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// 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.
*/
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