connect (): how to pass a parameter to the function

Alby

I have a program with an array with 9 buttons,

I need to link them to a function [check (int idCell)], but I also need to pass a parameter to the function for work it, I have already tried this (it doesn't work):

QSignalMapper* signalMapper = new QSignalMapper (this) ;

for(i = 0; i < 9; i++)
{
    connect(buttons[i], SIGNAL (clicked()), signalMapper, SLOT (map()));

    signalMapper->setMapping(buttons[i], i);

    connect(signalMapper, SIGNAL(mapped(int)), this, SLOT(check(int)));
}

and this too (it doesn't work):

  for(i = 0; i < 9; i++)
 {
    connect(buttons[i],  &QAction::triggered, this, [this]{ check(1); });
}

help.

J.Hilk

@Alby it doesn't work, isn't really an adequate error description.

Anyway I assume this would do what you want it to:

for(i = 0; i < 9; i++)
 {
    connect(buttons[i],  &QAction::triggered, this, [=]()->void{ check(i); }); // = -> we capture everything by copy, that includes the value of I during the creation of this connect
}

Alby

@J-Hilk Unfortunately it gives me the following error:

error: no matching member function for call to 'connect'

I forgot to tell you that I run that piece of code inside the builder of a class, if it can help you i can send you all the code

J.Hilk

@Alby what exactly is the type of buttons ?

Alby

@J-Hilk It is QPushButton

J.Hilk

@Alby said in connect (): how to pass a parameter to the function:

@J-Hilk It is QPushButton

well, no.
But I assume the pointers stored in the list/vector/array are of type QPushButton*

for(i = 0; i < 9; i++)
 {
    connect(buttons[i],  &QPushButton::clicked, this, [=]()->void{ check(i); }); // = -> we capture everything by copy, that includes the value of I during the creation of this connect
}

KroMignon

@Alby said in connect (): how to pass a parameter to the function:

connect(buttons[i], &QAction::triggered, this, [this]{ check(1); });

This cannot work, because you have to specify valid lambda function, you have forgotten the parenthesis.

For example, this should work:

 for(i = 0; i < 9; i++)
 {
    // capture this and i in lambda to be able to use it
    connect(buttons[i],  &QAction::triggered, this, [this, i](){ 
       check(i); 
    });
}

Alby

@J-Hilk Yes, I was wrong I meant * QPushButton.

I tried your solution, first with the buttons inside the array, it didn't work, so I tried to create a separate button:

  QPushButton * btn = new QPushButton ();

I set its geometry and created a connection:

 connect (btn, & QPushButton :: clicked, this, [=] () -> void {check (i);});

With this button, your solution works.

So the problem is the button array, which doesn't work, how do I fix it?

Other information:

the array is instantiated in the class's .h file, like so:

QPushButton * buttons [9];

and then in the .ccp file, in a method of the same class, I did this:

for (i = 0; i <9; i ++)
{
    for (y = 0; y <= height * 2; y = y + height)
    {
        for (x = 0; x <= width * 2; x = x + width)
        {
            buttons [i] = new QPushButton (grill);
            buttons [i] -> setObjectName ("cell" + QString :: number (i));
            buttons [i] -> setGeometry (x, y, width, height);
        }
   }
}

In addition I forgot to mention that the buttons be inside a QFrame as you can see from the code above, I don't know if the QFrame can be the cause of the problem.

The QFrame is always instantiated in the .h file of the class, like this:

QFrame * grill = new QFrame ();

J.Hilk

@Alby said in connect (): how to pass a parameter to the function:

I tried your solution, first with the buttons inside the array, it didn't work

again, it didn't work is not a sufficient error description.
Whats the compiler error?

So the problem is the button array, which doesn't work, how do I fix it?

it should work with or without the array, important is only the pointer to the object instance.

~~just in case:~~
~~you can do the following:~~

for (i = 0; i <9; i ++)
{
    for (y = 0; y <= height * 2; y = y + height)
    {
        for (x = 0; x <= width * 2; x = x + width)
        {
            auto btn = new QPushButton (grill);
            connect (btn, & QPushButton :: clicked, this, [=] () -> void {check (i);});
            buttons [i] = btn
            buttons [i] -> setObjectName ("cell" + QString :: number (i));
            buttons [i] -> setGeometry (x, y, width, height);
        }
   }
}

~~if that doesn't work either, than you're probably mixing your buttons up, and are actually not pressing, the button you're think you're pressing.~~

Never mind, reading is a virtue.
see @KroMignon 's post, you need to bring your loops into order.

KroMignon

@Alby said in connect (): how to pass a parameter to the function:

for (i = 0; i <9; i ++)
{
   for (y = 0; y <= height * 2; y = y + height)
   {
       for (x = 0; x <= width * 2; x = x + width)
       {
           buttons [i] = new QPushButton (grill);
           buttons [i] -> setObjectName ("cell" + QString :: number (i));
           buttons [i] -> setGeometry (x, y, width, height);
       }
  }
}

What are you doing here?!?
This don't make sense to me.
Are you aware that you will only have in buttons[] the last QPushButton (with highest X and Y values). All other instance references are lost.

Alby

@KroMignon I am creating a small game (tick-tack-toe), those loops I need to create the grid of buttons, where players enter X or 0

I honestly did not know, I thought that inside the array, I inserted the various instances of the buttons, without losing them.

But then how do I create a series of buttons inside an array, without losing the various references to the instance.

KroMignon

@Alby said in connect (): how to pass a parameter to the function:

I am creating a small game (tick-tack-toe), those loops I need to create the grid of buttons, where players enter X or 0
I honestly did not know, I thought that inside the array, I inserted the various instances of the buttons, without losing them.
But then how do I create a series of buttons inside an array, without losing the various references to the instance.

Why do you have 3 loop?
I can understand loop over x and y to build the matrix, but why i?

J.Hilk

@Alby

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QApplication app(argc, argv);

    QWidget frame;
    frame.resize(600,600);
    const int col = 3;
    const int row = 3;
    const int width = frame.width() / col;
    const int height = frame.width() / row;

    QList<QPushButton*> buttons;

    auto check = [](int i)->void{qDebug() << i;};

    int i{0};
    for(int y{0}; y < row; y++){
        for(int x{0}; x < col; x++){
            auto btn = new QPushButton(&frame);
            btn->setGeometry(x * width, y *height, width, height);
            QObject::connect(btn, &QPushButton::clicked, [=]()->void{check(i);});
            buttons.append(btn);
            i++;
        }
    }
    frame.show();

    return app.exec();
}

you should seriously consider using a QGridLayout for this.

Alby

@KroMignon I realized it now, sorry I was wrong that cycle is not needed

Alby

@J-Hilk

I will try to use QGridLayout, the last things () about your code:

What is 'auto'? , in the line:
auto btn = new QPushButton(&frame);
I saw that you make use of this syntax ex: int y {0}, what is it?
What does this mean:
[] (int i) ?
in the line:
auto check = [] (int i) -> void {qDebug () << i;};
What does this mean?:
[=] () -> void
in the line:
QObject :: connect (btn, & QPushButton :: clicked, [=] () -> void {check (i);});

Sorry for the many questions I asked, most likely stupid, I'm new to Qt and C ++, in any case thank you very much

J.Hilk

@Alby said in connect (): how to pass a parameter to the function:

Sorry for the many questions I asked, most likely stupid, I'm new to Qt and C ++, in any case thank you very much

don't worry, we all started sometime, somewhere.
That said, starting with Qt and c++ in parallel is a hard path, I did it myself, so it's not impossible :D but be warned.

What is 'auto'?

mmh, if you haven't heard of it, ignore it :P
its a place holder for compile time type deduction...
for more information:
https://en.cppreference.com/w/cpp/keyword/auto

I saw that you make use of this syntax ex: int y {0}, what is it?

it is one of the many, many ways to initialise a variable in c++ and the newest one and the one the c++ committee recommends to use.

What does this mean:
[] (int i) ?
in the line:
auto check = [] (int i) -> void {qDebug () << i;};

its a lambda, in this case an unnamed function inside a function
for more information
https://en.cppreference.com/w/cpp/language/lambda

QObject :: connect (btn, & QPushButton :: clicked, [=] () -> void {check (i);});

an other lambda to call the previous lambda, when the clicked signal is emitted!

🙈

Alby

@J-Hilk Thanks very much