QMap iteration gets inexisting item

[Andrew23]

When I iterate a QMap for the second time I get an inexisting item why?

typedef struct
{
    uint8_t connected;
    QByteArray address;
    uint8_t type;
    uint8_t enabled;

} item_t;

QMap<uint8_t, item_t> map;

void Foo( QByteArray address, uint8_t connectionState, uint8_t itemType, uint8_t id )
{
    if ( map.contains( id ) == false )
    {
        map.insert( id, { .connected = connectionState, .address = address, .type = itemType, .enabled = 0 } );
    }
    else
    {
        map[ id ].connected = connectionState;
    }

    foreach ( const uint8_t &key, map.keys() )
    {
        qDebug() << "id" << key << "-" << map[ key ].address.toHex( ':' ) << "- connectionState" << map[ key ].connected;
    }
}

If i run Foo( hereByteArray, 1, 1, 1 ) the output is

id 1 - "28:9e:39:68:72:e8" - connectionState 1

as expected. But If I continue to run Foo( hereByteArray, 0/1, 1, 1 ) the output become

id 0 - "" - connectionState 0
id 1 - "28:9e:39:68:72:e8" - connectionState 0

even if I never insert the item "0". Why ?

[Pl45m4]

@Andrew23 said in QMap iteration gets inexitent item:

But If I continue to run Foo( hereByteArray, 0/1, 1, 1 ) the output become
id 0 - "" - connectionState 0
id 1 - "28:9e:39:68:72:e8" - connectionState 0

even if I never insert the item "0". Why ?

At first glance, I would say you fell for the []-operator "mistake".
Try to avoid [] everywhere when dealing with QMaps.

Because if you try to look-up a key-value pair with [] and the key is not found, it will construct an empty, default key value pair at this position.

I could be wrong but this is what your [0] entry seems to be.
Replace every access to the map with .value(key) and it should not happen anymore

So instead of

map[ key ].address.toHex( ':' )

better (and everywhere else)

map.value(key).address.toHex( ':' )

[jsulm]

@Andrew23 To add to @Pl45m4 this behavior is explained in the documentation which you should read: https://doc.qt.io/qt-6/qmap.html#operator-5b-5d

[Andrew23]

@Pl45m4 thanks a lot. Now it works without output the '0' item. Can you tell me if on the other side it's ok to use the [] operator to write the map item value?

@jsulm thanks. I didn't see it.

[JonB]

@Andrew23 said in QMap iteration gets inexisting item:

it's ok to use the [] operator to write the map item value?

Yes, of course, since here you are writing to and/or creating the element, you don't have to worry about it auto-creating a "blank" one. What you should not do is read from map[...], e.g. if (map[...]) ... or qDebug() << map[...] since those would create the entry if it does not exist.

[Andrew23]

Ok. Thanks!