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c++qtsignals-slotsqpushbutton

Qt signal slot button

I need to change my text in QLabel every time when I clicked in order to my info updates. When I change the value of A, the value of B must change, too. I have two button that can change values in two QLabel (value of A, value of B).

main.cpp:
Counter A, B;
QObject::connect(&A, &Counter::changeValue, &B, &Counter::setValue);
QObject::connect(&A, &Counter::changeValue, &B, &Counter::Increment);
QObject::connect(&A, &Counter::changeValue, &B, &Counter::Decrement );    


QObject::connect(Add, &QPushButton::clicked, &A, &Counter::clickedAdd(QLabel* obj));
QObject::connect(Sub, &QPushButton::clicked, &B, &Counter::clickedSub(QLabel* obj));


class Counter: public QObject{
private: 
int count;
public slots:
int Increment () {
count++;
emit changeValue(count);
}
int Decrement () {
count--;
emit changeValue(count);
}
void clickedAdd(QLabel* obj){
int new_count = Increment();
obj_label->setText(QString::number(new_count));_
}
void clickedSub(QLabel* obj){
int new_count = Deccrement();
obj_label->setText(QString::number(new_count));_
}
void setValue(int new_count){
m_count = new_count;
emit changeValue(new_count);
}
public signals:
void changeValue(int);

How can I change my text in two QLabel's? Because in this way it stays const - 0.... When I try to connect:

QObject::connect(Add, &QPushButton::clicked, &A, &Counter::clickedAdd(QLabel* obj));

It writes an error: Call to non-static member function without an object argument.

But I pass the function an argument - QLabel*.

Solution

  • Assuming OP wants an application with

    • two counters which can be incremented and decremented
    • a GUI
      • displaying the counter values
      • buttons to increment/decrement the counters interactively.

    From this requirement, I would derive the structure of the program:

    • a class for the Counter (as already exposed by OP)
    • a GUI.

    For the latter, I often saw classes as well in numerous sample codes but I believe: for such a minimal GUI / application, this could even all be done in main() directly.

    testQCounter.cc:

    #include <iostream>
#include <string>

// Qt header:
#include <QtWidgets>

// OPs Counter Class
class Counter : public QObject {
  Q_OBJECT

  private:
    int count = 0;
  public slots:
    int Increment() {
      count++;
      emit changeValue(count);
      return count;
    }
    int Decrement() {
      count--;
      emit changeValue(count);
      return count;
    }
    int getValue() const { return count; }
    void setValue(int new_count) {
      count = new_count;
      emit changeValue(new_count);
    }
  signals:
    void changeValue(int);
};

#include "testQCounter.moc"

void setLabelValue(QLabel& qLbl, int value)
{
  qLbl.setText(QString::number(value));
}

// main application
int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup data
  Counter a, b;
  // setup GUI
  QWidget qWinMain;
  qWinMain.setWindowTitle("Counter Sample");
  QGridLayout qGrid;
  QLabel qLblATitle("Counter A");
  qGrid.addWidget(&qLblATitle, 0, 0);
  QPushButton qBtnIncA("+");
  qGrid.addWidget(&qBtnIncA, 1, 0);
  QLabel qLblA;
  qLblA.setAlignment(Qt::AlignRight);
  qLblA.setFrameStyle(QLabel::Box);
  qGrid.addWidget(&qLblA, 2, 0);
  QPushButton qBtnDecA("-");
  qGrid.addWidget(&qBtnDecA, 3, 0);
  QLabel qLblBTitle("Counter B");
  qGrid.addWidget(&qLblBTitle, 0, 1);
  QPushButton qBtnIncB("+");
  qGrid.addWidget(&qBtnIncB, 1, 1);
  QLabel qLblB("");
  qLblB.setAlignment(Qt::AlignRight);
  qLblB.setFrameStyle(QLabel::Box);
  qGrid.addWidget(&qLblB, 2, 1);
  QPushButton qBtnDecB("-");
  qGrid.addWidget(&qBtnDecB, 3, 1);
  qWinMain.setLayout(&qGrid);
  qWinMain.show();
  setLabelValue(qLblA, a.getValue());
  setLabelValue(qLblB, b.getValue());
  // install signal handlers
  // connect clicked signal of buttons to counter a
  QObject::connect(&qBtnDecA, &QPushButton::clicked, &a, &Counter::Decrement);
  QObject::connect(&qBtnIncA, &QPushButton::clicked, &a, &Counter::Increment);
  // connect changeValue signal of counter a to a function
  QObject::connect(&a, &Counter::changeValue,
    [&](int value) { setLabelValue(qLblA, value); });
  // connect clicked signal of buttons to counter b
  QObject::connect(&qBtnDecB, &QPushButton::clicked, &b, &Counter::Decrement);
  QObject::connect(&qBtnIncB, &QPushButton::clicked, &b, &Counter::Increment);
  // connect changeValue signal of counter b to b function
  QObject::connect(&b, &Counter::changeValue,
    [&](int value) { setLabelValue(qLblB, value); });
  // runtime loop
  return app.exec();
}

    Output:

    Qt Version: 5.15.1

    Animated Snapshot of testQCounter.exe

    Notes:

    1. Concerning the update of the counter value label:
      The signature of QLabel::setText() (the potential slot) is void QLabel::setText(const QString&).
      The signature of the signal to connect is void Counter::changeValue(int).
      Obviously, these signatures are not compatible.

      For convenience, I introduced a function

      void setLabelValue(QLabel& qLbl, int value)
{
  qLbl.setText(QString::number(value));
}

      but this doesn't fix the incompatibility because the function still has another parameter QLabel& which is not in the emitted signal.

      This is a very usual case and the very usual solution is to bind the resp. QLabel reference to the signal. It can be done most easily using a lambda.

      When I first saw lambdas in C++, I found the syntax non-intuitive and somehow scaring. However, after having read the doc. and tutorials I got used to it, and today, I couldn't imagine to live without. I must admit before I learnt about lambdas I had to fiddle with bind() and hide() (in gtkmm with sigc++). This was a real nightmare…

    2. Counter defines a signal. (I fixed the wrong syntax of OP.)

      To make this linking properly, I had to add some things:

      • Q_OBJECT
      • #include "testQCounter.moc"
      • support for the Qt moc in my build script.

      My build script is a Visual Studio project which I prepared with a CMake script. I had to extend my CMakeLists.txt for moc (as I usually build without moc).

      The CMakeLists.txt used to build the build script for testQCounter.cc:

      project(QCounter)

cmake_minimum_required(VERSION 3.10.0)

set_property(GLOBAL PROPERTY USE_FOLDERS ON)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

find_package(Qt5 COMPONENTS Widgets REQUIRED)

set(CMAKE_AUTOMOC ON)
set(CMAKE_INCLUDE_CURRENT_DIR ON)
include_directories("${CMAKE_SOURCE_DIR}")

add_executable(testQCounter testQCounter.cc)
target_link_libraries(testQCounter Qt5::Widgets)

    For my production, I would use separate classes for the GUI stuff as well, of course.

    So, considering that the sample has two counters with nearly identical GUIs, it may make sense to introduce a counter widget – CounterEdit.

    testQCounter2.cc:

    #include <iostream>
#include <string>

// Qt header:
#include <QtWidgets>

// OPs Counter Class
class Counter : public QObject {
  Q_OBJECT

  private:
    int count = 0;
  public slots:
    int Increment() {
      count++;
      emit changeValue(count);
      return count;
    }
    int Decrement() {
      count--;
      emit changeValue(count);
      return count;
    }
    int value() const { return count; }
    void setValue(int new_count) {
      count = new_count;
      emit changeValue(new_count);
    }
  signals:
    void changeValue(int);
};

#include "testQCounter2.moc"

class CounterEdit : public QWidget {
  private:
    Counter* pCounter = nullptr;
    QVBoxLayout qVBox;
    QLabel qLblTitle;
    QPushButton qBtnInc;
    QLabel qLblValue;
    QPushButton qBtnDec;

    QMetaObject::Connection connectionInc;
    QMetaObject::Connection connectionDec;
    QMetaObject::Connection connectionValue;

  public:
    CounterEdit(const QString& title, QWidget* pQParent = nullptr) :
      QWidget(pQParent),
      qLblTitle(title),
      qBtnInc("+"),
      qLblValue(""),
      qBtnDec("-")
    {
      qLblTitle.setAlignment(Qt::AlignCenter);
      qVBox.addWidget(&qLblTitle);
      qVBox.addWidget(&qBtnInc);
      qLblValue.setAlignment(Qt::AlignRight);
      qLblValue.setFrameStyle(QLabel::Box);
      qVBox.addWidget(&qLblValue);
      qVBox.addWidget(&qBtnDec);
      setLayout(&qVBox);
    }

    virtual ~CounterEdit()
    {
      QObject::disconnect(connectionInc);
      QObject::disconnect(connectionDec);
      QObject::disconnect(connectionValue);
    }

    CounterEdit(const CounterEdit&) = delete;
    CounterEdit& operator=(const CounterEdit&) = delete;

    Counter* counter() { return pCounter; }
    const Counter* counter() const { return pCounter; }

    void updateValue();
    void updatevalue(int) { updateValue(); }

    void setCounter(Counter* pCounter);

};

void CounterEdit::updateValue()
{
  if (pCounter) {
    qLblValue.setText(QString::number(pCounter->value()));
  } else {
    qLblValue.setText(QString());
  }
}

void CounterEdit::setCounter(Counter* pCounter)
{
  QObject::disconnect(connectionInc);
  QObject::disconnect(connectionDec);
  QObject::disconnect(connectionValue);
  this->pCounter = pCounter;
  if (pCounter) {
    qLblValue.setText(QString::number(pCounter->value()));
    connectionInc
      = QObject::connect(&qBtnInc, &QPushButton::clicked, pCounter, &Counter::Increment);
    connectionDec
      = QObject::connect(&qBtnDec, &QPushButton::clicked, pCounter, &Counter::Decrement);
    connectionValue
      = QObject::connect(pCounter, &Counter::changeValue, this, &CounterEdit::updateValue);
  }
}

// main application
int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup data
  Counter a, b;
  // setup GUI
  QWidget qWinMain;
  qWinMain.setWindowTitle("Counter Sample");
  QHBoxLayout qHBox;
  CounterEdit editA("Counter A:");
  qHBox.addWidget(&editA);
  CounterEdit editB("Counter B:");
  qHBox.addWidget(&editB);
  qWinMain.setLayout(&qHBox);
  qWinMain.show();
  editA.setCounter(&a);
  editB.setCounter(&b);
  // runtime loop
  return app.exec();
}

    Output:

    Qt Version: 5.15.1

    Snapshot of testQCounter2.exe

    Notes:

    1. As data model and GUI are not anymore hardwired by design, I changed the management of the signal-slot connections a bit:

      • The connections are done on demand (in CounterEdit::setCounter()).
      • Connections are disconnected when not anymore needed.

      It's not strictly necessary to store the connections like I did in the sample. In Qt, a connection may be disconnected as well by providing signal and slot like in connect(). I don't like this for two reasons:

      1. I'm paranoid.
      2. This won't work for lambdas.

    2. While the actual member function for update (CounterEdit::updateValue()) is parameter-less, I provided a second flavor (CounterEdit::updateValue(int)) which is just a wrapper.

      However, this 2nd flavor has an identical signature like Counter::changeValue() and, hence, can be used as slot without an adapter.