Please help with lambda sytax

[JonB]

@AnneRanch
As @jsulm has said. As long as you use the old-style SIGNAL & SLOT() macros, you are liable to have code which compiles OK but does not behave correctly at runtime. With the new style syntax, if the signal/slot code is wrong, you get told at compile-time. It might seem harder to get it right, but at least you know that when it does compile it will do what you expect when you run it; if it does not compile, you have to sort it out to be correct at that stage.

[Anonymous_Banned275]

@jsulm said in Please help with lambda sytax:

But this will also not work as QPlainTextEdit does not have a slot appendPlainText which takes a QListWidgetItem* as parameter (use new Qt5 connect syntax and a lambda)...

I do appreciate the post.
It may seems strange, but at this point I am NOT after "it" working.
I just need to figure out WHY adding this "connect" , in any form or format , crashes the program.
After I figure out the crasher I will work on form / format/ style.
Of course I could verify the "connect" using separate test program.

[jsulm]

@AnneRanch said in Please help with lambda sytax:

function on_pushButton_11_clicked
@line 206

Can you show your on_pushButton_11_clicked? The crash is there at line 206.

[Pl45m4]

@AnneRanch

Back to lambdas:

You said, you are struggling to understand lambda connections correctly.
It's not as complicated as it might look.

• You have your sender (first argument)

  connect(ui->list,.....)
  

• You have your sender's class + signal

   connect(ui->list, &QListWidget::itemDoubleClicked,.....)
  

• Then you can define a scope for your lambda

  connect(ui->list, &QListWidget::itemDoubleClicked, this,......)
  

• the last part consists of the capture, the function or slot arguments and the actual code itself, using the passed and captured values

  • Capture

    • "[ ]": None of the variables from current class / scope are captured and can be used inside lambda "code"
    • " [=]": Captures everything (by value!)
    • "[this] ": (current instance) or any combination of variables (e.g. [x, a]), to pick variables you need explicitely.
    • "[&]" or "[ &someVar ]": Captures by reference. Changes made inside lambda will affect and change the captured variable outside the lambda as well ( Thx, @JonB )

  • Signal / Slot arguments after capture inside ( .... ), e.g. if signal sends a QListWidgetItem, you need a (QListWidgetItem *item) here.

  • lambda code inside { .... }

    • do whatever you like and what you would put in your slot here.

One big advantage using lambda connections is, that you can connect signals to slots (or "code"), that would be incompatible using a "direct" connection because of any reason (e.g. parameter mismatch).

Simple example:

// signal
void signal(QString s);
// slot
void onSignalFired(int i);

A direct connection of these two won't work neither using the SIGNAL(...) / SLOT( ... ) syntax
(-> crash / error at runtime),
nor using the function based connection &Class::signal... &Class::onSignalFired
( -> wont compile).

With lambda, you can make it work:

connect(this, &Class::signal, [] (QString s) {  onSignalFired( s.toInt() ); } ); 

Instead of calling the slot, you could also skip the slot and do whatever you want to do with this string inside the lambda directly, e.g. send your number to a QLineEdit ("A" would lead to a 65, simple ASCII).

@AnneRanch said in Please help with lambda sytax:

Here is my current - under construction connect to copy the "list" lines.
All it does - just compiles and cannot be tested , in existing code , due to this issue.

... and this is exactly where you can make use of lambdas ( as @jsulm already said ).
Pass your QListWidgetItem from signal to your lambda, get the text from your item using text() (Qt Doc), then pass the text to QPlainTextEdit's appendPlainText(QString).

[JonB]

@Pl45m4
Since you've laid this out so neatly, may I suggest you add [&] to your Capture section, and note how it differs from [=]? Because some day someone's going to need it... :)

[Anonymous_Banned275]

@jsulm said in Please help with lambda sytax:

@AnneRanch said in Please help with lambda sytax:

function on_pushButton_11_clicked
@line 206

Can you show your on_pushButton_11_clicked? The crash is there at line 206.

There it is.

I have left in all the trace / debug stuff .
And highlighted the line 206

code// scan for nearby BT devices

void MainWindow_CAT_TAB::on_pushButton_11_clicked()
{
#ifdef TRACE
    qDebug() << "QDEBUG TRACE ";
    qDebug() << "QDEBUG TRACE TASK \n\t\t void MainWindow_CAT_TAB::on_pushButton_11_clicked()  \n";
     qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
    qDebug() << "file     " << __FILE__;
    qDebug() << "function "<<__FUNCTION__;
    qDebug() << "@line    " << __LINE__;
    qDebug()<<"TEMPORARY EXIT ";
    //exit(99);
#endif
//   ui->listWidget_5->(" addItem(on_pushButton_11_clicked()");
//     ui->listWidget_5->(addItem(on_pushButton_11_clicked());
             ui->listWidget_5->addItem(" addItem(on_pushButton_11_clicked()");
             ui->listWidget_5->addItem(" addItem(on_pushButton_11_clicked()");
             ui->listWidget_5->addItem("information_main.count())");


#ifdef TRACE
    qDebug() << "QDEBUG TRACE ";
    qDebug() << "QDEBUG TRACE TASK \n\t\t START imoplement Device DiscoveryDialog  \n";
     qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
    qDebug() << "file     " << __FILE__;
    qDebug() << "function "<<__FUNCTION__;
    **qDebug() << "@line    " << __LINE__;**
    qDebug()<<"TEMPORARY EXIT ";
    //exit(99);
#endif

// instatiate Devievce Discovery Dialog as in btscanner example
DeviceDiscoveryDialog *DDD = new DeviceDiscoveryDialog();

DDD->startScan();  // takes time 2 seconds for each device 

DDD->show();

#ifdef BYPASS
// test add connect here
connect(
            DDD->ui->list,
 //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
              SIGNAL(itemChanged(TEST_LABEL)),
            DDD->ui->plainTextEdit,
 //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
            SLOT(appendPlainText(TEST_LABEL))
        );
#endif


#ifdef TRACE
    qDebug() << "QDEBUG TRACE ";
    qDebug() << "QDEBUG TRACE TASK \n\t\t END imoplement Device DiscoiveryDialog  \n";
     qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
    qDebug() << "file     " << __FILE__;
    qDebug() << "function "<<__FUNCTION__;
    qDebug() << "@line    " << __LINE__;
    qDebug()<<"TEMPORARY EXIT ";
    //exit(99);
#endif


}_text

[Christian Ehrlicher]

@AnneRanch said in Please help with lambda sytax:

SIGNAL(itemChanged(TEST_LABEL)),

And you really want to tell us that you read at least one sentence from what we're telling you here??

[Anonymous_Banned275]

Partial success.
Using both old and new "connect" style throws "segmentation fault" at either one.

It is my understanding that "connect" code placement is not critical - however my crash occurs exactly on "connect" which I have placed in a secondary dialog constructor. My main window dialog is primary.

RTFM implies that passing null pointer may be the problem...

[Anonymous_Banned275]

@AnneRanch DONE!
Ready to tackle lambda .
Moral of the story - stay on original issue - it was my shallow knowledge of "connect" , my wrong syntax and my implementation of "connect" - applied in wrong place.

Help from everybody assisting is very much appreciated.

[Pablo J. Rogina]

@AnneRanch said in Please help with lambda sytax:

@AnneRanch DONE!

great, so please don't forget to mark your post as solved!