qt-everywhere-src-5.14.1/qtdoc/doc/src/signalslotsyntaxes.qdoc

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/*!
\page signalsandslots-syntaxes.html
\title Differences between String-Based and Functor-Based Connections
\brief Compares the two syntaxes for making signal-slot connections in C++.

From Qt 5.0 onwards, Qt offers two different ways to write \l{Signals and Slots}
{signal-slot connections} in C++: The \e{string-based} connection syntax and the
\e{functor-based} connection syntax. There are pros and cons to both syntaxes.
The table below summarizes their differences.


\table
\header
    \li
    \li String-based
    \li Functor-based
\row
    \li Type checking is done at...
    \li Run-time
    \li Compile-time
\row
    \li Can perform implicit type conversions
    \li
    \li Y
\row
    \li Can connect signals to lambda expressions
    \li
    \li Y
\row
    \li Can connect signals to slots which have more arguments than the signal (using default parameters)
    \li Y
    \li
\row
    \li Can connect C++ functions to QML functions
    \li Y
    \li
\endtable

The following sections explain these differences in detail and demonstrate how
to use the features unique to each connection syntax.


\section1 Type Checking and Implicit Type Conversions

String-based connections type-check by comparing strings at run-time. There are
three limitations with this approach:

\list 1
\li Connection errors can only be detected after the program has started running.
\li Implicit conversions cannot be done between signals and slots.
\li Typedefs and namespaces cannot be resolved.
\endlist

Limitations 2 and 3 exist because the string comparator does not have access to
C++ type information, so it relies on exact string matching.

In contrast, functor-based connections are checked by the compiler. The compiler
catches errors at compile-time, enables implicit conversions between compatible
types, and recognizes different names of the same type.

For example, only the functor-based syntax can be used to connect a signal that
carries an \c int to a slot that accepts a \c double. A QSlider holds an \c int
value while a QDoubleSpinBox holds a \c double value. The following snippet shows
how to keep them in sync:

\snippet snippets/signalsandslots/signalslotsyntaxes.cpp implicitconversion

The following example illustrates the lack of name resolution. QAudioInput::stateChanged()
is declared with an argument of type "QAudio::State". Thus, string-based
connections must also specify "QAudio::State", even if \c "State" is already
visible. This issue does not apply to functor-based connections because argument
types are not part of the connection.

\snippet snippets/signalsandslots/signalslotsyntaxes.cpp typeresolution


\section1 Making Connections to Lambda Expressions

The functor-based connection syntax can connect signals to C++11 lambda expressions,
which are effectively inline slots. This feature is not available with the
string-based syntax.

In the following example, the TextSender class emits a \c textCompleted() signal
which carries a QString parameter. Here is the class declaration:

\snippet snippets/signalsandslots/signalslotsyntaxes.h lambda

Here is the connection which emits \c{TextSender::textCompleted()} when the user
clicks the button:
\snippet snippets/signalsandslots/signalslotsyntaxes.cpp lambda

In this example, the lambda function made the connection simple even though
QPushButton::clicked() and \c TextSender::textCompleted() have incompatible
parameters. In contrast, a string-based implementation would require extra
boilerplate code.

\note The functor-based connection syntax accepts pointers to all functions,
including standalone functions and regular member functions. However, for the
sake of readability, signals should only be connected to slots, lambda expressions,
and other signals.


\section1 Connecting C++ Objects to QML Objects

The string-based syntax can connect C++ objects to QML objects, but the
functor-based syntax cannot. This is because QML types are resolved at run-time,
so they are not available to the C++ compiler.

In the following example, clicking on the QML object makes the C++ object print
a message, and vice-versa. Here is the QML type (in \c{QmlGui.qml}):
\snippet snippets/signalsandslots/QmlGui.qml crosslanguage

Here is the C++ class:
\snippet snippets/signalsandslots/signalslotsyntaxes.h crosslanguage

Here is the code that makes the signal-slot connections:
\snippet snippets/signalsandslots/signalslotsyntaxes.cpp crosslanguage

\note All JavaScript functions in QML take parameters of \c var type, which maps
to the QVariant type in C++.

When the QPushButton is clicked, the console prints,
\e{'QML received: "Hello from C++!"'}. Likewise, when the Rectangle is clicked,
the console prints, \e{'C++ received: "Hello from QML!"'}.

See \l{Interacting with QML Objects from C++} for other ways to let C++ objects
interact with QML objects.


\section1 Using Default Parameters in Slots to Connect to Signals with Fewer Parameters

Usually, a connection can only be made if the slot has the same number of arguments
as the signal (or less), and if all the argument types are compatible.

The string-based connection syntax provides a workaround for this rule: If the
slot has default parameters, those parameters can be omitted from the signal.
When the signal is emitted with fewer arguments than the slot, Qt runs the slot
using default parameter values.

Functor-based connections do not support this feature.

Suppose there is a class called \c DemoWidget with a slot \c printNumber() that
has a default argument:
\snippet snippets/signalsandslots/signalslotsyntaxes.h defaultparams

Using a string-based connection, \c{DemoWidget::printNumber()} can be connected
to QApplication::aboutToQuit(), even though the latter has no arguments. The
functor-based connection will produce a compile-time error:
\snippet snippets/signalsandslots/signalslotsyntaxes.cpp defaultparams

To work around this limitation with the functor-based syntax, connect the signal to
a lambda function that calls the slot. See the section above, \l{Making Connections
to Lambda Expressions}.


\section1 Selecting Overloaded Signals and Slots

With the string-based syntax, parameter types are explicitly specified. As a
result, the desired instance of an overloaded signal or slot is unambiguous.

In contrast, with the functor-based syntax, an overloaded signal or slot must
be casted to tell the compiler which instance to use.

For example, \l QLCDNumber has three versions of the \c display() slot:
\list 1
\li \c QLCDNumber::display(int)
\li \c QLCDNumber::display(double)
\li \c QLCDNumber::display(QString)
\endlist

To connect the \c int version to QSlider::valueChanged(), the two syntaxes are:

\snippet snippets/signalsandslots/signalslotsyntaxes.cpp overload

\sa qOverload()

*/