help to find "connect" error

Christian Ehrlicher

Do you really think spamming us with the same question over and over again will help you in any way? Maybe starting with basic c++ stuff would be a much better option.

Anonymous_Banned275

This post is deleted!

Christian Ehrlicher

As long as you won't format your code correctly noone can read your code.

JonB

@AnneRanch said in help to find "connect" error:

connect(subMenu[index], &QMenu::triggered, this ,= { this->processMenu(index,index_sub) ;});

It's hard to know what you are saying here, what you are surprised at or what you expect which you claim does not happen. Let's insert an extra line above the connect() and change the lambda slot slightly so it reads:

qDebug() << index << index_sub;
connect(subMenu[index], &QMenu::triggered, this, [=]() { qDebug() << index << index_sub; } );

When these two lines of code execute, i.e. the connect() statement is executed, you will see the values for index & index_sub at that instant. I don't know what they are, but let's say 1 & 3.
When whatever QMenu item was at subMenu[index] at the time this was executed is later triggered, you will see the same values for index & index_sub as they were when the connect() line was previously executed. In the above case they would be 1 and 3 again.

Of course, if you are executing the connect() inside a loop(s) where index and/or index_sub are changing you will end up with more than one lambda slot attached. You will (potentially) have more than one slot attached to a given QMenu item and/or slot(s) attached to more than one QMenu item.

Each time index changes you will be attaching to a different subMenu[index] item.
Each time index_sub changes you will be attaching to the same subMenu[index] item but with a different value for the qDebug() << index_sub in the lambda slot.

J.Hilk

@AnneRanch said in help to find "connect" error:

both indexes - index and index-sub are STATIC, they are NOT created when "click on submenu" is activated.

Well I'm not surprised you're lambda does not snapshot shot index or index_sub during the moment of the connect call but it will capture the reference to the member variables and look them up at the moment the triggered signal is emitted.

this will snapshot index and index_sub:

connect(subMenu[index], &QMenu::triggered, this , [this, i = index, i_s = index_sub]()->void{ this->processMenu(i,i_s) ;});

JonB

@J-Hilk said in help to find "connect" error:

Well I'm not surprised you're lambda does not snapshot shot index or index_sub during the moment of the connect call but it will capture the reference to the member variables and look them up at the moment the triggered signal is emitted.

I do not understand why you have written this. The OP's lambda, and my post, use [=] as the capture. That will "snapshot" (as you call it) the values at the instant the connect() statement is executed, as I wrote. So why are you saying

but it will capture the reference to the member variables and look them up at the moment the triggered signal is emitted

I do not see your [this, i = index, i_s = index_sub] will behave any differently from [=] in this case? Do you really want me to go try, are you saying I have misunderstood C++ lambdas/``[=]` ??

which it will not (as & was not used)?

WAIT
Are you saying index and index_sub are class member variables, not local ones wherever the code is?? I thought they were locals, I think they are, I think the OP is sjowing us some function where it has

// results
int submenu_index = -1;
int mainmenu_index= -1;

at the top, but it's hard to tell....

J.Hilk

@JonB

    QApplication app(argc,argv);

    static int index{0}, index_sub{0};

    auto lambda1 = [=]()->void{qDebug() << index << index_sub;};
    auto lambda2 = [=, i = index, i_s = index_sub]()->void{qDebug() << i << i_s;};

    lambda1();
    lambda2();

    index = 100;
    index_sub = 100;

    lambda1();
    lambda2();

->

Are you saying index and index_sub are class member variables, not local ones wherever the code is??

that's what the 2nd post is saying, at least what I understand it's saying.

JonB

@J-Hilk
Nope! Please read, you have an "edge" case of some kind in your code.

To do your static int index{0}, index_sub{0}; I had to change my default C++ compilation from c++11 to c++14. There may be a clue here (actually I'm not sure this is relevant). With c++11 and your code I get

/home/jon/QtTests/lambdacopy/main.cpp:11: warning: lambda capture initializers only available with ‘-std=c++14’ or ‘-std=gnu++14’
../lambdacopy/main.cpp: In function ‘int main(int, char**)’:
../lambdacopy/main.cpp:11:24: warning: lambda capture initializers only available with ‘-std=c++14’ or ‘-std=gnu++14’
   11 |     auto lambda2 = [=, i = index, i_s = index_sub]()->void{qDebug() << i << i_s;};
      |                        ^

Please change your code to remove the static, so it reads

/*static*/ int index{0}, index_sub{0};

Now the output reads

which is exactly what I expected.

The behaviour of [=] seems to be different on local variables which are static? But we have no evidence those would be static in OP's code, and I would never have tried with static.....

EDIT
Ohhhhh, now I see the OP's

both indexes - index and index-sub are STATIC, they are NOT created when "click on subnenu" is activated.

@AnneRanch
Now that I see this. (You had never posted the declarations of these variables, and no static keyword appears in any of your code, which is why we really need complete examples to help.) For your purposes you must NOT have static index, index_sub. This is why they are not delivering the values they had at the time of the connect() but rather the values they have from the last iteration. Or follow @J.Hilk's

[this, i = index, i_s = index_sub]()->void{ this->processMenu(i,i_s) ; }

to copy their current values into i & i_s (or whatever) so that the lambda gets a copy of their current value.

J.Hilk

@JonB what exactly is your point of content with my example/solution?

With c++11 and your code I get

of course, capture initalizeres [i = index] are a c++17 feature independently from index being static or not

Please change your code to remove the static, so it reads
Now the output reads

Of course, I would expect nothing else. I'm pretty sure one of either the capture initialisers or the capture by copy of the local variables are skipped by the compiler.

The behaviour of [=] seems to be different on local variables which are static? But we have no evidence those would be static in OP's code, and I would never have tried with static.....

well in this case, static and/or member variables. I was too lazy to write a whole class for this example. And like I quoted, her 2nd comment clearly states staticmember variables of index and index_sub

JonB

@J-Hilk said in help to find "connect" error:

And like I quoted, her 2nd comment clearly states static member variables of index and index_sub

Evidently easier to spot that by some (you) than by others (me)! There is a lot to wade through (unformatted), and it always helps to show the relevant code...! :)

Anyway, hopefully the OP has a couple of alternative ways to modify the code now to get the behaviour desired.