Qt Serialization

The purpose of serialization is to save the state of an object to be able to recreate it when needed. It allows you to perform actions like:

Sending the object to a remote application by using a web service
Passing the object as a JSON or XML string
Saving and restoring user information or sharing it across applications

The Qt API provides support for serialization for several use cases:

JSON support in Qt provides an easy to use C++ API to parse, modify and save JSON data. 在 Qt 中支持 CBOR is a compact form of binary data encoding that is a superset of JSON.
QDataStream provides serialization of binary data to a QIODevice
Qt XML C++ 类提供 C++ 实现的 XML 流和 DOM (文档对象模型) 标准对于 XML
CBOR is Qt's implementation for the CBOR serialization format.
QSettings provides a way of serializing and storing platform independent application settings.

Advantages of JSON and CBOR

When you use JSON information is stored in a QJsonObject 和 QJsonDocument takes care to stream values into a QByteArray .

例如

QJsonObject jobject;
jobject["SensorID"] = m_id;
jobject["AmbientTemperature"] = m_ambientTemperature;
jobject["ObjectTemperature"] = m_objectTemperature;
jobject["AccelerometerX"] = m_accelerometerX;
jobject["AccelerometerY"] = m_accelerometerY;
jobject["AccelerometerZ"] = m_accelerometerZ;
jobject["Altitude"] = m_altitude;
jobject["Light"] = m_light;
jobject["Humidity"] = m_humidity;
QJsonDocument doc( jobject );
return doc.toJson();

JSON has several advantages:

Textual JSON is declarative, which makes it readable to humans
The information is structured
Exchanging generic information is easy
JSON allows extending messages with additional values
Many solutions exist to receive and parse JSON in cloud-based solutions

CBOR is the Concise Binary Object Representation, a very compact form of binary data encoding that is a superset of JSON. It was created by the IETF Constrained RESTful Environments (CoRE) WG, which has been used in many new RFCs. CBOR shares many of the advantages of JSON, but sacrifices human readability for compactness.

Advantages of QDataStream Classes

QDataStream is a viable option when the whole flow of data is determined and not about to change. In addition, both the reader and the writer of the data must be written in Qt.

Adding support for this to a class requires two additional operators. For example, for a class named SensorInformation:

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

The implementation for the serialization is shown below:

QDataStream &operator<<(QDataStream &out, const SensorInformation &item)
{
    QDataStream::FloatingPointPrecision prev = out.floatingPointPrecision();
    out.setFloatingPointPrecision(QDataStream::DoublePrecision);
    out << item.m_id
        << item.m_ambientTemperature
        << item.m_objectTemperature
        << item.m_accelerometerX
        << item.m_accelerometerY
        << item.m_accelerometerZ
        << item.m_altitude
        << item.m_light
        << item.m_humidity;
    out.setFloatingPointPrecision(prev);
    return out;
}

Deserialization works similarly, but using the >> operator. For example, out >> item.m_id , and so on.

Usually, using QDataStream is faster than using textual JSON.

Advantages of Qt XML C++ Classes

Qt provides both DOM classes and stream-based classes to read and write XML content.

Qt 提供 QDomDocument class that represents the XML document and two classes for reading and writing the XML through a simple streaming API: QXmlStreamReader and QXmlStreamWriter .

The DOM XML Classes

QDomDocument class represents the entire XML document. It is the root of the document tree and provides primary access to the document's data.

The Stream-Based XML Classes

A stream reader reports an XML document as a stream of tokens. This differs from SAX as SAX applications provide handlers to receive XML events from the parser, whereas the QXmlStreamReader drives the loop, pulling tokens from the reader when they are needed. This pulling approach makes it possible to build recursive descent parsers, allowing XML parsing code to be split into different methods or classes.

QXmlStreamReader a parser for well-formed XML 1.0, excluding external parsed entities. Hence, data provided to the stream reader adheres to the W3C's criteria for well-formed XML, or an error will be raised. Functions such as atEnd() , error() ，和 hasError() can be used to test for such errors and obtain a description of them.

The QXmlStreamWriter is a streaming API that takes care of prefixing namespaces, when the namespaceUri is specified when writing elements or attributes.

Classes that Provide Serialization

<QtCborCommon>	Contains definitions common to both the streaming classes (QCborStreamReader and QCborStreamWriter) and to QCborValue
QCborArray	用于保持 CBOR (简明二进制对象表示) 元素的数组
QCborMap	用于保持以 CBOR 表示的关联容器
QCborStreamReader	操作 QByteArray 或 QIODevice 的简单 CBOR 流解码器
QCborStreamWriter	操作单向流的简单 CBOR 编码器
QCborValue	封装值在 CBOR 中
QDataStream	把二进制数据序列化到 QIODevice
QJsonArray	封装 JSON 数组
QJsonDocument	读写 JSON 文档的办法
QJsonObject	封装 JSON 对象
QJsonParseError	用于在 JSON 剖析期间报告错误
QJsonValue	把值封装在 JSON 中
QTextStream	用于读写文本的方便接口
QXmlStreamReader	用于凭借简单流化 API 读取格式良好的 XML 的快速剖析器
QXmlStreamWriter	采用简单流化 API 的 XML 写入器