[Development] Question about the lifetime of references taken from QHash

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Thu Mar 11 12:36:02 CET 2021

Il 11/03/21 12:30, Jaroslaw Kobus ha scritto:
> Provided I have only one instance of my QHash (so, no detaches are being done), and the only API of the QHash I'm using is: insert(), remove(), contains() and operator[], how long I may rely on the reference to value returned by QHash::operator[]?
> E.g.:
>      QHash<int, MyValue> hash;
>      hash.insert(1, MyValue(...));
>      hash.insert(2, MyValue(...));
>      MyValue &val = hash[1];
>      hash.remove(2);
>      // May I rely that now my reference is still valid? Wondering, if any internal rehash on remove could copy internal data, so that it invalidates the reference obtained earlier.
> Assuming that I keep and use this reference only until I explicitly called hash.remove() for the key associated with my reference - may I assume that this reference will be valid?

The quick answer is: in Qt 5 `val` is still valid (modifications of the 
container only affect iterators/references to the modified elements), 
while in Qt 6 it's not (modifications of the container invalidates all 
iterators and references).

So don't rely on that if you want to future-proof your code. (Or, if you 
really want to rely on that, then switch to std::unordered_map).

Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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