[Interest] QMultiHash / QMultiMap: insert QList?

[Tony Rietwyk]

Hi,

On 8/10/2026 6:24 pm, Schimkowitsch Robert wrote:
>
> Hi there,
>
> I saw this data structure:
>
> QHash<SomeKey, QList<SomeValue>>
>
> …and I was wondering: Why not use QMultiHash?
>
> The rationale was that it’s easier to extract and re-insert whole 
> blocks (i.e. all values sharing the same key), which is indeed a 
> relevant use case for the surrounding code.
>
> There IS QHash::values(key) -> QList<value>, but what’s missing is the 
> opposite function: insert(key, QList<value>)
>
QMultiHash has the interesting property that the multiple values are 
implicitly ordered with the last to be inserted returned first in 
.values().  This presents problems when inserting or replacing from a 
list.  A naive forward implementation would end up reversing the order 
when later fetched by .values():

hash.insert(1, v1)
hash.insert(1, v2)
auto list = hash.values() returns (V2, V1)
A new function hash.insert(1, list), followed by hash.values() now 
returns (V1, V2, V2, V1)!
Ditto with a new replace(1, list), followed by hash.values() now returns 
(V1, V2).

That reversal would probably catch a lot of people off guard.

Regards,

Tony
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