(C++) From Qt signal to Boost signal

 

This article describes why and how to move from using Qt signals to using Boost signals.

 

 

 

 

 

Introduction

 

The first paragraph contains arguments why to move from Qt signals to Boost signals and when you might refrain from it. The second paragraph and beyond follow the transition in code for two very simple classes.

 

 

 

 

 

Why move from Qt signals to Boost signals?

 

My personal reasons for moving from Qt signals to Boost signals, are:

• When a library supports Boost, but conflicts with Qt
• Boost signals do not require the seperation of a class in a header (.h)file and implementation (.cpp) file (because Boost signals do not require the use of moc)
• Because the STL, TR1 and Boost libraries are 'the number one libraries' [1][2]
• I prefer using standard C++ keywords only

 

Arguments against moving from Qt signals to Boost signals are:

• The Qt signal syntax is easier to read and write
• When using only Qt, adding the Boost signals libraries increase compile time

 

 

 

 

 

The program's goal

 

The program implements two classes: an emitter and a receiver. After connecting the signals and slots, the emitter emits a signal, received by the receiver.

 

main should be:

 

---

int main() {   //Create emitter   //Create receiver   //Connect emitter's signal to receiver   //Let emitter emit its signal }

 

Screen output should be like:

 

---

Emitter: emitting signal Receiver: received signal

 

 

 

 

 

CppFromQtSignalToBoostSignal1: the ideal starting point

 

• Download the Qt Creator project 'CppFromQtSignalToBoostSignal1' (zip)

 

The code below would be the ideal starting point. But because moc requires the seperation of a class in a header (.h)file and implementation (.cpp) file, this code results in the link error undefined reference to 'vtable for [...]'. Due to this, next step is to seperate the class in a working example.

 

---

#include <iostream> #include <QObject> class QtEmitter : public QObject {   Q_OBJECT public:   void DoEmit()   {     std::clog << "QtEmitter: emitting signal\n";     emit signal_emit();   } signals:   void signal_emit(); }; class QtReceiver : public QObject {   Q_OBJECT public slots:   void OnReceive()   {     std::clog << "QtReceiver: received signal\n";   } }; int main() {   //Create emitter   QtEmitter e;   //Create receiver   QtReceiver r;   //Connect emitter's signal to receiver   QObject::connect(&e,SIGNAL(signal_emit()),&r,SLOT(OnReceive()));   //Let emitter emit its signal   e.DoEmit(); }

 

 

 

 

 

CppFromQtSignalToBoostSignal2: the working starting point

 

• Download the Qt Creator project 'CppFromQtSignalToBoostSignal2' (zip)

 

The code below would be the working starting point. The next step is to replace the Qt signals by Boost signals.

 

main.cpp

 

---

#include "qtemitter.h" #include "qtreceiver.h" int main() {   //Create emitter   QtEmitter e;   //Create receiver   QtReceiver r;   //Connect emitter's signal to receiver   QObject::connect(&e,SIGNAL(signal_emit()),&r,SLOT(OnReceive()));   //Let emitter emit its signal   e.DoEmit(); }

 

qtemitter.cpp

 

---

#include <iostream> #include "qtemitter.h" void QtEmitter::DoEmit() {   std::clog << "QtEmitter: emitting signal\n";   emit signal_emit(); }

 

qtemitter.h

 

---

#ifndef QTEMITTER_H #define QTEMITTER_H #include <QObject> class QtEmitter : public QObject {   Q_OBJECT public:   void DoEmit(); signals:   void signal_emit(); }; #endif // QTEMITTER_H

 

qtreceiver.cpp

 

---

#include <iostream> #include "qtreceiver.h" void QtReceiver::OnReceive() {   std::clog << "QtReceiver: received signal\n"; }

 

qtreceiver.h

 

---

#ifndef QTRECEIVER_H #define QTRECEIVER_H #include <QObject> class QtReceiver : public QObject {   Q_OBJECT public slots:   void OnReceive(); }; #endif // QTRECEIVER_H

 

 

 

 

 

CppFromQtSignalToBoostSignal3: the conversion to Boost signals

 

• Download the Qt Creator project 'CppFromQtSignalToBoostSignal3' (zip)

 

In the code below, the Qt signal is replaced by using a Boost signal. The next step is: can we put all this code in a single file?

 

emitter.cpp

 

---

#include <iostream> #include "emitter.h" void Emitter::DoEmit() {   std::clog << "Emitter: emitting signal\n";   m_signal(); }

 

emitter.h

 

---

#ifndef EMITTER_H #define EMITTER_H #include <boost/signals2.hpp> struct Emitter {   void DoEmit();   boost::signals2::signal<void ()> m_signal; }; #endif // EMITTER_H

 

main.cpp

 

---

#include "emitter.h" #include "receiver.h" int main() {   //Create emitter   Emitter e;   //Create receiver   Receiver r;   //Connect emitter's signal to receiver   e.m_signal.connect(     boost::bind(       &Receiver::OnReceive,       r));   //Let emitter emit its signal   e.DoEmit(); }

 

receiver.cpp

 

---

#include <iostream> #include "receiver.h" void Receiver::OnReceive() {   std::clog << "Receiver: received signal\n"; }

 

receiver.h

 

---

#ifndef RECEIVER_H #define RECEIVER_H struct Receiver {   void OnReceive(); }; #endif // RECEIVER_H

 

 

 

 

 

CppFromQtSignalToBoostSignal4: Boost signals in a single file

 

• Download the Qt Creator project 'CppFromQtSignalToBoostSignal4' (zip)

 

In the code below, all the code is placed a single file, and it works fine!

 

---

#include <iostream> #include <boost/signals2.hpp> struct Emitter {   void DoEmit()   {     std::clog << "Emitter: emitting signal\n";     m_signal();   }   boost::signals2::signal<void ()> m_signal; }; struct Receiver {   void OnReceive()   {     std::clog << "Receiver: received signal\n";   } }; int main() {   //Create emitter   Emitter e;   //Create receiver   Receiver r;   //Connect emitter's signal to receiver   e.m_signal.connect(     boost::bind(       &Receiver::OnReceive,       r));  //Let emitter emit its signal   e.DoEmit(); }

 

 

 

 

 

Conclusion

 

There are reasons to move from using Qt signals to using Boost signals. This article shows that this is easy.

 

 

 

 

 

References

 

1. Scott Meyers. Effective C++ (3rd edition). ISBN: 0-321-33487-6. Item 53: Familiarize yourself with the standard library, including TR1
2. Scott Meyers. Effective C++ (3rd edition). ISBN: 0-321-33487-6. Item 54: Familiarize yourself with Boost