QDataStream Class

QDataStream's binary format has evolved since Qt 1.0, and is likely to continue evolving to reflect changes done in Qt. When inputting or outputting complex types, it's very important to make sure that the same version of the stream (version()) is used for reading and writing. If you need both forward and backward compatibility, you can hardcode the version number in the application:

stream.setVersion(QDataStream::Qt_4_0);

If you are producing a new binary data format, such as a file format for documents created by your application, you could use a QDataStream to write the data in a portable format. Typically, you would write a brief header containing a magic string and a version number to give yourself room for future expansion. For example:

QFile file("file.xxx");
file.open(QIODevice::WriteOnly);
QDataStream out(&file);

// Write a header with a "magic number" and a version
out << (quint32)0xA0B0C0D0;
out << (qint32)123;

out.setVersion(QDataStream::Qt_4_0);

// Write the data
out << lots_of_interesting_data;

Then read it in with:

QFile file("file.xxx");
file.open(QIODevice::ReadOnly);
QDataStream in(&file);

// Read and check the header
quint32 magic;
in >> magic;
if (magic != 0xA0B0C0D0)
    return XXX_BAD_FILE_FORMAT;

// Read the version
qint32 version;
in >> version;
if (version < 100)
    return XXX_BAD_FILE_TOO_OLD;
if (version > 123)
    return XXX_BAD_FILE_TOO_NEW;

if (version <= 110)
    in.setVersion(QDataStream::Qt_3_2);
else
    in.setVersion(QDataStream::Qt_4_0);

// Read the data
in >> lots_of_interesting_data;
if (version >= 120)
    in >> data_new_in_XXX_version_1_2;
in >> other_interesting_data;

You can select which byte order to use when serializing data. The default setting is big-endian (MSB first). Changing it to little-endian breaks the portability (unless the reader also changes to little-endian). We recommend keeping this setting unless you have special requirements.

You may wish to read/write your own raw binary data to/from the data stream directly. Data may be read from the stream into a preallocated char * using readRawData(). Similarly data can be written to the stream using writeRawData(). Note that any encoding/decoding of the data must be done by you.

A similar pair of functions is readBytes() and writeBytes(). These differ from their raw counterparts as follows: readBytes() reads a quint32 which is taken to be the length of the data to be read, then that number of bytes is read into the preallocated char *; writeBytes() writes a quint32 containing the length of the data, followed by the data. Note that any encoding/decoding of the data (apart from the length quint32) must be done by you.

The Qt container classes can also be serialized to a QDataStream. These include QList, QSet, QHash, and QMap. The stream operators are declared as non-members of the classes.

In addition to the overloaded stream operators documented here, any Qt classes that you might want to serialize to a QDataStream will have appropriate stream operators declared as non-member of the class:

    QDataStream &operator<<(QDataStream &, const QXxx &);
    QDataStream &operator>>(QDataStream &, QXxx &);

For example, here are the stream operators declared as non-members of the QImage class:

    QDataStream & operator<< (QDataStream& stream, const QImage& image);
    QDataStream & operator>> (QDataStream& stream, QImage& image);

To see if your favorite Qt class has similar stream operators defined, check the Related Non-Members section of the class's documentation page.

When a data stream operates on an asynchronous device, the chunks of data can arrive at arbitrary points in time. The QDataStream class implements a transaction mechanism that provides the ability to read the data atomically with a series of stream operators. As an example, you can handle incomplete reads from a socket by using a transaction in a slot connected to the readyRead() signal:

in.startTransaction();
QString str;
qint32 a;
in >> str >> a; // try to read packet atomically

if (!in.commitTransaction())
    return;     // wait for more data

If no full packet is received, this code restores the stream to the initial position, after which you need to wait for more data to arrive.

See also QTextStream, QVariant

The byte order used for reading/writing the data.

The precision of floating point numbers used for reading/writing the data. This will only have an effect if the version of the data stream is Qt_4_6 or higher.

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This enum describes the current status of the data stream.

This enum provides symbolic synonyms for the data serialization format version numbers.

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Constructs a data stream that has no I/O device.

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Constructs a data stream that uses the I/O device d.

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Constructs a data stream that operates on a byte array, a. The mode describes how the device is to be used.

Alternatively, you can use QDataStream(const QByteArray &) if you just want to read from a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

Constructs a read-only data stream that operates on byte array a. Use QDataStream(QByteArray*, int) if you want to write to a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

Destroys the data stream.

The destructor will not affect the current I/O device, unless it is an internal I/O device (e.g. a QBuffer) processing a QByteArray passed in the constructor, in which case the internal I/O device is destroyed.

Aborts a read transaction.

This function is commonly used to discard the transaction after higher-level protocol errors or loss of stream synchronization.

If called on an inner transaction, aborting is delegated to the outermost transaction, and subsequently started inner transactions are forced to fail.

For the outermost transaction, discards the restoration point and any internally duplicated data of the stream. Will not affect the current read position of the stream.

Sets the status of the data stream to

This function was introduced in Qt 5.7.

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Returns true if the I/O device has reached the end position (end of the stream or file) or if there is no I/O device set; otherwise returns false.

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Returns the current byte order setting – either BigEndian or LittleEndian.

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Completes a read transaction. Returns true if no read errors have occurred during the transaction; otherwise returns false.

If called on an inner transaction, committing will be postponed until the outermost commitTransaction(), rollbackTransaction(), or abortTransaction() call occurs.

Otherwise, if the stream status indicates reading past the end of the data, this function restores the stream data to the point of the startTransaction() call. When this situation occurs, you need to wait for more data to arrive, after which you start a new transaction. If the data stream has read corrupt data or any of the inner transactions was aborted, this function aborts the transaction.

This function was introduced in Qt 5.7.

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Returns the I/O device currently set, or nullptr if no device is currently set.

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Returns the floating point precision of the data stream.

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Reads the buffer s from the stream and returns a reference to the stream.

The buffer s is allocated using new []. Destroy it with the delete [] operator.

The l parameter is set to the length of the buffer. If the string read is empty, l is set to 0 and s is set to nullptr.

The serialization format is a quint32 length specifier first, then l bytes of data.

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Reads at most len bytes from the stream into s and returns the number of bytes read. If an error occurs, this function returns -1.

The buffer s must be preallocated. The data is not decoded.

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Resets the status of the data stream.

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Reverts a read transaction.

This function is commonly used to rollback the transaction when an incomplete read was detected prior to committing the transaction.

If called on an inner transaction, reverting is delegated to the outermost transaction, and subsequently started inner transactions are forced to fail.

For the outermost transaction, restores the stream data to the point of the startTransaction() call. If the data stream has read corrupt data or any of the inner transactions was aborted, this function aborts the transaction.

If the preceding stream operations were successful, sets the status of the data stream to

This function was introduced in Qt 5.7.

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Sets the serialization byte order to bo.

The bo parameter can be QDataStream::BigEndian or QDataStream::LittleEndian.

The default setting is big-endian. We recommend leaving this setting unless you have special requirements.

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void QDataStream::setDevice(QIODevice *d)

Sets the I/O device to d, which can be nullptr to unset to current I/O device.

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Sets the floating point precision of the data stream to precision. If the floating point precision is DoublePrecision and the version of the data stream is Qt_4_6 or higher, all floating point numbers will be written and read with 64-bit precision. If the floating point precision is SinglePrecision and the version is Qt_4_6 or higher, all floating point numbers will be written and read with 32-bit precision.

For versions prior to Qt_4_6, the precision of floating point numbers in the data stream depends on the stream operator called.

The default is DoublePrecision.

Note that this property does not affect the serialization or deserialization of qfloat16 instances.

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Sets the status of the data stream to the status given.

Subsequent calls to setStatus() are ignored until resetStatus() is called.

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Sets the version number of the data serialization format to v, a value of the Version enum.

You don't have to set a version if you are using the current version of Qt, but for your own custom binary formats we recommend that you do; see Versioning in the Detailed Description.

To accommodate new functionality, the datastream serialization format of some Qt classes has changed in some versions of Qt. If you want to read data that was created by an earlier version of Qt, or write data that can be read by a program that was compiled with an earlier version of Qt, use this function to modify the serialization format used by QDataStream.

The Version enum provides symbolic constants for the different versions of Qt. For example:

QDataStream out(file);
out.setVersion(QDataStream::Qt_4_0);

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Skips len bytes from the device. Returns the number of bytes actually skipped, or -1 on error.

This is equivalent to calling readRawData() on a buffer of length len and ignoring the buffer.

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Starts a new read transaction on the stream.

Defines a restorable point within the sequence of read operations. For sequential devices, read data will be duplicated internally to allow recovery in case of incomplete reads. For random-access devices, this function saves the current position of the stream. Call commitTransaction(), rollbackTransaction(), or abortTransaction() to finish the current transaction.

Once a transaction is started, subsequent calls to this function will make the transaction recursive. Inner transactions act as agents of the outermost transaction (i.e., report the status of read operations to the outermost transaction, which can restore the position of the stream).

When an error occurs during a transaction (including an inner transaction failing), reading from the data stream is suspended (all subsequent read operations return empty/zero values) and subsequent inner transactions are forced to fail. Starting a new outermost transaction recovers from this state. This behavior makes it unnecessary to error-check every read operation separately.

This function was introduced in Qt 5.7.

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Returns the status of the data stream.

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Returns the version number of the data serialization format.

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Writes the length specifier len and the buffer s to the stream and returns a reference to the stream.

The len is serialized as a quint32, followed by len bytes from s. Note that the data is not encoded.

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Writes len bytes from s to the stream. Returns the number of bytes actually written, or -1 on error. The data is not encoded.

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

This is an overloaded function.

Writes an unsigned byte, i, to the stream and returns a reference to the stream.

This is an overloaded function.

Writes a signed 16-bit integer, i, to the stream and returns a reference to the stream.

This is an overloaded function.

Writes an unsigned 16-bit integer, i, to the stream and returns a reference to the stream.

This is an overloaded function.

Writes a signed 32-bit integer, i, to the stream and returns a reference to the stream.

This is an overloaded function.

Writes an unsigned integer, i, to the stream as a 32-bit unsigned integer (quint32). Returns a reference to the stream.

This is an overloaded function.

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

This is an overloaded function.

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

This is an overloaded function.

Simulates writing a std::nullptr_t, ptr, to the stream and returns a reference to the stream. This function does not actually write anything to the stream, as std::nullptr_t values are stored as 0 bytes.

This function was introduced in Qt 5.9.

Writes a boolean value, i, to the stream. Returns a reference to the stream.

This is an overloaded function.

Writes a floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.

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This is an overloaded function.

Writes a floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.

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This is an overloaded function.

Writes the '\0'-terminated string s to the stream and returns a reference to the stream.

The string is serialized using writeBytes().

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This is an overloaded function.

Writes a character, c, to the stream. Returns a reference to the stream

This function was introduced in Qt 6.0.

This is an overloaded function.

Writes a character, c, to the stream. Returns a reference to the stream

This function was introduced in Qt 6.0.

Reads a signed byte from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads an unsigned byte from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads a signed 16-bit integer from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads an unsigned 16-bit integer from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads a signed 32-bit integer from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads an unsigned 32-bit integer from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads a signed 64-bit integer from the stream into i, and returns a reference to the stream.

This is an overloaded function.

Reads an unsigned 64-bit integer from the stream, into i, and returns a reference to the stream.

This is an overloaded function.

Simulates reading a std::nullptr_t from the stream into ptr and returns a reference to the stream. This function does not actually read anything from the stream, as std::nullptr_t values are stored as 0 bytes.

This function was introduced in Qt 5.9.

Reads a boolean value from the stream into i. Returns a reference to the stream.

This is an overloaded function.

Reads a floating point number from the stream into f, using the standard IEEE 754 format. Returns a reference to the stream.

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This is an overloaded function.

Reads a floating point number from the stream into f, using the standard IEEE 754 format. Returns a reference to the stream.

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This is an overloaded function.

Reads the '\0'-terminated string s from the stream and returns a reference to the stream.

The string is deserialized using readBytes().

Space for the string is allocated using new [] – the caller must destroy it with delete [].

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This is an overloaded function.

Reads a 16bit wide char from the stream into c and returns a reference to the stream.

This function was introduced in Qt 6.0.

This is an overloaded function.

Reads a 32bit wide character from the stream into c and returns a reference to the stream.

This function was introduced in Qt 6.0.

Writes the pair pair to stream out.

This function requires the T1 and T2 types to implement operator<<().

This function was introduced in Qt 6.0.

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Reads a pair from stream in into pair.

This function requires the T1 and T2 types to implement operator>>().

This function was introduced in Qt 6.0.

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