Data Input Output
This page provides an overview of various ways of retrieving and saving data using Qt.
Saving and Loading Data
The QIODevice class is the base class for all file and data storage devices in Qt Core. All classes that are used for reading and writing data inherit from it.
Examples of devices are QFile, QBuffer, QTcpSocket, and QProcess. QFile is used for reading and writing text, binary files, and resources, using the QIODevice interface. A QFile may be used by itself or, more conveniently, with a QTextStream or QDataStream.
The QBuffer class provides a QIODevice interface for a QByteArray. QTcpSocket enables the developer to establish a TCP connection and transfer streams of data. QProcess is used to start external programs, and to read from and write to that process.
- Input/Output and Networking (list of I/O related classes)
- File and Datastream Functions
The Qt API provides support for data serialization for several use cases.
- JSON support in Qt provides an easy to use C++ API to parse, modify and save JSON data
- QDataStream class provides serialization of binary data to a QIODevice
- Qt XML C++ Classes provide C++ implementations of the XML Streaming and DOM standards for XML
The Qt SQL module uses driver plugins to communicate with several database APIs. Qt has drivers for SQLite, MySQL, DB2, Borland InterBase, Oracle, ODBC, and PostgreSQL. It is also possible to develop your own driver if Qt does not provide the driver needed.
Qt's SQL classes can be divided in 3 layers:
With all the SQL drivers, with the exception of SQLite, you can connect to a server that is hosting your database system. If you use the embedded MySQL Server, you do not need a MySQL server in order to use that database system.
For instructions on how to build the SQL database drivers, see SQL Database Drivers.
The Qt SQLite plugin is very suitable for local storage. SQLite is a relational database management system contained in a small (~350 KiB) C library. In contrast to other database management systems, SQLite is not a separate process that is accessed from the client application, but an integral part of it. SQLite operates on a single file, which must be set as the database name when opening a connection. If the file does not exist, SQLite will try to create it.
SQLite has some restrictions regarding multiple users and multiple transactions. If you are reading or writing on a file from different transactions, your application might freeze until one transaction commits or rolls back.
Once the driver part is set up, the data is accessible using the classes, QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel. QSqlTableModel and QSqlRelationalTableModel provide editable models that can used with Qt's item views. QSqlTableModel has read/write access to a single table, whereas QSqlRelationalTableModel has read/write access to the main table (not to the table with the foreign key).
The following pages contain information about incorporating SQL into applications:
- SQL Programming
XML Support in Qt
Qt provides APIs to read and parse XML streams, and also to write to these streams. The following key classes facilitate these actions by providing the necessary infrastructure:
- QDomDocument class represents the entire XML document as a document tree and provides primary access to the document's data.
- It has several functions for creating document data, for example, createElement(), createTextNode(), createComment(), createCDATASection(), createProcessingInstruction(), createAttribute(), and createEntityReference().
- QXmlStreamReader class provides a parser to read XML. It is a well-formed XML 1.0 parser that does not include external parsed entities.
- It understands and resolves XML namespaces. For example, in case of a StartElement, namespaceUri() returns the namespace the element is in, and name() returns the element's local name. The combination of namespaceUri() and name() uniquely identifies an element.
- It is not CPU-intensive, as it doesn't store the entire XML document tree in memory. It only stores the current token at the time it is reported.
- The QXmlStreamWriter class provides an XML writer with a simple streaming API. It is the counterpart to QXmlStreamReader for writing XML, and it operates on a QIODevice specified with setDevice().
- It is a simple API that provides a dedicated function for every XML token or event you want to write.
- It takes care of prefixing namespaces based on the namespaceUri specified while writing elements or attributes. If you have to use certain standardized prefixes, you can force the writer to use them by declaring the namespaces manually with either writeNamespace() or writeDefaultNamespace().
- It can automatically format the generated XML data by adding line-breaks and indentation, making it readable. This feature can be turned on with the auto-formatting property.
The following topics provide more insight into Qt XML support:
JSON in Qt
An object can take 2 forms:
These databases are user-specific and QML-specific, but accessible to all QML applications. They are stored in the Databases subdirectory of QDeclarativeEngine::offlineStoragePath() as SQLite databases (SQL Database Drivers).
The API conforms to the Synchronous API of the HTML5 Web Database API, W3C Working Draft 29 October 2009 (HTML5 Web Database API).
See Qt Quick Examples - Local Storage for a basic demonstration of using the Local Storage API.
The QSettings class provides persistent storage of application settings. An application usually remembers its settings from the previous session.
Settings are stored differently on different platforms. For example, on Windows they are stored in the registry, whereas on macOS they are stored in XML files.
QSettings enable you to save and restore application settings in a portable manner. Constructing and destroying a QSettings object is lightweight and fast. While creating an object of QSettings, it is a good practice to specify not only the name of the application, but also the name of your organization. For example:
QSettings settings("MyCompany", "Accountancy");
The Qt Resource System is a platform-independent mechanism for storing binary files in the application's executable. This is handy if your application frequently needs a certain file, or set of files. It also protects against loss of that particular file .
Resource data can either be compiled into the binary and accessed immediately in the application code, or a binary resource can be created dynamically and registered with the resource system by the application.
By default, resources are accessible from the application code by the same file name as they are stored in the source tree, with a
:/ prefix, or by a URL with a qrc scheme.
An archive file is a collection of files or directories which are generally compressed in order to reduce the space they would otherwise consume on a drive. Examples of archive files are ZIP, TAR, RAR and 7z.
Qt has support for archives produced by zlib (see qCompress() and qUncompress()).
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