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

[elfring]

…, if you have an idea for an implementation then please go ahead and propose it to the community.

I guess that progress will depend on this basic clarification:
Are you familiar with the usage of placement new?

[VRonin]

The project maintainers are seasoned (15-20 years experience) developers and are familiar with all aspects of standard C++ (especially its oldest parts like placement new).

It's safe to assume a total mastery of the placement new concept by people reviewing code, don't worry

[elfring]

It's safe to assume a total mastery of the placement new concept by people reviewing code, don't worry

This information is very promising.

• Unfortunately, I could not extract corresponding indications of understanding for my proposal so far.
• How would you like to clarify a possible mapping from data model indexes to pointers further?

[VRonin]

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

How would you like to clarify a possible mapping from data model indexes to pointers further?

That's what we are asking you to propose.
We can't think of a way unfortunately

[elfring]

We can't think of a way unfortunately

Why do you stumble on limitations in your imaginations here?

[sierdzio]

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

Unfortunately, I could not extract corresponding indications of understanding for my proposal so far.

Because you have not proposed anything. Show an API and it will be judged. Show a usage example of that API and it will help us know if the API is convenient. Measure with benchmark and we'll know if it improves performance.

Without concrete foundations, any idea can be argued endlessly without result.

[J.Hilk]

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

We can't think of a way unfortunately

Why do you stumble on limitations in your imaginations here?

I guess that will depend on this basic clarification:
Are you familiar with the usage of placement new?

[VRonin]

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

Why do you stumble on limitations in your imaginations here?

Honestly I just think I'm not smart enough to get into this. It wouldn't be the first time. On the other hand I'd be really happy to see how it could be implemented so I could learn something new

[elfring]

Show an API and it will be judged.

Can this application programming interface be just “placement new” (which got the parameters “row” and “column” passed)?

[VRonin]

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

Can this application programming interface be just “placement new” (which got the parameters “row” and “column” passed)?

See, I struggle already, what would the return type be? (void* is a bit useless...)

[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?