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Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

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Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

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The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

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All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

A Voz do Brasil ('The Voice of Brazil') is a governmental radio program in Brazil produced by Empresa Brasil de Comunicação, the country's public broadcaster. The programme must be aired at any one-hour slot between the time frame of 7:00pm to 10:00pm by all Brazilian radio stations every weeknight excluding national holidays and other occasions.^[1] It is the oldest radio program in the country and the longest-running in the Southern Hemisphere.^[2]

History[edit]

The first national radio program in Brazil hit the airwaves on July 22, 1935, known as Programa Nacional (National Program). It was presented by Luis Jatobá.^[1]

On January 3, 1938, the program, retitled A Hora do Brasil (The Brazil Hour), went national and became a mandatory broadcast.^[2][3] It featured the speeches and actions of the president, initially Getúlio Vargas, as well as cultural programming including music and art, with 70 percent of the music output coming from Brazilian composers. The program was initially produced by the National Propaganda Department (DNP), which was replaced in 1939 by the Department of Press and Propaganda [pt]. Especially after Brasília was instated as the capital in 1960, A Hora do Brasil was known for its opening, reciting the time in the capital: Em Brasília, 19 horas. Such is the notoriety of the line that when a former director of Radiobrás published a book about his time at the public broadcaster, it was titled Em Brasília, 19 Horas.^[4]

1962 changes[edit]

1962 saw the passage of the Brazilian Telecommunications Code [pt], a law that governed telecommunications and broadcasting. Under the new Code, the program was restructured, with the legislature taking up the second half-hour; in 1971 it was retitled Voz do Brasil.^[2] The legislature's entrance into the program opened the door for other portions of the federal government: the judiciary now occupies the last five minutes of the first half hour, and segments from the Tribunal de Contas da União (TCU) air on Mondays, Wednesdays and Fridays.^[5] During the military regime of the 1970s and 1980s, the legislative segment of the program was often the only airtime in the control of opposition forces, and after the return of democracy, the program took on a more journalistic format and focus.^[6]

The program was later produced by the Empresa Brasileira de Notícias (EBN) and Empresa Brasileira de Radiodifusão (Radiobrás). In 2007, the provisional decree that created the Empresa Brasil de Comunicação (EBC, 'Brazilian Communication Company') transferred the production of this program to this new public broadcaster.^[7]

Longevity[edit]

In 1995, the Guinness Book of World Records certified A Voz do Brasil as the oldest radio program in Brazil.^[2]

Format[edit]

A Voz do Brasil is an hour-long program divided into two components. The first 25 minutes are presented live via Embratel and produced by EBC from Rádio Nacional de Brasília. This segment features the content from the executive branch. The opening theme tune comes from the opera Il Guarany by Antônio Carlos Gomes; while the program used the original for decades, it has been re-recorded in variants such as samba and capoeira, and more recently a classical version was commissioned for the program's 2016 relaunch.^[2]

The remaining 35 minutes are pre-recorded and inserted by EBC:

• Judiciary: Notícias do Poder Judiciário (7:25-7:30)
• Senate: Jornal do Senado (7:30-7:40)
• Chamber of Deputies: Jornal Câmara dos Deputados (7:40-8:00)

Additionally, the TCU's Minuto do TCU (TCU Minute) airs on Mondays, Wednesdays and Fridays at the end of the program.^[5]

In 2016, the first 25 minutes of A Voz do Brasil was refreshed with a format including more audience participation. Additionally, new hosts, Airton Medeiros and Gláucia Gomes, were introduced.^[2] In July 2017, Airton and Gláucia were replaced by Nasi Brum and Gabriela Mendes, until then occasional presenters.

Dissent from commercial broadcasters[edit]

The 7 p.m. hour slot occupied by A Voz do Brasil and its mandatory broadcast (except on weekends and holidays^[2]) have caused friction with commercial broadcasters. These broadcasters consider that the program has lost relevance over the years with the rise of other media. In addition, A Voz do Brasil runs during the evening rush hour in many large cities, so stations cannot present traffic information or news updates during the program. Some stations have held preliminary injunctions that allowed them to run the program in the early morning. Rádio Jovem Pan in Paraná ran the program at 5 a.m. for more than 12 years until its authorization to do so was revoked in January 2012, though the station still refused to transmit A Voz do Brasil in its habitual 7 p.m. timeslot.^[8] Additionally, the Paraná Broadcasters' Association held a general injunction, applicable to all stations in the state, from 2008 to 2009.^[8] From 2006 to 2010, stations in Rio Grande do Sul also could move the program thanks to an injunction obtained by that state's broadcasters association; Grupo RBS stations continued carrying alternative programming on Internet streams and pay television even after the Supreme Federal Court closed the door to timeshifting of the program.^[9]

In São Paulo, Rádio Metropolitana Paulista was sanctioned with a mandatory one-day suspension of programming in 2013 for not taking A Voz do Brasil in its normal timeslot.^[10]

While attempts at allowing stations more latitude in when they carry the program have been debated in the Chamber of Deputies,^[8] so-called 'flexibilization' policies have been enacted on special occasions of national interest, responding to petitions from the Brazilian Association of Radio and Television Stations (ABERT) and state broadcasters' associations. On June 12, 2014, Dilma Rousseff enacted a provisional measure allowing stations to carry the program between 7 and 10pm, regardless of whether they were carrying matches of the 2014 FIFA World Cup.^[11]Michel Temer authorized an identical measure during the 2016 Summer Olympics and 2016 Summer Paralympics.^[12] Finally, on April 4, 2018, Temer signed Law 13.644, allowing this measure permanently for all stations except educational stations and legislative stations when the respective legislatures are not in session.^[13]

The negative reputation of A Voz do Brasil inspired a feature in the mobile app of radio station Paradiso FM, which broadcasts to Rio de Janeiro. If the app detects that the user is driving above the speed limit, it imposes a 'sound penalty' and subjects the user to one minute of the program, which a station spokesman describes as 'a nightmare for all Brazilians'.^[14]

See also[edit]

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• La Hora Nacional, a similar program in Mexico

References[edit]

1. ^ ^ab'Programa surgiu em 1935, durante o governo de Getúlio Vargas'. Correio do Estado. 22 August 2010. Retrieved 2 June 2017.
2. ^ ^abcdefgBrandão, Marcelo (31 October 2016). 'A Voz do Brasil estreia hoje em novo formato' [A Voz do Brasil premieres new format today]. Retrieved 2 June 2017.
3. ^Nava, Carmen; Lauerhass, Ludwig (2006). Brazil in the Making: Facets of National Identity. Rowman & Littlefield. p. 222.
4. ^Lins da Silva, Carlos Eduardo (5 April 2008). 'Bucci narra conflitos na Radiobrás' [Bucci describes conflicts at Radiobrás]. Folha de S. Paulo (in Portuguese). Retrieved 4 June 2017.
5. ^ ^ab'A Voz do Brasil'. SECOM. 3 December 2013. Retrieved 2 June 2017.
6. ^Sant'Anna, Chico; Almeida, Beto (25 February 2011). 'Em Brasília, 19 horas: Pela preservação da Voz do Brasil' [Em Brasilia, 19 horas: For the preservation of Voz do Brasil]. Carta Maior. Retrieved 2 June 2017.
7. ^'Produtos Sustentáveis no seu dia a dia'. www.ekological.com.br. 30 November 2019.
8. ^ ^abcFavretto, Viviane (16 February 2012). 'Voz do Brasil pode ter novo horário' [Voz do Brasil could have new timeslot]. Gazeta do Povo. Retrieved 2 June 2017.
9. ^'Rádios da RBS mantêm programas alternativos à A Voz do Brasil' [RBS radio stations maintain alternative programs to A Voz do Brasil]. 17 May 2010. Archived from the original on 9 June 2017. Retrieved 2 June 2017.
10. ^'Rádio é suspensa por um dia por não transmitir A Voz do Brasil na hora certa' [Radio station suspended for a day for not transmitting A Voz do Brasil at the right time]. Migalhas. 6 November 2013. Retrieved 2 June 2017.
11. ^'Começa período especial da flexibilização da Voz do Brasil' [Special period of flexibilization for Voz do Brasil begins]. ABERT. 10 June 2014. Retrieved 2 June 2017.
12. ^'Governo flexibiliza horário de 'A Voz do Brasil' durante Olimpíada, diz Abert' [Government makes flexible timeslot for 'A Voz do Brasil' during Olympics, according to Abert]. G1. 21 July 2016. Retrieved 2 June 2017.
13. ^Câmara dos Deputados (4 April 2018). 'Sancionada lei que flexibiliza horário do programa 'A Voz do Brasil'' [Signed into law bill that flexibilizes time slot of program 'A Voz do Brasil']. Jusbrasil (in Portuguese). Retrieved 5 June 2020.
14. ^'In Rio the penalty for speeding is having to listen to the worst radio on Earth'. Radio Info. 23 May 2017. Retrieved 2 June 2017.

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