QMAP is suddenly Empty! Bug or I'm I using it wrong?

Benjamin B.

Hi all,

I've a map of 'courses' called mapK. Each 'course' (class D_Kurs) has its own (mapSchiene). Due to debugging I made 'mapSchiene' public. So here is my code and the weird result.

    QList<D_Kurs> listK = mapK.values();

    foreach(D_Kurs dk,  listK) {
        dk.mapSchiene["la"] =  7;
        qDebug() << dk.getName() << " --A--> " << dk.mapSchiene.size();
    }

    foreach(D_Kurs dk,  listK) {
        qDebug() << dk.getName() << " --B--> " << dk.mapSchiene.size();
    }

I don't understand, why the individual QMaps are empty in the second iteration.

"PJ-BI-PJK1"  --A-->  1
"D-LK1"  --A-->  1
"D-LK2"  --A-->  1
...
"PJ-BI-PJK1"  --B-->  0
"D-LK1"  --B-->  0
"D-LK2"  --B-->  0

Could you please help me. Thx.

Christian Ehrlicher

@Benjamin-B. said in QMAP is suddenly Empty! Bug or I'm I using it wrong?:

foreach(D_Kurs dk,  listK) {
    dk.mapSchiene["la"] =  7;

You're adding calues to a local object here.
You may want to do

for(D_Kurs &dk : listK) {
  dk.mapSchiene["la"] =  7;
}
or
for(auto it = listK.begin(); it != listK.end(); ++it) {
  D_Kurs &dk = it.value(); 
  dk.mapSchiene["la"] =  7;
}

foreach won't work here at all because it's creating a copy of the container internally iirc.

sierdzio

@Benjamin-B. said in QMAP is suddenly Empty! Bug or I'm I using it wrong?:

foreach(D_Kurs dk, listK) {

Here foreach is making a copy of each element from listK. So when you modify that copy in dk.mapSchiene["la"] = 7; it is destroyed when it goes out of scope (the closing bracket }).

Then when you read your listK again, you read (again) the original list - without the changes you've made.

In order to modify the map in-place, you can do 2 things:

use pointers in mapK / listK. Just remember you are responsible for object lifetime (need to call delete when objects are not needed anymore)
replace the objects after you modify them. So, you take a value from your map / list, modify it, then call insert() or replace() on your ma to overwrite the original value

Another thing to remember: when you call mapK.values() you are also making a copy of the values. So if you replace the items in listK, the original items in mapK will still remain the same (unless you use pointers).

Benjamin B.

Thank you for your comments...its really hard for me to handle pointers, since I am used to JAVA.

I wanted to store in mapK pointers so I did.

QMap<int, D_Kurs*> mapK;

adding to it like:

    int kID = 0;
    foreach (QString line, listK) {
        D_Kurs dk(kID, line);
        mapK.insert(kID, &dk);
        kID++;
    }

and accessing it:

    for(D_Kurs *dk : mapK.values()) {
        dk->mapSchiene["la"] =  7; // HERE ERROR
        qDebug() <<  dk->getName() << " --A--> " << dk->mapSchiene.size();
    }

    for(D_Kurs *dk : mapK.values()) {
        qDebug() <<  dk->getName() << " --B--> " << dk->mapSchiene.size();
    }

But I get an error. Could you help?

sierdzio

@Benjamin-B. said in QMAP is suddenly Empty! Bug or I'm I using it wrong?:

foreach (QString line, listK) {
D_Kurs dk(kID, line);
mapK.insert(kID, &dk);
kID++;
}

You create dk on stack, so it gets automatically destroyed when scope ends (}). Create a pointer instead:

auto dk = new D_Kurs(kID, line);
mapK.insert(kID, dk);
[...]

And btw. if you are only storing indexes in keys, then you may want to use QVector or QList instead of QMap. QMap/QHash make more sense when the keys are something different (strings, non-continuous integers, negative nhumbers etc.).

Benjamin B.

...ahhhhh (Stack!) I'm starting to understand (starting to remember: some time ago i studied computer science)!

"And btw. if you are only storing indexes in keys, then you may want to use QVector or QList instead of QMap. QMap/QHash make more sense when the keys are something different (strings, non-continuous integers, negative nhumbers etc.)."

...yes i know, the keys may later be arbitrary (when keys get deleted).

Thank you a lot!!!

KillerSmath

Don't forget to delete the dynamically allocated itens.

// Deleting the itens
qDeleteAll(mapK); // remove the itens from memory
mapK.clear(); // remove the pointers to itens from map