets the source of data to device , resetting the decoder to its initial state.

See also

device()

PySide2.QtCore.QCborStreamReader.toBool()
Return type

bool

Returns the boolean value of the current element.

This function does not perform any type conversions, including from integer. Therefore, it may only be called if isTrue() , isFalse() or isBool() returned true; calling it in any other condition is an error.

PySide2.QtCore.QCborStreamReader.toDouble()
Return type

double

Returns the 64-bit double-precision floating point value of the current element.

This function does not perform any type conversions, including from other floating point types or from integer values. Therefore, it may only be called if isDouble() is true; calling it in any other condition is an error.

See also

isDouble() toFloat16() toFloat()

PySide2.QtCore.QCborStreamReader.toFloat()
Return type

float

Returns the 32-bit single-precision floating point value of the current element.

This function does not perform any type conversions, including from other floating point types or from integer values. Therefore, it may only be called if isFloat() is true; calling it in any other condition is an error.

See also

isFloat() toFloat16() toDouble()

PySide2.QtCore.QCborStreamReader.toInteger()
Return type

qint64

Returns the integer value of the current element, be it negative, positive or zero. If the value is larger than 2 63 - 1 or smaller than -2 63 , the returned value will overflow and will have an incorrect sign. If handling those values is required, use toUnsignedInteger() or toNegativeInteger() instead.

This function does not perform any type conversions, including from boolean or CBOR tag. Therefore, it may only be called if isInteger() is true; calling it in any other condition is an error.

See also

isInteger() toUnsignedInteger() toNegativeInteger()

PySide2.QtCore.QCborStreamReader.toSimpleType()
Return type

QCborSimpleType

Returns value of the current simple type.

This function does not perform any type conversions, including from integer. Therefore, it may only be called if isSimpleType() is true; calling it in any other condition is an error.

PySide2.QtCore.QCborStreamReader.toUnsignedInteger()
Return type

quint64

Returns the unsigned integer value of the current element.

This function does not perform any type conversions, including from boolean or CBOR tag. Therefore, it may only be called if isUnsignedInteger() is true; calling it in any other condition is an error.

This function may be used to obtain numbers beyond the range of the return type of toInteger() .

PySide2.QtCore.QCborStreamReader.type()
Return type

Type

Returns the type of the current element. It is one of the valid types or Invalid.