<![CDATA[QModelIndex::constInternalPointer returns nullptr unexpectedly]]>

  • code

    QModelIndex MyModel::index(int row, int column, const QModelIndex & parent) const
{
    if (parent.isValid())
    {
        const void * p_parent_data = parent.constInternalPointer();
        assert(p_parent_data != nullptr);
        ...
        assert(p != nullptr);
        return createIndex(row, column, p);
    }
    else
    {
        ...
        assert(p != nullptr);
        return createIndex(row, column, p);
    }
}

  • Question
    QModelIndex::constInternalPointer in return value of MyModel::index is always non-null, how could assert(p_parent_data != nullptr) in the above codes fail?

    • ]]>    https://forum.qt.io/topic/131429/qmodelindex-constinternalpointer-returns-nullptr-unexpectedlyRSS for NodeSun, 07 Jun 2026 09:42:54 GMTTue, 26 Oct 2021 06:23:41 GMT60<![CDATA[Reply to QModelIndex::constInternalPointer returns nullptr unexpectedly on Tue, 26 Oct 2021 07:19:26 GMT]]>@jronald said in QModelIndex::constInternalPointer returns nullptr unexpectedly:

    how could assert(parent) in the above codes fail?

    "It can't!"

    How do you know void * parent is non-null? If you are looking at it in debugger watch window, don't rely on that, it could be being held in a register and the value of parent be unreliable. Print out the value of parent on the line before the assert().

    Try making your void * parent be const void * parent (I'm surprised you don't get a compiler warning)?

    Meanwhile I find your

    void * parent = parent.constInternalPointer();

    "strange". The right-hand side uses the const QModelIndex & parent while the left-hand side uses the new void * parent declaration. This is at best "confusing". Suggest rename the local pointer variable to something else.

    Finally, are you sure you are compiling for debug? Else assert() is a no-op....

    ]]>    https://forum.qt.io/post/686744https://forum.qt.io/post/686744Tue, 26 Oct 2021 07:19:26 GMT    <![CDATA[Reply to QModelIndex::constInternalPointer returns nullptr unexpectedly on Tue, 26 Oct 2021 17:49:39 GMT]]>@JonB said in QModelIndex::constInternalPointer returns nullptr unexpectedly:

    "It can't!"

    Alright

    The problem for me is that not only each QModelIndex in the first column may have parent
    but each QModelIndex in the remaining columns should also have the same parent as the QModelIndex in the first column of the same row.

    Thanks

    ]]>    https://forum.qt.io/post/686866https://forum.qt.io/post/686866Tue, 26 Oct 2021 17:49:39 GMT    <![CDATA[Reply to QModelIndex::constInternalPointer returns nullptr unexpectedly on Tue, 26 Oct 2021 08:16:16 GMT]]>@JonB
    I've refined the code.

    Run in debug mode, and assert(p_parent_data != nullptr) fails.

    The problem might be in QModelIndex MyModel::parent(const QModelIndex & child) const
    because there are 3 columns and only the first column might have children, for other columns MyModel::parent always returns QModelIndex()

    ]]>    https://forum.qt.io/post/686754https://forum.qt.io/post/686754Tue, 26 Oct 2021 08:16:16 GMT    <![CDATA[Reply to QModelIndex::constInternalPointer returns nullptr unexpectedly on Tue, 26 Oct 2021 07:19:26 GMT]]>@jronald said in QModelIndex::constInternalPointer returns nullptr unexpectedly:

    how could assert(parent) in the above codes fail?

    "It can't!"

    How do you know void * parent is non-null? If you are looking at it in debugger watch window, don't rely on that, it could be being held in a register and the value of parent be unreliable. Print out the value of parent on the line before the assert().

    Try making your void * parent be const void * parent (I'm surprised you don't get a compiler warning)?

    Meanwhile I find your

    void * parent = parent.constInternalPointer();

    "strange". The right-hand side uses the const QModelIndex & parent while the left-hand side uses the new void * parent declaration. This is at best "confusing". Suggest rename the local pointer variable to something else.

    Finally, are you sure you are compiling for debug? Else assert() is a no-op....

    ]]>    https://forum.qt.io/post/686744https://forum.qt.io/post/686744Tue, 26 Oct 2021 07:19:26 GMT