Increasing usage for C++ new operators based on data model indexes?

[elfring]

See, I struggle already, what would the return type be?

C++ new operators are returning non-void-pointer types, don't they?

[VRonin]

I still can't see a way forward.
The simplest example would probably be QStringListModel. Could you help me understand how the placement new operator would work in that case?

[elfring]

Could you help me understand how the placement new operator would work in that case?

Can you understand already that “placement new” provides a pointer to an existing object?

[VRonin]

Yes, I can't see a safe way to use that pointer though

[elfring]

Yes,

I find this answer confusing in combination with the subsequent information.

I can't see a safe way to use that pointer though

You are used to the application of ordinary pointers.

auto x(new my_ball);

How many ball variants would you manage by your QStringListModel example?

[VRonin]

of, let's say you have a new that takes the row as an int parameter (QStringListModel has only 1 column).

I imagine that the implementation would check that the argument is within the range (row>=0 && row < lst.size() ) and then return something like &lst[row] (which is of type QString*).

Now we are back to the point we discussed here. How can we make sure that if the QString is modified then the dataChanged signal is sent?

[elfring]

of, let's say you have a new that takes the row as an int parameter (QStringListModel has only 1 column).

I imagine that the implementation would check that the argument is within the range (row>=0 && row < lst.size()) and then return something like &lst[row] (which is of type QString*).

This kind of feedback fits also to my imaginations.

How can we make sure that if the QString is modified then the dataChanged signal is sent?

Corresponding solutions will become interesting if you would like to modify the determined string object at all.

[VRonin]

@elfring said in Increasing usage for C++ new operators based on data model indexes?:

Corresponding solutions will become interesting

I agree but, once again I have no idea how to implement solutions. Do you?

[elfring]

…, once again I have no idea how to implement solutions.

I find this hard to believe. It might take another while until you feel more comfortable with related software design approaches.

• A class can still offer functions which perform a specific change alone (as before the programming interface extension).
• The user class should take responsibility for mutable C++ references (as usual). Will it put special function calls into destructor implementations?

[VRonin]

I find this hard to believe.

I'm not joking, I really can't think of a decent way

The user class should take responsibility for mutable C++ references

I disagree. This is a recipe for disaster

A class can still offer functions which perform a specific change alone

This might work for QStringListModel as all the elements are QString but as soon as you move just 1 step further and look at QListModel (the model behind QListWidget) where the stored data can be of any type, even a custom one defined by the user, your argument kinda falls apart, doesn't it?

[kshegunov]

@elfring said in Increasing usage for C++ new operators based on data model indexes?:

Does your software development experience include the usage of placement new?

Yes, but then I wonder does yours?

You are used to the application of ordinary pointers.

auto x(new my_ball);

How many ball variants would you manage by your QStringListModel example?

You don't seem to understand that the model is the boundary between the application-level code (i.e. user programmers) and the system-level code (for brevity only, it's the Qt library not the whole system). At that boundary the system code has to provide the means for the application code to map the data, and at the same time the library provides the display of said data.

At the point when the system is compiled the application-level code does not exist, so the system-level programmer (in this case the Qt Project contributor) can not and will not use anything of type that's unknown to the system. Unknown types include every user type the application provides itself, granted the exception the system has put in place a way for the type to be made known to the system. The latter is done through the meta-type system in Qt, and QVariant is aware of it.

Is your ball known (i.e. defined) when the models are developed? Of course not. Then the models can't in any conceivable way create that type. The models are generic and use QVariant so they can map multitude of types, not only your own ball.
Again, provide code that demonstrates your idea, so we can have a reasonable discussion.

[elfring]

This is a recipe for disaster

How many questionable applications do you notice then for the QVector class (for comparison)?

…, your argument kinda falls apart, doesn't it?

• This model is for private use by the widget so far, isn't it?
• Can you eventually get a desire to fiddle with list elements from a data model directly?

[VRonin]

@elfring said in Increasing usage for C++ new operators based on data model indexes?:

How many questionable applications do you notice then for the QVector class (for comparison)?

QVector is not a QObject subclass. it doesn't have to take care of sending signals to notify other parts of the program that something changed/was added/was removed/was moved.

This model is for private use by the widget so far, isn't it?

No. Absolutely not. I often use (and abuse) the model framework even in application that do not have a view.
It's also used in QML.

Can you eventually get a desire to fiddle with list elements from a data model directly?

I don't because I realise how badly it can break the entire framework. The internals of the data model should be handled by the model only, not get exposed otherwise it becomes simply unmanageable

[elfring]

Is your ball known (i.e. defined) when the models are developed?

Models can be implemented in the way that this can happen.

Again, provide code that demonstrates your idea, …

How do you think about to discuss concrete consequences for passing a pair of integer types (row, column) to a “placement new”?

[kshegunov]

@elfring said in Increasing usage for C++ new operators based on data model indexes?:

Is your ball known (i.e. defined) when the models are developed?

Models can be implemented in the way that this can happen.

No they can't.

How do you think about to discuss concrete consequences for passing a pair of integer types (row, column) to a “placement new”?

I don't. I already stated directly and quite clearly what is needed to discuss - a proof of concept in code.

[VRonin]

I'll try to explain in layman terms why we are a bit frustrated by this and the related topics with a small parallel.
Imagine you walk into a car factory and say to the manager: "you should put wings on your cars"
The manager answers: "what would that achieve?"
you - "do you know what wings are?"
manager - "yes but I don't see how they can be useful on a car"
you - "wouldn't it be cool to have wings on the cars?!"
manager - "yes but wings are not enough to make the cars fly"
you - "that's right, you need to also make the cars fly"
manager - "fine but how am I supposed to do that?"
you - "It's something you should think about"
manager - "well if you don't have any idea how to make the cars fly, I don't see why i should put wings on my cars"
you - "wouldn't it be cool to have wings on the cars?!"

This is exactly what you are doing to us!

[elfring]

No they can't.

Why do you try to exclude the possibility that various data can be loaded into models?

… - a proof of concept in code.

Do any other aspects hinder you to discuss software design possibilities around returning pointers (or C++ references) from functions after a few parameters were passed?

[elfring]

Why would you get frustrated by topics (which I presented)?

[kshegunov]

@elfring said in Increasing usage for C++ new operators based on data model indexes?:

Why do you try to exclude the possibility that various data can be loaded into models?

I don't, and it already can through QVariant.

Do any other aspects hinder you to discuss software design possibilities around returning pointers (or C++ references) from functions after a few parameters were passed?

No. I want you to meet the burden or proof - if you claim something, have the balls to back it up. You claim to have an idea that's better than the currently available one, put it on paper, so to speak, and we can discuss it.

[elfring]

You claim to have an idea that's better than the currently available one,

I got another software development idea which I find useful.

put it on paper,

I have described it in a few variants for this feature request already.

so to speak, and we can discuss it.

Another approach

• Function input:
  Parameters like row and column
• Function output:
  • Pointer ⇒ How likely is it that this function variant can be called “C++ new operator”?
  • C++ reference ⇒ Would you like to use direct access for an object?