C / C++ / MFC

Re: "connect" with "lambda" C++ code analysis , please.

3-Feb-24 15:55

Best guess? Wrong forum. AFAICT this is a QT issue. QT ≠ C++. While QT does use C++ as a base, it's a framework on top of C++. You are more likely to get an answer either from a QT specific forum, or from the site you got your examples from.

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

Salvatore Terress3-Feb-24 16:44

Re: "connect" with "lambda" C++ code analysis , please.

3-Feb-24 16:44

So you do not think this is my fault - wrong usage of C++?

"connect " is Qt specific , but lambda is C++ or am I wrong?
I did post same on QT forum and I fully expect somebody to say _ this is C++ code problem "...

But if I continue this discussion here ... I will end up banned......
.,..in that case..

Thanks and have a swell day...

k50544-Feb-24 4:34

Re: "connect" with "lambda" C++ code analysis , please.

4-Feb-24 4:34

I'm basing this on the runtime error, which you believe is something to do with "wrong sender". That seems to me to be a QT issue, not a C++ issue. AFAIK, there's no such thing as a "sender" in the C++ standard. Which points to QT, not C++

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

Salvatore Terress4-Feb-24 4:49

Re: "connect" with "lambda" C++ code analysis , please.

4-Feb-24 4:49

OK, it is Qt issue..
I'll give few days for somebody else to respond , then I
will take it down.
Is that OK?

k50544-Feb-24 5:30

Re: "connect" with "lambda" C++ code analysis , please.

4-Feb-24 5:30

I'd just leave it, unless more senior members disagree. Otherwise, there's a discussion that someone else might stumble on and wonder what was going on. Who knows, maybe someone will stumble on this at some time in the future and be able to explain what the issue is.

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

Salvatore Terress4-Feb-24 13:36

Re: "connect" with "lambda" C++ code analysis , please.

4-Feb-24 13:36

Here is my analysis in a nutshell.
The "issue" is with Qt "connect" function / process...
I have the "connect" working as expected , but ...
I am passing parameters / indexes - main menu and submenu...
...that is what I thought I was doing , but ...
the "parameters " values are NOT dynamic AKA
used when "connect" is activated - click on sub menu.
In my test - the main menu should be 0
and the submenu anything from 0 to 7

The code I have found uses lambda
and it analyze the "connect" AND
RETRIEVES the dynamic values of both indexes.
I need the indexes to do what the menu / submenu
is about - process something...

I have modified the code because i
CANNOT debug / step thru the lambda code !

So - the lambda code does work BUT ...
it only works/ retrieves main menu index of 0

That is the real error and I am seeking assistance
to find out why
it only works on FIRST menu.

My uneducated guess is -
the obvious place
to look is where
the code step thru "main menu list"...

C++

    if(parent)
        {
        QMenu* menu=qobject_cast<QMenu*>(parent);
        int index=0;
        const QList<QAction*> actions=menu->actions();
        for(const QAction *act : actions)
            {
            if(act
->text()==title)
                {
                path+=QString("(%1)").arg(index);
                break;
                }
            index++;
            }
        }

k50545-Feb-24 4:25

Re: "connect" with "lambda" C++ code analysis , please.

5-Feb-24 4:25

Salvatore Terress wrote:
I have modified the code because i
CANNOT debug / step thru the lambda code !

That does seem to be an issue with lambdas - at least gdb doesn't seem to be able to step into lambdas. I haven't tried it myself, so I can only go by what I've seen elsewhere on the subject.

But.

Why don't you just extract the function definition from the lambda and define it as a normal function. I think that should work

C++

foo( a, b, c, [](int x, int y) { /*  do something; */ return val; } );

becomes

C++

int lambda_f( int x, int y)
{
   // body from lambda called in foo(), above
   return val;
}

// ...
foo(a, b, c lambda_f);

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

Salvatore Terress5-Feb-24 6:30

Re: "connect" with "lambda" C++ code analysis , please.

5-Feb-24 6:30

Yes, I am now using plain function to analyze results of "connect".
My code is still pretty messy , hard to read , since I added debug...
But I can step thru it , so the debug is "leftover from lambda"...

I got my code doing what I need...
In retrospect - the problem was what I suspected from the start

understanding "connect" and PLACING it into correct place.
PS
As far as getting help from forums - it was ABOUT 80% of "RTFM" and other
"fill the blanks" to make "small talk"...

Richard MacCutchan4-Feb-24 6:16

Richard MacCutchan

4-Feb-24 6:16

Salvatore Terress wrote:
then I
will take it down.
Is that OK?

No, because that leaves orphaned messages which make little sense to others reading it.

Incidentally, I did look at this code earlier and tried to make it readable, by removing all those extra braces in the first sample, but it still made little sense. As to the second sample, why are you trying to use a lambda expression for all that code? Use a proper function/member subroutine so you can see better what is (supposed to be) happening. And, as k5054 says, this is most likely a Qt issue rather than simple C++.

Re: "connect" with "lambda" C++ code analysis , please.

jschell5-Feb-24 5:33

Re: "connect" with "lambda" C++ code analysis , please.

5-Feb-24 5:33

Richard MacCutchan wrote:
why are you trying to use a lambda expression for all that code?

I agree with that strongly.

If the developer can't figure out the lambda code then they shouldn't be using it in the first place. That isn't being denigrating since to me they are almost always confusing so I never voluntarily use them. They do nothing but make the code obtuse and the more complicated it is the more that becomes true.

It also has the following impacts.
- Maintenance programmers must figure it out.
- I have seen more than one senior developer use it incorrectly making the code less efficient.

Seems almost like there could be a coding guideline in that. If methods must be, for example, less than 100 lines of code, then seems that lambda expressions should also be limited perhaps to something like 10 clauses (or less.)

Salvatore Terress5-Feb-24 6:38

Re: "connect" with "lambda" C++ code analysis , please.

5-Feb-24 6:38

Please read my previous reply.

It was not my choice to use lambda - I took the code from another forum contributor...
Using lambda was not that difficult - the code flow was pretty logical.
The issue was - for whatever reason I could not "step thru" the lambda code...

k50545-Feb-24 7:10

Re: "connect" with "lambda" C++ code analysis , please.

5-Feb-24 7:10

I fully understand that. What I'm saying is that there is no reason that you to have to use a lambda. You can extract the function body and use a normal function pointer instead. And a normal function can be debugged by putting a break point a the function entry point.

I'm not sure what your stumbling point is here. Consider:

C++

struct S {
  int i;
  double d;
};

bool cmp_double(const S& s1,  const S&s2) {
   return s1.d < s2.d;
}

int main()
{
    std::vector<S>  vs;
    // ... add some data to vs ... 

     // sort the vector using the cmp_double function: 
    std::sort( vs.begin(), vs.end(), cmp_double);

    // sort the vector using an anonymous lambda function
    std::sort( vs.begin(), vs.end(), [](const S& x, const S&y) { return x.d < y.d } );
}

The point is, the two sorts do exactly the same thing. So there's no reason you can't extract the lambda and use the named function as a parameter. If you're having trouble understanding this, you need to go back to your study material and review lambdas. I can recommend Justin Turners C++ weekly you-tube blog [https://www.youtube.com/playlist?list=PLs3KjaCtOwSZ2tbuV1hx8Xz-rFZTan2J1](https://www.youtube.com/playlist?list=PLs3KjaCtOwSZ2tbuV1hx8Xz-rFZTan2J1) Just follow the link and then search for lambda, and watch the videos. It might be worth doing even if you do understand lambdas. It's not like another perspective can't help with the understanding.

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

jschell6-Feb-24 4:40

6-Feb-24 4:40

Salvatore Terress wrote:
It was not my choice to use lambda - I took the code from another forum contributor

Pretty sure that is a choice. As suggested in the other post you can rewrite it.

Not sure I have ever used a code sample where I did not modify it.

strlen innards

mike74112-Feb-24 12:02

mike7411

2-Feb-24 12:02

Can someone explain to me in layman's terms how this function works?

https://codebrowser.dev/glibc/glibc/string/strlen.c.html

Thank you.

Re: strlen innards

Richard MacCutchan2-Feb-24 21:52

Richard MacCutchan

2-Feb-24 21:52

It counts the initial characters in the string until the pointer is aligned on a longword boundary. It then uses some clever bit manipulations to count the rest of the characters four at a time. If you run that code in the debugger you will be able to see exactly what happens.

Re: strlen innards

jschell5-Feb-24 5:58

Missing "type specifier " ?

5-Feb-24 5:58

Presumably already clear what the 4/8 block is used in the first place.

Googling with following provides some answers.

explain strlen himagic lomagic

Following line is the key

if (((longword - lomagic) & ~longword & himagic) != 0)

I tried eye balling that and using examples in my head but still not clear. I believe it is relying on arithmetic overflow.

If I wanted to understand that I would write up some test code with examples characters (4 blocks) with zero at the end (position 4, 3, 2, 1). Then have it print the results of each clause in the above if using binary representation (1 and 0) to see how the bits look for each different example and for each part of the clause.

Perhaps as a learning experience also copy that code into your own space and then write a test jig to time results via that and using the more straightforward (just by char) code. You should start with a large number of runs like probably at least 100,000.

Salvatore Terress2-Feb-24 5:37

2-Feb-24 5:37

More update
Found the crasher - wrong "exit" from "for" loop looking for main menu.

Having issues (expected ) using other than first main menu...

UPDATE

I am posting this to show SOME progress.
I did pepper the original with handful of "debug"...
( I need to add comments about what the code does...)

So far it works with main menu = 0 and all its sub menus - SUCCESS.

I got the sub menu index - so part of my goal is done.

Now I need to find the "main menu" index and then find the "crasher"..

BTW the author deserves LARGE credit for the code..

C++

// NOTES placed AFTER menu / submenu
int  MainWindow_Bluetooth::setupLambda ( void )
{
#ifdef LAMBDA
    text = "START TASK DEBUG lambda ... ";
    text += Q_FUNC_INFO;
    text += QString::number(__LINE__);
    qDebug() << text;
#endif
    QObject obj;

    qDebug() << " index_sub " << index_sub ; // bogus
    obj.connect(subMenu[index_sub],&QMenu::triggered,subMenu[index_sub  ],[](QAction* action)
// obj.connect(subAction[index_sub],QAction::triggered(bool),subAction[index_sub ],[](QAction* action)

    {// lambda

        QString path=action->text();

#ifdef LAMBDA
       qDebug() << "TEST need index ?? " << path;
       qDebug()<<"path (sub menu trigger ) "<< path ;
#endif
        QWidget* parent=action->parentWidget();
        // !!!!  submenu  trigger index   !!!
        path = QString("(%1)").arg(parent->actions().indexOf(action));
        //path += QString("(%1)").arg(parent->actions().indexOf(action));
        //qDebug() << " index ??" << parent->actions().indexOf(actions);

#ifdef LAMBDA
        qDebug()<<"path (sub menu ) with index TOK  "<< path ;
#endif
        while(parent)
            {
            // main menu
            QMenu* menu=qobject_cast<QMenu*>(parent);
            QString title=menu->title();
 #ifdef LAMBDA
            qDebug()<<"title (main menu )  "<< title ;
#endif
            path+="->"+title;
            qDebug()<<"path (title) "<< path ;
            // ??
            parent=parent->parentWidget();
#ifdef LAMBDA
           qDebug()<<"TRACE  "<< __LINE__ ;
#endif
            if(parent)
                {
                QMenu* menu=qobject_cast<QMenu*>(parent);
#ifdef LAMBDA
           qDebug()<<"TRACE  "<< __LINE__ ;
#endif
                int index=0;
                const QList<QAction*> actions=menu->actions();
                for(const QAction *act : actions)
                    {
#ifdef LAMBDA
           qDebug()<<"TRACE  "<< __LINE__ ;
#endif

                    if(act->text()==title)
                        {
                        path+=QString("(%1)").arg(index);
#ifdef LAMBDA
                        qDebug()<<"path (main menu -> sub menu) "<< path ;
#endif
                        break;
                        }
#ifdef LAMBDA
           qDebug()<<"TRACE  "<< __LINE__ ;
#endif

                    index++;
                    }
                }
            }
        qDebug()<<" FINAL  (??) "<< path;

    }// lambda
    );
#ifdef LAMBDA
    text = "END TASK DEBUG lambda... ";
    text += Q_FUNC_INFO;
    text += QString::number(__LINE__);
    qDebug() << text;
#endif
    return 0;
}

OUTPUT

11:57:03: Starting /mnt/A_BT_DEC10/BT__PROGRAMS/A_JAN11/A_BT_LIBRARY/mdi/mdi...
Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
QObject::connect(QAction, Unknown): invalid nullptr parameter
"START TASK DEBUG lambda ... int MainWindow_Bluetooth::setupLambda()89"
 index_sub  0
"END TASK DEBUG lambda... int MainWindow_Bluetooth::setupLambda()167"
TEST need index ??  "SubWindow LOCAL terminal all options          #0"
path (sub menu trigger )  "SubWindow LOCAL terminal all options          #0"
path (sub menu ) with index TOK   "(0)"
title (main menu )   "terminal      # 0"
path (title)  "(0)->terminal      # 0"
TRACE   128
TRACE   134
TRACE   141
TRACE   153
TRACE   141
TRACE   153
TRACE   141
TRACE   153
TRACE   141
TRACE   153
TRACE   141
TRACE   153
TRACE   141
path (main menu -> sub menu)  "(0)->terminal      # 0(5)"
title (main menu )   "Window control"
path (title)  "(0)->terminal      # 0(5)->Window control"
TRACE   128
TRACE   134
11:57:22: /mnt/A_BT_DEC10/BT__PROGRAMS/A_JAN11/A_BT_LIBRARY/mdi/mdi crashed.

I do not get what is missing - the error "points" at "action".

Code snippet :

C++

QObject::connect(&MainWindow_Bluetooth::subMenu,&QMenu::triggered,&MainWindow_Bluetooth::subMenu,[](QAction* action)
{// lambda
    QString path=action->text();

QWidget* parent=action->parentWidget();
path+=QString("(%1)").arg(parent->actions().indexOf(action));

while(parent)
    {
    QMenu* menu=qobject_cast<QMenu*>(parent);
    QString title=menu->title();
    path+="->"+title;
    parent=parent->parentWidget();

    if(parent)

ERROR :

/mnt/A_BT_DEC10/BT__PROGRAMS/A_JAN11/A_BT_LIBRARY/terminal_Bluetooth/mainwindow_Bluetooth_copy.cpp:42: error: C++ requires a type specifier for all declarations
mainwindow_Bluetooth_copy.cpp:42:10: error: C++ requires a type specifier for all declarations
QObject::connect(&MainWindow_Bluetooth::subMenu,&QMenu::triggered,&MainWindow_Bluetooth::subMenu,[](QAction* action)
         ^

modified 3-Feb-24 16:02pm.

Mircea Neacsu2-Feb-24 5:52

Mircea Neacsu

2-Feb-24 5:52

No, the error is on QObject::connect(.... You cannot call a member function of a class(*). You have to insantiate an object of the class and call the member function on that object. Something like:

C++

QObject obj(/*constructor params*/);
obj.connect(/*...*/);

[*] Unless the member function is a static member function. I doubt that is the case here.

Mircea

jschell2-Feb-24 5:53

2-Feb-24 5:53

Certainly looks to me that at a minimum you are missing a paren in the first line/statement.

k50542-Feb-24 7:28

2-Feb-24 7:28

That's what I thought too, but then I saw the comment //lambda just after the opening brace. I think that the following code is all a lambda expression as a parameter to the QObject::connect() call. I think the OP could have cleaned up the submission with something like

C++

QObject::connect(&MainWindow_Bluetooth::subMenu,&QMenu::triggered,&MainWindow_Bluetooth::subMenu,[](QAction* action){ /* lamda, body omitted */ });

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

RedDk2-Feb-24 7:39

RedDk

2-Feb-24 7:39

I suggest looking through all your code (there really is no "points" when something comes to an "action" especially when ALL of the code is not available) for "typedef". Then, assuming these errors are sallying forth from the mingw/gnu/etc compiler you're using, and have no code associated with them to differentiate one from the other, change any typedefs of the form:

(this only an example, not the code you have)

std::vector<int>::size_type

To

std::vector<int>::size_type size

See if any errors go away.

modified 2-Feb-24 14:34pm.

Salvatore Terress2-Feb-24 8:02

2-Feb-24 8:02

Thanks, the code is an example I found, so I am just starting to modify it for my use.
I stopped when I received the error.
I actually got more , but I just do not see how to fix the "action".
The "problem" is - the original error "pointed " on "connect" and this error "points " on "action"

Let me work on this...maybe I can actually clean up the whole "lambda".

Salvatore Terress2-Feb-24 9:05

2-Feb-24 9:05

Here is modified , incomplete , code .

int  MainWindow_Bluetooth::setupLambda ( void )
{
#ifdef LAMBDA
    text = "START TASK DEBUG LAMBDA... ";
    text += Q_FUNC_INFO;
    text += QString::number(__LINE__);
    qDebug() << text;
#endif
    QObject obj();  // no parameters , all variables are class members , is that OK ?

    obj.connect(&subMenu[index],&QMenu::triggered,&subMenu[index],[](QAction* action)
    {// lambda

    }// lambda
    );
#ifdef LAMBDA
    text = "END TASK DEBUG LAMBDA... ";
    text += Q_FUNC_INFO;
    text += QString::number(__LINE__);
    qDebug() << text;
#endif



    return 0;
}

Now I am getting this error

/mnt/A_BT_DEC10/BT__PROGRAMS/A_JAN11/A_BT_LIBRARY/terminal_Bluetooth/mainwindow_Bluetooth_copy.cpp:92: error: base of member reference is a function; perhaps you meant to call it with no arguments?

I am still lost...

(I do not like lambda - too cryptic )