QList<QVector< . >>: how to reserve

Davide87

Hello everybody.

In my application I have to manage several vectors of float numbers and I figured I would organize them in a QList.
Since such vectors may become quite large (several kbytes up to few Mbytes each), I though it would be better to use the reserve() function to avoid memory issues.

If, for example, I want N vectors with maximum allowed size M, what is the best way to reserve my memory for it?

For context, I have in my mainwindow.h

QList<QVector<float>*> *myList;

and in mainwindow.cpp

int N = ...
int M = ...

myList = new QList<QVector<float>*>();

After that, what is the best way to reserve myList?

myList->clear();
myList->reserve(N);

for(int i=0; i<N; i++)
{
    myList->append(new QVector<float>());
    myList->at(i)->reserve(M);
}

or should I myList->reserve(N*M);?
(consider that M may be several thousends)

Are there better ways to do it that I didn't consider?

Christian Ehrlicher

@Davide87 said in QList<QVector< . >>: how to reserve:

QList<QVector<float>*> *myList;

Why a pointer here? Unneeded

or should I myList->reserve(N*M);?

It depends on how many items you want to insert into the subvectors - from your descriptions they contain max 'M' elements so why would you reserve N*M then?

JonB

@Davide87 said in QList<QVector< . >>: how to reserve:

or should I myList->reserve(N*M);?

No, the QList and QVector are quite separate from one another, not related. Any reservations must be done on each one separately, and for the elements of the list which are vectors in each vector individually, as your code does.

 myList->at(i)->reserve(M);

Not sure you will be able to do this (or have you already done it and it compiles)? const T &QList::at(int i) const returns a const reference to the (QVector) element, so you won't be able to call QList::reserve() on it? Create a temporary for the new QVector<float>(), call reserve() on that, then add it to your QList.

Christian Ehrlicher

@JonB said in QList<QVector< . >>: how to reserve:

Create a temporary for the new QVector<float>(), call reserve() on that, then add it to your QList.

or use operator[] :)

JonB

@Christian-Ehrlicher
I could have suggested that, but to me I should not have to think about how I am addressing the element in the list ([] vs at()) when all I want to do is set reserve() on the QVector I am adding. I also have a feeling it will be 1 nanosecond slower to do it via []/on an element I have already put into the list than if I do it do it beforehand (though i amy be wrong on this)... ;-)

If you can afford to actually create all the elements (value 0.0) initially and overwrite them instead of reserve()ing (because it will be fixed size), Qt6 at least seems to have QList::QList(qsizetype size) constructor which you can use.

Davide87

Thank you guys.

@Christian-Ehrlicher
In my application N is in the order of few tens. The subvectors may be instead hundreds or thousands of elements, depending on the configuration of the hardware that sends me the data to store/display.
I thought I may have to reserve the list depending on the actual maximum number of bytes it is going to contain.
The size of the vectors can vary, but I would prefer to set a maximum size to avoid my application to take too much memory.

@JonB
myList->at(i)->reserve(M); compiles without issues.
I'm actually having some memory issues in my application, but they are probably related to other parts of may code. That's why I opened this topic.

JonB

@Davide87 said in QList<QVector< . >>: how to reserve:

@JonB
myList->at(i)->reserve(M); compiles without issues.

@Christian-Ehrlicher could you explain, am I having a mind block? Given it's const T &QList::at(int i) const, doesn't the const T & prevent me from calling QList::reserve() on it?

@Davide87
I still suspect reserve() won't make that much difference, Qt's list/vector allocation will be reasonable even without it. You would have to test.

The size of the vectors can vary, but I would prefer to set a maximum size to avoid my application to take too much memory.

I don't think reserve() will have any effect on the maximum size of the list/vector. That will happen dynamically whether you reserve or not. It's only to do with efficient use of memory during re-allocations as list grows.

Christian Ehrlicher

@JonB said in QList<QVector< . >>: how to reserve:

Given it's const T &QList::at(int i) const, doesn't the const T & prevent me from calling QList::reserve() on it?

Because he is working with heap-allocated QList/QVector instances for unknown reaons / he wants to debug memory leaks later on because he forgets to delete them afterwards...

Davide87

@JonB said in QList<QVector< . >>: how to reserve:

@Davide87 said in QList<QVector< . >>: how to reserve:

@JonB
myList->at(i)->reserve(M); compiles without issues.

@Christian-Ehrlicher could you explain, am I having a mind block? Given it's const T &QList::at(int i) const, doesn't the const T & prevent me from calling QList::reserve() on it?

It may be because myList is a list of pointers. QList::at(int i) returns a pointer in this case. Therefore ->reserve() works in this case. Maybe.

@Davide87
I still suspect reserve() won't make that much difference, Qt's list/vector allocation will be reasonable even without it. You would have to test.

The size of the vectors can vary, but I would prefer to set a maximum size to avoid my application to take too much memory.

I don't think reserve() will have any effect on the maximum size of the list/vector. That will happen dynamically whether you reserve or not. It's only to do with efficient use of memory during re-allocations as list grows.

Ok, thank you. I will test that.

JonB

@Davide87 said in QList<QVector< . >>: how to reserve:

It may be because myList is a list of pointers. QList::at(int i) returns a pointer in this case. Therefore ->reserve() works in this case. Maybe.

Ah, yes indeed, I did not notice that. Only the pointer is const then, it does not make *const_pointer be const :)

Christian Ehrlicher

Again: don't use pointers here - they are completely unneeded and will get you in trouble later on when you hunt for memory leaks

Davide87

Ok Christian. Thank you.

J.Hilk

@JonB said in QList<QVector< . >>: how to reserve:

No, the QList and QVector are quite separate from one another, not related

alt text

Not in Qt6

https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/tools/qvector.h

you're technically correct in relation to the OP post. And technically correct is the best kind of correct

JonB

@J-Hilk
I did not say QList and QVector are not related to one another! I said THE QList and THE QVector in OP's

QList<QVector<float>*> *myList;

declaration are unrelated to each other (Qt4, 5, 6 or anything else). The OP asked whether

or should I myList->reserve(N*M);?

where he thinks a list/vector of lists/vectors are "one blob" which can be reserved like that....

J.Hilk

@JonB

@J-Hilk said in QList<QVector< . >>: how to reserve:

you're technically correct in relation to the OP post. And technically correct is the best kind of correct