[Interest] QMultiMap as QVariant

[Thomas Sevaldrud]

> On 15 Apr 2023, at 12:07, Volker Hilsheimer <volker.hilsheimer at qt.io> wrote:
> 
>> On 14 Apr 2023, at 23:11, Thomas Sevaldrud <thomas at silentwings.no> wrote:
>> 
>> Hi, in my code I have a hierarchical QMultiMap of QVariants, where a map value can be a new multimap of variants. This does not compile anymore in Qt 6. I.e. something like this:
>> 
>>  	QMultiMap<QString, QVariant> varMap;
>>   	QMultiMap<QString, QVariant> subMap;
>>   	varMap.insert("key", subMap); // Compile error here, since there is no conversion to QVariant
>> 
>> This worked in 5.15, but not in 6.5
>> 
>> If I change it to QMap it works, but I need multimap for this. Is this change by design, or is it an omission? Any ideas for a workaround?
>> 
>> Cheers,
>> Thomas
> 
> 
> In Qt 5, QMultiMap was a subclass of QMap, so implicitly constructing a QVariant from a QMultiMap was possible because it was possible from a QMap. That happened to work because QMultiMap just added API on top of a QMap, and QMap’s data structure could handle multiple values for the same key. So no slicing occurred. However, if you run:
> 
> #include <QtCore>
> 
> int main(int argc, char *argv[])
> {
>    QMultiMap<QString, QVariant> varMap;
>    QMultiMap<QString, QVariant> subMap;
>    varMap.insert("key", subMap);
>    qDebug() << varMap.value("key").typeName();
> 
>    return 0;
> }
> 
> then you would get QVariantMap (ie a QMap<QString, QVariant>), not QMultiMap. So modifying the value you got out of the map would not give you multi-map semantics.
> 
> 
> In Qt 6, QMultiMap is not a QMap subclass anymore (and that was a design decision), and since it’s such a rarely used type there is also no implicit constructor or conversion operator, and no built-in metatype defined for QMultiMap.
> 
> But since QVariant can be constructed from custom types via the fromValue template, you can use:
> 
> varMap.insert("key", QVariant::fromValue(subMap));
> 
> and now you can’t get it out as a QVariantMap anymore, and the code above will print “QMultMap<QString,QVariant>”, like it should.
> 
> Volker
> 


Cool, thanks! That makes sense. We probably never had any submaps in this structure with non-unique keys, so we never got this problem. This is of course much better. 

Thanks again!

- Thomas