custom-types.qdoc

Go to the documentation of this file.

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page custom-types.html
    \title Creating Custom Qt Types
    \brief How to create and register new types with Qt.
 
    \ingroup how-to
 
    \tableofcontents
 
    \section1 Overview
 
    When creating user interfaces with Qt, particularly those with specialized controls and
    features, developers sometimes need to create new data types that can be used alongside
    or in place of Qt's existing set of value types.
 
    Standard types such as QSize, QColor and QString can all be stored in QVariant objects,
    used as the types of properties in QObject-based classes, and emitted in signal-slot
    communication.
 
    In this document, we take a custom type and describe how to integrate it into Qt's object
    model so that it can be stored in the same way as standard Qt types. We then show how to
    register the custom type to allow it to be used in signals and slots connections.
 
    \section1 Creating a Custom Type
 
    Before we begin, we need to ensure that the custom type we are creating meets all the
    requirements imposed by QMetaType. In other words, it must provide:
 
    \list
    \li a public default constructor,
    \li a public copy constructor, and
    \li a public destructor.
    \endlist
 
    The following \c Message class definition includes these members:
 
    \snippet customtype/customtypeexample.cpp custom type definition
 
    The class also provides a constructor for normal use and two public member functions
    that are used to obtain the private data.
 
    \section1 Declaring the Type with QMetaType
 
    The \c Message class only needs a suitable implementation in order to be usable.
    However, Qt's type system will not be able to understand how to store, retrieve
    and serialize instances of this class without some assistance. For example, we
    will be unable to store \c Message values in QVariant.
 
    The class in Qt responsible for custom types is QMetaType. To make the type known
    to this class, we invoke the Q_DECLARE_METATYPE() macro on the class in the header
    file where it is defined:
 
    \snippet customtype/customtypeexample.cpp custom type meta-type declaration
 
    This now makes it possible for \c Message values to be stored in QVariant objects
    and retrieved later:
 
    \snippet customtype/customtypeexample.cpp storing a custom value
    \dots
    \snippet customtype/customtypeexample.cpp retrieving a custom value
 
    The Q_DECLARE_METATYPE() macro also makes it possible for these values to be used as
    arguments to signals, but \e{only in direct signal-slot connections}.
    To make the custom type generally usable with the signals and slots mechanism, we
    need to perform some extra work.
 
    \section1 Creating and Destroying Custom Objects
 
    Although the declaration in the previous section makes the type available for use
    in direct signal-slot connections, it cannot be used for queued signal-slot
    connections, such as those that are made between objects in different threads.
    This is because the meta-object system does not know how to handle creation and
    destruction of objects of the custom type at run-time.
 
    To enable creation of objects at run-time, call the qRegisterMetaType() template
    function to register it with the meta-object system. This also makes the type
    available for queued signal-slot communication as long as you call it before you
    make the first connection that uses the type.
 
    The \l{Queued Custom Type} example declares a \c Block class which is registered
    in the \c{main.cpp} file:
 
    \snippet threads/queuedcustomtype/main.cpp main start
    \dots
    \snippet threads/queuedcustomtype/main.cpp register meta-type for queued communications
    \dots
    \snippet threads/queuedcustomtype/main.cpp main finish
 
    This type is later used in a signal-slot connection in the \c{window.cpp} file:
 
    \snippet threads/queuedcustomtype/window.cpp Window constructor start
    \dots
    \snippet threads/queuedcustomtype/window.cpp connecting signal with custom type
    \dots
    \snippet threads/queuedcustomtype/window.cpp Window constructor finish
 
    If a type is used in a queued connection without being registered, a warning will be
    printed at the console; for example:
 
    \code
    QObject::connect: Cannot queue arguments of type 'Block'
    (Make sure 'Block' is registered using qRegisterMetaType().)
    \endcode
 
    \section1 Making the Type Printable
 
    It is often quite useful to make a custom type printable for debugging purposes,
    as in the following code:
 
    \snippet customtype/customtypeexample.cpp printing a custom type
 
    This is achieved by creating a streaming operator for the type, which is often
    defined in the header file for that type:
 
    \snippet customtype/customtypeexample.cpp custom type streaming operator declaration
 
    The implementation for the \c Message type here goes to some effort to make the
    printable representation as readable as possible:
 
    \snippet customtype/customtypeexample.cpp custom type streaming operator
 
    The output sent to the debug stream can, of course, be made as simple or as
    complicated as you like. Note that the value returned by this function is
    the QDebug object itself, though this is often obtained by calling the
    \l{QDebug::}{maybeSpace()} member function of QDebug that pads out the stream with space
    characters to make it more readable.
 
    \section1 Further Reading
 
    The Q_DECLARE_METATYPE() macro and qRegisterMetaType() function documentation
    contain more detailed information about their uses and limitations.
 
    The \l{Queued Custom Type} example shows how to implement a custom type with
    the features outlined in this document.
 
    The \l{Debugging Techniques} document provides an overview of the debugging
    mechanisms discussed above.
*/