Support for constructing QStandardItem objects from QVariant references?

[elfring]

The function “QStandardItem::setData()” is provided. This programming interface specifies that an object based on the data structure “QVariant” should be passed. I find it interesting that custom data types are also supported by calling the function “QVariant::fromValue()” there.

Now I notice once more that the class “QVariant” does not belong to the supported construction parameters so far.
Can it make sense to support it there, too?

[Christian Ehrlicher]

Why should it? The ctor with an QString sets the EditRole, the one with two parameters the Decoration and EditRole. Why should a QVariant passed there?
With setData() you can set any role you want.

[elfring]

Why should it?

• The rule value can be another construction parameter, can't it?
• I imagine that it would be nice if Qt standard items can be created based on QVariant references (which support also custom data types).

[VRonin]

I feel your pain, I even tried to fix it (here the reference for the same thing in QTableWidgetItem https://bugreports.qt.io/browse/QTBUG-65555)

Unfortunately it can't be implemented without breaking binary and source code compatibility and this means It's close to impossible to change.

To give you a brief explanation, to make it so that the constructor accepting QVariant doesn't become ambiguous you have to remove both QStandardItem(QString) and QStandardItem(int,int). The latter is the deal breaker, basically you'd change the behaviour of things like new QStandardItem(3) and that's almost always unacceptable

Why should it? The ctor with an QString sets the EditRole

Exactly, what if I want to put numbers in my models?!

[elfring]

I feel your pain, …

Thanks for your background information.

How do you think about the support for object construction by passing an argument pair (or even a parameter pack?)

…, what if I want to put numbers in my models?!

How will this use case evolve?

[Christian Ehrlicher]

@VRonin said in Support for constructing QStandardItem objects from QVariant references?:

Exactly, what if I want to put numbers in my models?!

Use setData(num, Qt::EditRole) :)

[VRonin]

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

How do you think about the support for object construction by passing an argument pair (or even a parameter pack?)

Like for example?

@Christian-Ehrlicher said in Support for constructing QStandardItem objects from QVariant references?:

Use setData(num, Qt::EditRole) :)

Yep, but it can turn 1 line of code into 3. So, often, inexperienced people just use QString::number or QString::arg (like shown in the docs) and this leads to all kind of problems, first and foremost it's not sorted correctly

[elfring]

Like for example?

You pointed development concerns out around ambiguous constructor variants.
I imagine that it would be possible to combine the desired construction data (including QVariant objects) by other classes (like QPair to avoid the mentioned ambiguity).

[VRonin]

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

combine the desired construction data (including QVariant objects) by other classes

I'm lost, can you provide an example of what you'd do?

[elfring]

I'm lost, can you provide an example of what you'd do?

I am just curious if it will ever become supported to create Qt standard items like the following.

auto mcds(QVariant::fromValue(my_custom_data_structure));
auto si1(new QStandardItem(Qt::DisplayRole, mcds));
auto my_parameters(qMakePair(mcds, Qt::DisplayRole));
auto si2(new QStandardItem(my_parameters));
auto sim(new QStandardItemModel);
sim->appendRow(si1);
sim->appendRow(si2);

[VRonin]

Ok, now I get it.

auto si1(new QStandardItem(Qt::DisplayRole, mcds)); has the same problem as above. as (int,int) is a clash. You can go around it using sfinae but I still think it's a hard sell to add it

The qMakePair is actually unnecessarily limited, you can actually pass a whole QMap<int,QVariant> and forward it to QStandardItemPrivate::setItemData

[elfring]

The qMakePair is actually unnecessarily limited,

This can be useful when only two values should be passed by a single constructor argument instead of two parameters which are needed for calling the function “QStandardItem::setData”.
The generated data type should be unique just for the desired function overloading.

you can actually pass a whole QMap<int,QVariant>

I find that such a map would not be needed for the shown source code example.

[elfring]

… , you can actually pass a whole QMap<int,QVariant> and forward it to QStandardItemPrivate::setItemData

Can this possibility be added to the public construction parameters for the class “QStandardItem”?

[VRonin]

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

I find that such a map would not be needed for the shown source code example.

What I meant is that a map is a more generic solution:
new QStandardItem({std::make_pair<QVariant,int>(mcds, Qt::DisplayRole)}); would bind to the map version of the constructor

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

Can this possibility be added to the public construction parameters for the class “QStandardItem”?

Don't see why not. Open a ticket on https://bugreports.qt.io and post the link here. Make sure to mark the component as Core: Item Models. If David Faure gives the green light I'll add it

[elfring]

… Open a ticket …

I am curious on how the clarification will evolve for the topic “Addition of constructor variants for QStandardItem class”.

[JKSH]

@VRonin said in Support for constructing QStandardItem objects from QVariant references?:

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

Can this possibility be added to the public construction parameters for the class “QStandardItem”?

Don't see why not. Open a ticket on https://bugreports.qt.io and post the link here. Make sure to mark the component as Core: Item Models. If David Faure gives the green light I'll add it

Before going down this path... isn't it better to subclass QStandardItem? http://doc.qt.io/qt-5/qstandarditem.html#subclassing

I presume the new constructor is to make QStandardItem work with a custom data type. If this is the case, then it's way better to have a constructor (AND getter + setter) that takes the custom type directly, without having to do QVariant conversion:

class MyItem : public QStandardItem {
public:
    // NICE: Constructor for your custom type
    MyItem(MyData *value);
    
    // NICE: Getter and setter for your custom type. No need to convert to/from with QVariants
    const MyData *myData() const; // IMPORTANT! const MyData -- The pointer should not allow editing

    void setMyData(MyData *value) {
        // TODO: Ensure that old data is freed, or use smart pointers
        m_data = value;
        emitDataChanged(); // IMPORTANT! Notifies the view that the data has changed
    }
    
    // Allow the models/views to access your data through the standard interface
    QVariant data(int role) const override;
    void setData(const QVariant &value, int role) override;

    // ...
	
    // Other functions that might also be worth reimplementing are
    // Destructor, clone(), type(), read(), write(), operator<()

private:
    MyData *m_data;
};

Overall though, I don't like QStandardItemModel. Except for quick prototypes, I think custom data structures are much better served by subclassing QAbstract(Item|Table)Model directly.

[elfring]

… that takes the custom type directly, without having to do QVariant conversion:

I imagine that this another software development challenge if you need to work with the provided generic (or standard) programming interfaces.

class MyItem : public QStandardItem {
…
MyData *m_data;
};

I find the specification of this member variable questionable for such a software design approach because the base class should take care of the desired data storage.
You might add attributes there for other design reasons.

[JKSH]

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

… that takes the custom type directly, without having to do QVariant conversion:

I imagine that this another software development challenge if you need to work with the provided generic (or standard) programming interfaces.

Sorry, I didn't understand this. Could you rephrase it?

I find the specification of this member variable questionable for such a software design approach because the base class should take care of the desired data storage.

That's true, but you also didn't like converting/copying data in/out of QVariant. That's why I suggested this design, as a compromise to meet your different goals.

Like I mentioned before, QStandardItemModel is not well-suited for handling custom data structures. If you want a clean software design AND avoid converting/copying data, then avoid QStandardItemModel. Subclass QAbstractItemModel instead.

[elfring]

Could you rephrase it?

The class “QVariant” is a generic programming interface for the handling of known data structures.

That's true,

Thanks for your acknowledgement.

but you also didn't like converting/copying data in/out of QVariant.

Yes. - Thus I am looking again for useful software adjustments there.

Subclass QAbstractItemModel instead.

I would appreciate if I can reuse existing functionality from a higher level base class.

[VRonin]

@JKSH said in Support for constructing QStandardItem objects from QVariant references?:

QStandardItemModel is not well-suited for handling custom data structures.

I disagree. It is not performance-efficient but it is generic enough to handle all kinds of custom metatypes

@elfring said in Support for constructing QStandardItem objects from QVariant references?:

I would appreciate if I can reuse existing functionality from a higher level base class.

Since we are moving one step higher, why not be even more generic:
QAbstractItemModel* model = new QStandardItemModel(parent);

This allows you to:

1. use QStandardItemModel instead of subclassing your own
2. use your custom data types seamlessly as QAbstractItemModel always uses QVariant
3. Lets you abstract the implementation of the model by using the API that is guaranteed to be available in every model