How to make QList const from outside

Cocojambo

#include <QCoreApplication>
#include <QObject>
#include <QDebug>

class Item {
    public:
        QString value;
};

class Box {
    public:
        Box() {
            items.append((new Item()));
        }

        QList<Item*> getItems() {
            return items;
        }

    private:
        QList<Item*> items;
};

int main(int argc, char* argv[]) {
    QCoreApplication a(argc, argv);

    auto variable = new Box();
    auto items = variable->getItems();
    items.at(0)->value = "value"; // <- items shouldn't be edited from outside
    qDebug() << items.at(0)->value;

    return a.exec();
}

Is it possible to prevent changing items outside Box class?

guerinoni

@Cocojambo
You can return const QList<Item*> & and declare const your function;

Cocojambo

@guerinoni
If you meant code below then it doesn't work. Still able to change outside.

const QList<Item*>& getItems() const {
            return items;
}

guerinoni

@Cocojambo
ok, then you need to create a QList with every Item as const *Item const
But why don't you use a simple QList<Item>?

Cocojambo

@guerinoni

class Item {
    public:
        QString value;
};

class Box {
    public:
        Box() {
            auto itm = new Item();
            items.append(*itm);
            items.at(0).value = ""; // cannot change it now
        }
        const QList<Item>& getItems() const {
            return items;
        }
    private:
        QList<Item> items;
};

int main(int argc, char* argv[]) {
    QCoreApplication a(argc, argv);

    auto variable = new Box();
    auto items = variable->getItems();
}

I was afraid of copying items. But now I cannot change the value inside Box class.
My goal is get this list inside Box, give items for reading outside and change them only inside Box.

guerinoni

@Cocojambo
Yes because you miss that .at() method return a const reference, because of this you can't edit...

        items.append(Item{"hello"});
        items.at(0).value = ""; // items.at(0) is const
        items[0].value = "hello world";

Cocojambo

@guerinoni
Well, then we still have previous problem :)

int main(int argc, char* argv[]) { 
    ...
    auto variable = new Box();
    auto items = variable->getItems();
    items[0].value = "value"; // I can change it outside the class
    qDebug() << items[0].value;
}

guerinoni

@Cocojambo

#include <QtDebug>

class Item
{
public:
    QString value;
};

class Box
{
public:
    Box() { items.append(Item{"hello"}); }
    const QList<Item> &getItems() const { return items; }

    void printAll() const
    {
        qDebug() << "len ->" << items.size();
        for (auto i : items) {
            qDebug() << i.value;
        }
        qDebug() << "";
    }

private:
    QList<Item> items;
};

int main(int argc, char *argv[])
{
    Box variable;
    variable.printAll();

    auto items = variable.getItems();
    items.push_back(Item{}); // edit only on items and not in variable
    variable.printAll();

    items[0].value = "value"; // I can change it outside the class
    variable.printAll();

    // if you make local variable const, you can't change it
    const auto itemsConst = variable.getItems();
    itemsConst.push_back(Item{}); // error
}

Please, this is the basic of C++

Cocojambo

@guerinoni
Thank you. But ... :) in my case assigning to new variable is redundantly each time when I want to read items from the list.
For example, I read all the records from some mysql table to that private QList<Item> items and work with these records in any part of my code in read only mode (without creation such itemsConst variable).

Christian Ehrlicher

@Cocojambo said in How to make QList const from outside:

to new variable is redundantly each time when I want to read items from the list.

But that's the problem of your code. Your function returns a const ref but you copy it to a local object:

auto items = variable.getItems();

In Qt this is not that problematic due to implicit sharing, for pure c++ objects like e.g std::vector you should write

const auto &items = variable.getItems();

But again - c++ basics

Cocojambo

@Christian-Ehrlicher
"In Qt this is not that problematic due to implicit sharing"
As I understand Qt will create another copy of QList<Item> items when I change it.

Can I prohibit copying reference and cloning the source array as result in any way from my class? For example if someone use my class and I want to allow him only read these items (and copy some of them only via loop iteration);
Sorry if I am asking something stupid :)

Christian Ehrlicher

@Cocojambo said in How to make QList const from outside:

Can I prohibit copying reference and cloning the source array as result in any way from my class?

No - it's a copy so the 'user' can do what he wants.