Hi,
Some time ago I documented the guidelines on passing by value vs
const-ref:
https://wiki.qt.io/API_Design_Principles#Passing_by_const-ref_vs._Passing_by_value
This was discussed in #qt-labs at the time and informally +2'd there.
But the reality is that passing small structs by value is not very
popular and Qt's API mostly passes them by const-ref.
I think this is a good time to have a more formal conversation,
bikeshed+popcorn, then write a QUIP and hopefully change the API for Qt
6.
The guideline is very simple:
- If your type has a non-trivial copy-CTOR or non-trivial DTOR then pass
by const-ref, no matter how small it is, to avoid calling those methods.
*[1]
- If your type has trivial CTOR and trivial DTOR, pass by value if it's
small, otherwise const-ref. Small meaning <= 16 bytes.
The idea is that by value allows the struct's members to be passed in
CPU registers, see [2] for extensive research.
The good news is, even if you don't care about raw performance, passing
by value is more convenient, as you type less code, so IMO, more fun to
use.
[1] - Shared pointers should go by value though, I'll try to prove it in
the QUIP.
[2] -
http://www.macieira.org/blog/2012/02/the-value-of-passing-by-value/
Cheers,
--
Sérgio Martins | sergio.mart...@kdab.com | Senior Software Engineer
Klarälvdalens Datakonsult AB, a KDAB Group company
Tel: Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322)
KDAB - The Qt, C++ and OpenGL Experts
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