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Q&A connect with SLOT/SIGNAL: QPushButton clicked signal not received by main window

Qt's SLOT macro is very specific in what it accepts. When specifying a slot in the connect function, it's required to put parenthesis, so the correct code would be: connect(ui->pushButton, SIGNA...

posted 4y ago by jrh‭ · edited 4y ago by jrh‭

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#2: Post edited by

jrh‭ · 2020-09-14T15:03:31Z (about 4 years ago)
Added an example of a connect statement that does not work

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## Qt's `SLOT` macro is very specific in what it accepts.
When specifying a slot in the `connect` function, it's required to put parenthesis, so the correct code would be:
```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(pushButton_Clicked()));
```
With this syntax, the button `clicked()` signal will be received by `MainWindow`.
## Some background information
When you use the `SLOT()` macro, you aren't passing in a function pointer, it actually stringizes what you pass in behind the scenes.
So you could actually do something like this, and it would still be accepted.:
```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(gibberish));
```
Unfortunately there are no errors or warnings to help find typos in usages of the `SLOT` macro, be very careful with this macro.
## Alternatives
It's possible to use function pointers instead of using the `SIGNAL`/`SLOT` macros.
```
connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::pushButton_Clicked);
```
For overloaded functions, it's necessary to explicitly specify the type of function pointer, (`auto` is no help).
```
void (QComboBox::*comboBox_IndexChanged)(int) = &QComboBox::currentIndexChanged;
QObject::connect(itemComboBox, comboBox_IndexChanged, this, &MainWindow::OnCellComboBoxChanged, Qt::AutoConnection);
```
I think there are some other methods too, but these are the best options I was able to find in Qt 5.7.0

## Qt's `SLOT` macro is very specific in what it accepts.
When specifying a slot in the `connect` function, it's required to put parenthesis, so the correct code would be:
```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(pushButton_Clicked()));
```
With this syntax, the button `clicked()` signal will be received by `MainWindow`.
## Some background information
When you use the `SLOT()` macro, you aren't passing in a function pointer, it actually stringizes what you pass in behind the scenes.
So you could actually do something like this, and it would still be accepted.:
```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(gibberish));
```
Unfortunately there are no errors or warnings to help find typos in usages of the `SLOT` macro, be very careful with this macro.
## Alternatives
It's possible to use function pointers instead of using the `SIGNAL`/`SLOT` macros.
```
connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::pushButton_Clicked);
```
For overloaded functions, it's necessary to explicitly specify the type of function pointer, (`auto` is no help).
```
void (QComboBox::*comboBox_IndexChanged)(int) = &QComboBox::currentIndexChanged;
QObject::connect(itemComboBox, comboBox_IndexChanged, this, &MainWindow::OnCellComboBoxChanged, Qt::AutoConnection);
// note:
// QObject::connect(itemComboBox, &QComboBox::currentIndexChanged(int), this, &MainWindow::OnCellComboBoxChanged(int));
// does NOT work. Unfortunately you cannot specify an overloaded method like this.
```
I think there are some other methods too, but these are the best options I was able to find in Qt 5.7.0

#1: Initial revision by

jrh‭ · 2020-09-02T21:07:42Z (about 4 years ago)

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## Qt's `SLOT` macro is very specific in what it accepts. 

When specifying a slot in the `connect` function, it's required to put parenthesis, so the correct code would be:

```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(pushButton_Clicked()));
```

With this syntax, the button `clicked()` signal will be received by `MainWindow`.

## Some background information

When you use the `SLOT()` macro, you aren't passing in a function pointer, it actually stringizes what you pass in behind the scenes.

So you could actually do something like this, and it would still be accepted.:

```
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(gibberish));
```


Unfortunately there are no errors or warnings to help find typos in  usages of the `SLOT` macro, be very careful with this macro.

## Alternatives

It's possible to use function pointers instead of using the `SIGNAL`/`SLOT` macros.

```
connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::pushButton_Clicked);
```

For overloaded functions, it's necessary to explicitly specify the type of function pointer, (`auto` is no help).

```
void (QComboBox::*comboBox_IndexChanged)(int) = &QComboBox::currentIndexChanged;

QObject::connect(itemComboBox, comboBox_IndexChanged, this, &MainWindow::OnCellComboBoxChanged, Qt::AutoConnection);
```

I think there are some other methods too, but these are the best options I was able to find in Qt 5.7.0

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