[Interest] Inquiry Regarding the Implementation Method of QT COW Containers

[Thiago Macieira]

On Tuesday, 11 November 2025 17:44:39 Pacific Standard Time xiaochaoyang0214 
wrote:
> I'm considering a approach similar to how Qt wrapped std::map for QMap:
> creating a straightforward wrapper layer around std::vector to add COW
> functionality, rather than building a completely custom implementation. Do
> you think this would be a feasible and reasonable approach? Are there any
> particular challenges or considerations I should be aware of when
> implementing COW semantics around std::vector in this manner?

QMap in Qt 6 is proof that is feasible and reasonable for some use-cases.

The challenge of wrapping a type instead of building it in from the scratch is 
that you need to add an extra layer of indirection. So your MyVector class, 
like QMap, doesn't access the data directly, but instead holds a pointer to 
the underlying structure, then dereferences it.

Take for example this very simple function: https://qt.godbolt.org/z/bP5Es945G
(at the time of writing this email, Godbolt's Qt support was broken)

Please ignore the fact that they return slightly different things coming from 
the same source. std::map generates two loads from memory:
        movq    24(%rdi), %rax
        movq    32(%rax), %rax
        retq

Whereas QMap inserts a third:
        movq    (%rdi), %rax
        movq    32(%rax), %rax
        movl    36(%rax), %eax
        retq

We figured this trade-off was acceptable, given how QMap isn't as often used as 
QVector and QHash, and usually isn't very performance sensitive anyway. It was 
offset with the decrease in our cost of maintenance. Besides, the old Qt 5 
implementation may have had worse performance than this extra indirection.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCG - Platform & Sys. Eng.
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