Referencing to a QList from a QList<QList<x>> *

Niagarer

Hi!
This is surely some kind of stupid question :)
It is actually a C++ question, but I am getting confused with Qt's behaviour of myList->at(i) and myList[i] and (*myList)[i]

// allNodeInstances = QList<QList<NodeInstance*>>*
// so, allNodeInstances is a pointer to a 2D QList
QList<NodeInstance*> l = (*allNodeInstances)[i];
qDebug() << "l.length(): " << l.length();
qDebug() << "(*allNodeInstances)[i].length(): " << (*allNodeInstances)[i].length();
l.removeOne(ni);
qDebug() << "l.length(): " << l.length();
qDebug() << "(*allNodeInstances)[i].length(): " << (*allNodeInstances)[i].length();

the output by running this code is

l.length():  2
(*allNodeInstances)[i].length():  2
l.length():  1
(*allNodeInstances)[i].length():  2

So, what I actually want to do here is getting a reference to a QList from allNodeInstance-list (at index i).
The documentation says that the &QList::operator[] returns a modifiable reference. But the output shows, that l isn't a right reference (cuz it doesn't change allNodeInstances).

QList<NodeInstance*> l = allNodeInstances[i];

causes the following error

error: conversion from 'QList<QList<NodeInstance*> >' to non-scalar type 'QList<NodeInstance*>' requested
         QList<NodeInstance*> l = allNodeInstances[i];
                                  ~~~~~~~~~~~~~~~~~~^

which is strange, because the [] operator already specifies the index ... surely because allNodeInstances is a pointer to a 2D QList. But if this doesn't work, I don't know, how to achieve what I need.
So, question is as simple as that: how can I get a working reference to a QList from my 2D QList pointer?
Thanks for answers!

mrjj

Hi
Is is just paste mistake thats its not a reference ?

QList<NodeInstance*> & l = allNodeInstances[i];

note the &
as the docs shows
T &QList::operator[](int i)
it returns ref to type T

Niagarer

@mrjj
Oh yes, you are right, thank you!
But using allNodeInstances[i] as the reference doesn't work, because it's just a pointer and I get an error, because the compiler still sees a 2D QList

error: invalid initialization of reference of type 'QList<NodeInstance*>&' from expression of type 'QList<QList<NodeInstance*> >'
         QList<NodeInstance*> &l = allNodeInstances[i];
                                   ~~~~~~~~~~~~~~~~~~^

so I need to do

QList<NodeInstance*> &l = (*allNodeInstances)[i];
l.removeOne(ni);

and it works. I need to specity the new list as a reference (&) and I have to take the data where to pointer points to in order to access the actual lists.

(&(*C++))->confusing(true)

update
We can break it down to this:
after this assignment

test_struct *sp = new test_struct;

these two are equal:

sp->a = 12;
(*sp).a = 12;

So first option is

QList<NodeInstance*> &l = (*allNodeInstances)[i];

because there is no arrow with the [] operator, the data the pointer points to can only be accessed via the *
and second (as @mrjj pointed out)

QList<NodeInstance*> &l = allNodeInstances->operator [](i);

mrjj

@Niagarer
yes, sorry missed it was pointer to the outer list.
so (*allNodeInstances)[i] should always be used.
you could wrap it in a function for nicer look.

Also its possible without deference it.
but its not pretty

list->operator[](index)