std::vector::reserve + std::vector::push_back in loop is sub-optimal, because push_back needs to check for re-allocation, but that never comes.
std::vector::resize + std::vector::operator[] in loop is also sub-optimal, because resize default-initializes all elements only to be overwritten soon anyway.
This article’s author suggests push_back_unchecked.
I suggest std::vector::insert with pair of random access iterators with custom dereference operator that does the “transform element” or “generate element” functionality. The standard will have resize_and_overwrite hopefully soon.
Moar discussion:
https://codingnest.com/the-little-things-the-missing-performance-in-std-vector/
https://twitter.com/horenmar_ctu/status/1695823724673466532
https://twitter.com/horenmar_ctu/status/1695331079165489161
https://www.reddit.com/r/cpp/comments/162tohr/the_little_things_the_missing_performance_in/
https://www.reddit.com/r/cpp/comments/162tohr/the_little_things_the_missing_performance_in/jy21hgd/
https://twitter.com/basit_ayantunde/status/1644895468399337473
https://twitter.com/MarekKnapek/status/1645272474517422081
https://www.reddit.com/r/cpp/comments/cno9ep/improving_stdvector/
Couldn’t this be solved by having
push_backbeing an inline function (or at least the check on capacity being inlined and the rest of the non-trivial part being in a sub non-inline function)?I don’t know about C++, but in Rust the push is inline, and still doesn’t always optimize checks away due to an annoying edge case: integer overflow. Reserving (old_len + new_len) could give you a smaller buffer than new_len. The optimizer sees it and is pedantic about it.
In C++ integer overflow is UB so this edge case cannot exist
Only signed overflow. size_t is unsigned.
That’s totally right but I thought you were talking about signed numbers since you said “integer overflow”. I forgot that
lenis usually unsigned in C++.
Ok, but why did you not use emplace?
emplacecontrols the construction of the object added to the collection. It’s also important but not related to the problem exposed by OP which is “how to remove the capacity check when we know statically that there is enough space”.