[Interest] How to make the QString "10" behind "9" in QMap's key order

[Konstantin Tokarev]

> Hi,
> 
> you need a "lessThan"-function, tailored for your needs. An
> example from me is this:
> 
> bool Person::lessThanFamilyFirst(QSharedPointer<Person>
> p1, QSharedPointer<Person>
> p2)
> 
> {
> 
> return (QString::localeAwareCompare(p1->familyName(),p2->familyName())<0)
> 
> || (( p1->familyName() == p2->familyName())
> 
> && (QString::localeAwareCompare(p1->firstName(),p2->firstName())<0));
> 
> }
> 
> Now you can pass this as a functor to a sort method like this:
> 
> std::stable_sort(pupils.begin(), pupils.end(), Person::lessThanFamilyFirst);

FWIW, if this is not a list displayed in UI, or other place where preserving order of
equal elements matters, you should consider std::sort

> 
> Please notice, that there are no () when calling the lessThan functor.

Original question was dealing with QMap. While sorted vector-like container can replace
map (there is even convenience wrapper flat_map in Boost, which allows to work with
sorted array just like it was a "true" map), it doesn't always fit, e.g. inserts in such
"flat map" are very expensive.

So, you can just replace QMap with std::map and pass your comparator, e.g.

auto comparator = [](const QString &a, const QString &b) -> bool {
    // Your comparison function here
};
std::map<QString, QGraphicsItem *, decltype(comparator)> myMap(comparator);

See [1] for more examples.

Yet another approach is to use your own type as a key of QMap instead of QString,
and create desired operator< for it. This has a downside that your access to internal
QString will be more complicated, and you'll have to invent different key type each time
you want to change comparator.

And, last but not least: you probably want to know *how* to write comparator for your
particular sorting order. You can do this with QCollator, see [2].

[1] http://en.cppreference.com/w/cpp/container/map/map
[2] https://forum.qt.io/topic/68910/natural-sort-using-qcollator/2

> 
> HTH
> Sebastian
> 
> Am 07.01.2018 um 09:01 schrieb jack ma:
> 
>> Hi,
>>
>> there is a QMap<QString,QGraphicsItem *> type, then
>> insert values with：
>>
>> type.insert("U1",nullptr),
>>
>> type.insert("U2",nullptr),
>>
>> …………
>>
>> type.insert("U9",nullptr),
>>
>> type.insert("U10",nullptr),
>>
>> I want get the values follow the order of U1, U2 ... U10，
>> but the default order is U1, U10 ,……U9
>>
>> I know this is a common string sorting problem, but I do
>> not know how to solve it well.
>>
>> Any suggestions are very grateful !
>>
>> Thanks.
>>
>> _______________________________________________
>> Interest mailing list
>> Interest at qt-project.org
>> http://lists.qt-project.org/mailman/listinfo/interest
> 
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Regards,
Konstantin