QList of QScopedPointers

Question

I'm trying to store QScopedPointers in a QList.

I found this comment

One can also use QList >. – Kuba Ober Jan 14 '14 at 18:17

(first comment on this answer: https://stackoverflow.com/a/21120575/3095014)

and this post https://forum.qt.io/topic/59338/solved-qlist-of-qscopedpointers which implies that this should work. But if I try to compile the code of the second link, I'm getting this errors:

E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qlist.h(404) : error C2248: 'QScopedPointer<Label,QScopedPointerDeleter<T>>::QScopedPointer' : cannot access private member declared in class 'QScopedPointer<Label,QScopedPointerDeleter<T>>'
    with
    [
        T=Label
    ]
    E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qscopedpointer.h(170) : see declaration of 'QScopedPointer<Label,QScopedPointerDeleter<T>>::QScopedPointer'
    with
    [
        T=Label
    ]
    E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qlist.h(403) : while compiling class template member function 'void QList<QScopedPointer<Label,QScopedPointerDeleter<T>>>::node_construct(QList<QScopedPointer<T,QScopedPointerDeleter<T>>>::Node *,const QScopedPointer<T,QScopedPointerDeleter<T>> &)'
    with
    [
        T=Label
    ]
    E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qlist.h(553) : see reference to function template instantiation 'void QList<QScopedPointer<Label,QScopedPointerDeleter<T>>>::node_construct(QList<QScopedPointer<T,QScopedPointerDeleter<T>>>::Node *,const QScopedPointer<T,QScopedPointerDeleter<T>> &)' being compiled
    with
    [
        T=Label
    ]
    E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qlist.h(794) : while compiling class template member function 'QList<QScopedPointer<Label,QScopedPointerDeleter<T>>>::~QList(void)'
    with
    [
        T=Label
    ]
    ..\tableview_row_dnd\main.cpp(13) : see reference to function template instantiation 'QList<QScopedPointer<Label,QScopedPointerDeleter<T>>>::~QList(void)' being compiled
    with
    [
        T=Label
    ]
    ..\tableview_row_dnd\main.cpp(20) : see reference to class template instantiation 'QList<QScopedPointer<Label,QScopedPointerDeleter<T>>>' being compiled
    with
    [
        T=Label
    ]
E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qlist.h(405) : error C2248: 'QScopedPointer<Label,QScopedPointerDeleter<T>>::QScopedPointer' : cannot access private member declared in class 'QScopedPointer<Label,QScopedPointerDeleter<T>>'
    with
    [
        T=Label
    ]
    E:\Qt\Qt5Enterprise\5.5\msvc2013\include\QtCore/qscopedpointer.h(170) : see declaration of 'QScopedPointer<Label,QScopedPointerDeleter<T>>::QScopedPointer'
    with
    [
        T=Label
    ]

Why isn't this working for me?

Answer 1

Values stored in Qt containers should be of assignable data types. That means they should have a default constructor, a copy constructor, and an assignment operator.

QScopedPointer has its copy constructor and assignment operator disabled. You can't assign two pointers to each other, but you can explicitly transfer the ownership of the underlying raw pointer using QScopedPointer::reset, QScopedPointer::swap or QScopedPointer::take.

At some point a move constructor and a move assignment operator were added to QScopedPointer. New move semantics made this possible:

QList<QScopedPointer<Label>> mLabels;
mLabels.append(QScopedPointer<Label>(new Label));

Here a temporary value is added to a list and the new list item is created using the move constructor.

Later they reverted that change:

Adding a move contructor to QScopedPointer makes no sense, because moving means 'escaping the scope', which breaks the fundamental point of QScopedPointer.

If you really want to have a list of smart pointers, you can use QSharedPointer which is assignable or std::unique_ptr which supports move semantics.

And if you talk about tracking lifetime of QObjects subclasses and especially widgets, I would suggest to use Qt child-parent mechanism rather than smart pointers.