Direct connect a built-in signal from one class to a slot on another class

JonB

@Habibie
This looks reasonable. You are indeed allowed to chain one signal onto another, as you have done.

If you are a newbie: do yourself a favour, drop your SIGNAL/SLOT macros, change over to New Signal Slot Syntax. That will give you edit-time autocomplete on picking signals/slots, compile-time errors on wrong connections and will allow you to use lambdas in due course for more powerful slots.

Habibie

@JonB said in Direct connect a built-in signal from one class to a slot on another class:

@Habibie
This looks reasonable. You are indeed allowed to chain one signal onto another, as you have done.

Reasonable could mean there can be some improvements, am I right? If so, I sure don't mind to learn more about the improvement if you and/or anyone is willing to help.

If you are a newbie: do yourself a favour, drop your SIGNAL/SLOT macros, change over to New Signal Slot Syntax. That will give you edit-time autocomplete on picking signals/slots, compile-time errors on wrong connections and will allow you to use lambdas in due course for more powerful slots.

I am not sure what exactly did you mean. Did you mean the commented SIGNAL/SLOT macros I put in the constructor of src/mainwindow.cpp as shown below?

// Using string-based QObject::connect syntax.
//    connect(D1, SIGNAL(indexChanged(int)), this, SLOT(outputToLineEdit(int)));
//    connect(D1, SIGNAL(indexChanged(int)), D2, SLOT(outputToLineEdit(int)));

JonB

@Habibie said in Direct connect a built-in signal from one class to a slot on another class:

Reasonable could mean there can be some improvements, am I right?

No, it means fine.

I am not sure what exactly did you mean. Did you mean the commented SIGNAL/SLOT macros I put in the constructor of src/mainwindow.cpp as shown below?

No, it referred to

connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));

Use new style instead of any SIGNAL or SLOT macros in connect()s.

Habibie

@JonB said in Direct connect a built-in signal from one class to a slot on another class:

No, it referred to

connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));

Use new style instead of any SIGNAL or SLOT macros in connect()s.

I tried the following:

connect(this, &DialogOne::ui->comboBox->currentIndexChanged, this, &DialogOne::indexChanged);

and got some errors as shown below:

../src/dialogone.cpp: In constructor ‘DialogOne::DialogOne(QWidget*)’:
../src/dialogone.cpp:12:96: error: no matching function for call to ‘DialogOne::connect(DialogOne*, <unresolved overloaded function type>, DialogOne*, void (DialogOne::*)(int))’
   12 |     connect(this, &DialogOne::ui->comboBox->currentIndexChanged, this, &DialogOne::indexChanged);

Christian Ehrlicher

@Habibie said in Direct connect a built-in signal from one class to a slot on another class:

and got some errors as shown below:

And the documentation tells you why and how to circumvent it...

Habibie

@Christian-Ehrlicher said in Direct connect a built-in signal from one class to a slot on another class:

And the documentation tells you why and how to circumvent it...

When I tried the suggestion in the above link, I keep getting the following error.

g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DGIT_VERSION=\"\" -DGIT_HASH=\"febc5d8c3a77a4a9039601f638b0ce220c3b3189\" -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../SignalsAndSlots-01 -I. -I../include -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o dialogone.o ../src/dialogone.cpp
../src/dialogone.cpp: In constructor ‘DialogOne::DialogOne(QWidget*)’:
../src/dialogone.cpp:13:22: error: expected primary-expression before ‘,’ token
   13 |     connect(QComboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index) {
      |                      ^
../src/dialogone.cpp: In lambda function:
../src/dialogone.cpp:14:13: error: ‘class Ui::DialogOne’ has no member named ‘lineEdut’
   14 |         ui->lineEdut->setText(QString::number(index));
      |             ^~~~~~~~
make: *** [Makefile:566: dialogone.o] Error 1

Here is (the changes in) the constructor of src/mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

Here is (the changes in) the constructor of src/dialogone.cpp:

#include "dialogone.h"
#include "ui_dialogone.h"

DialogOne::DialogOne(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogOne)
{
    ui->setupUi(this);

    init();
    connect(comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index) {
        ui->lineEdit->setText(QString::number(index));
    });
}

And, (the changes in) the constructor of src/dialogtwo.cpp

#include "dialogtwo.h"
#include "ui_dialogtwo.h"

DialogTwo::DialogTwo(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogTwo)
{
    ui->setupUi(this);
    connect(ui->comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), this, &DialogOne::indexChanged);
}

However, if I do the following (I don't know if this is what you really mean), it works just fine, except I am not able to connect the DialogOne::indexChange(int index) signal to the lineEdit in DialogTwo class using lambda.

src/mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    D1 = new DialogOne(this);
    D2 = new DialogTwo(this);

    // Using functor-based QObject::connect syntax with lambda
    connect(D1, &DialogOne::indexChanged, this, [this](int index) {
        ui->lineEdit->setText(QString::number(index));
    });

   // Using functor-based QObject::connect syntax
   connect(D1, &DialogOne::indexChanged, D2, &DialogTwo::outputToLineEdit);
}

src/dialogone.cpp:

#include "dialogone.h"
#include "ui_dialogone.h"

DialogOne::DialogOne(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogOne)
{
    ui->setupUi(this);

    init();
    connect(ui->comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), this, &DialogOne::indexChanged);
}

src/dialogtwo.cpp:

#include "dialogtwo.h"
#include "ui_dialogtwo.h"

DialogTwo::DialogTwo(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogTwo)
{
    ui->setupUi(this);
}

Christian Ehrlicher

What Qt version/ compiler do you use? I would say an ancient one... then you should also take a look at the Qt version documentation you use. e.g. for Qt5.5: https://doc.qt.io/archives/qt-5.5/qcombobox.html#currentIndexChanged

Habibie

@Christian-Ehrlicher said in Direct connect a built-in signal from one class to a slot on another class:

What Qt version/ compiler do you use? I would say an ancient one... then you should also take a look at the Qt version documentation you use. e.g. for Qt5.5: https://doc.qt.io/archives/qt-5.5/qcombobox.html#currentIndexChanged

I'm doing all of these on a 64-bit Dell Inspiron-3252 (a rather old) computer running on an uBuntu 21.04 (Hirsute) OS with Qt-5.15.2 (I think this is pretty up-to-date for Qt-5.x) and gcc-10.3.0.

~ qmake --version
QMake version 3.1
Using Qt version 5.15.2 in /usr/lib/x86_64-linux-gnu
~
~ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 10.3.0-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-10 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-10-gDeRY6/gcc-10-10.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-gDeRY6/gcc-10-10.3.0/debian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-mutex
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.3.0 (Ubuntu 10.3.0-1ubuntu1)

Christian Ehrlicher

I see that you compiler error does not fit to your pasted code:

connect(QComboBox, QOverload<int>::of(&QComboBox::currentIndexChanged),

I don't see a variable named QComboBox in your code.

Habibie

@Christian-Ehrlicher said in Direct connect a built-in signal from one class to a slot on another class:

I see that you compiler error does not fit to your pasted code:

connect(QComboBox, QOverload<int>::of(&QComboBox::currentIndexChanged),

I don't see a variable named QComboBox in your code.

You are right and sorry about that.

Now, I put it exactly like the documentation link you provided and filled the dotted line with ui->lineEdit(QString::number(index)); in the constructor of DialogOne class as shown below.

#include "dialogone.h"
#include "ui_dialogone.h"

DialogOne::DialogOne(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogOne)
{
    ui->setupUi(this);

    init();
    connect(comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index){
        ui->lineEdit(QString::number(index));
    });
}

The compilation of the above code generates the following error:

g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DGIT_VERSION=\"\" -DGIT_HASH=\"febc5d8c3a77a4a9039601f638b0ce220c3b3189\" -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../SignalsAndSlots-01 -I. -I../include -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o dialogone.o ../src/dialogone.cpp
../src/dialogone.cpp: In constructor ‘DialogOne::DialogOne(QWidget*)’:
../src/dialogone.cpp:12:13: error: ‘comboBox’ was not declared in this scope; did you mean ‘QComboBox’?
   12 |     connect(comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index){
      |             ^~~~~~~~
      |             QComboBox
../src/dialogone.cpp: In lambda function:
../src/dialogone.cpp:13:13: error: ‘class Ui::DialogOne’ has no member named ‘lineEdit’
   13 |         ui->lineEdit(QString::number(index));
      |             ^~~~~~~~
make: *** [Makefile:566: dialogone.o] Error 1

Christian Ehrlicher

C++ basics missing - you don't have a member named 'comboBox' on your class - so how do you suppose that this works? I would guess your combobox is somewhere in your ui struct.

Habibie

@Christian-Ehrlicher said in Direct connect a built-in signal from one class to a slot on another class:

C++ basics missing - you don't have a member named 'comboBox' on your class - so how do you suppose that this works? I would guess your combobox is somewhere in your ui struct.

Exactly.

I ONLY have a comboBox in DialogOne class. From there, whenever there is a change in the index (currentIndexChanged), I want it to trigger the external signal indexChanged(int index) (also declared in the header file of DialogOne class). This is done through the old (string) style SIGNAL/SIGNAL connect in the constructor of DialogOne class as shown below and it works just fine.

#include "dialogone.h"
#include "ui_dialogone.h"

DialogOne::DialogOne(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DialogOne)
{
    ui->setupUi(this);

    init();
    connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));
}

However, since in the constructor of MainWindow, I use the new functional style connect, you suggested in one of your posts above to drop your SIGNAL/SLOT macros, change over to New Signal Slot Syntax. TBH, I was so happy and thrill to see such a suggestion mainly because I wasn't able to do it before and someone like you made such a recommendation giving me a hope, especially after I read the documentation link you suggested to read. So, now it looks like that it's not possible, is it? I am still keeping my fingers crossed to hope the answer is yes.

The documentation link you suggested to read really gives me an impression that we can just drop the external indexChange(in index) signal (declared in the header file of DialogOne class) and use lambda to connect the built-in currentIndexChangedsignal from thecomboBoxdirectly to the twolineEditdeclared in bothMainWindowandDialogTwo` classes. If this can be done, I am all ears.

JonB

@Habibie
You say this works:

connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));

and it's inside DialogOne::DialogOne().

Then you say you changed to

connect(comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index){

still inside DialogOne::DialogOne().

The compiler says

../src/dialogone.cpp:12:13: error: ‘comboBox’ was not declared in this scope

Why have you changed from ui->comboBox to comboBox?

Habibie

@JonB said in Direct connect a built-in signal from one class to a slot on another class:

@Habibie
You say this works:
connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));
and it's inside DialogOne::DialogOne().

The above uses the old string SIGNAL/SIGNAL connection style.

Then you say you changed to
connect(comboBox, QOverload<int>::of(&QComboBox::currentIndexChanged), [=](int index){
still inside DialogOne::DialogOne().

The compiler says

../src/dialogone.cpp:12:13: error: ‘comboBox’ was not declared in this scope

Why have you changed from ui->comboBox to comboBox?

The above uses a new functional connection with lambda.

BTW, with an old style string SIGNAL/SIGNAL connection, if I change from this

    connect(ui->comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));

to this

    connect(comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));

the compilation spits out the following error messages.

g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DGIT_VERSION=\"\" -DGIT_HASH=\"febc5d8c3a77a4a9039601f638b0ce220c3b3189\" -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../SignalsAndSlots-01 -I. -I../include -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o dialogone.o ../src/dialogone.cpp
../src/dialogone.cpp: In constructor ‘DialogOne::DialogOne(QWidget*)’:
../src/dialogone.cpp:12:13: error: ‘comboBox’ was not declared in this scope; did you mean ‘QComboBox’?
   12 |     connect(comboBox, SIGNAL(currentIndexChanged(int)), this, SIGNAL(indexChanged(int)));
      |             ^~~~~~~~
      |             QComboBox
make: *** [Makefile:566: dialogone.o] Error 1

SGaist

Hi,

That's exactly the point of @JonB, there's no reason to remove the ui-> from your code.

Habibie

@SGaist said in Direct connect a built-in signal from one class to a slot on another class:

Hi,

That's exactly the point of @JonB, there's no reason to remove the ui-> from your code.

Got it and thank you for pointing it out.