this post was submitted on 28 Aug 2023
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Programming

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[–] robinm 6 points 1 year ago (2 children)

Couldn't this be solved by having push_back being 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)?

[–] kornel 6 points 1 year ago* (last edited 1 year ago) (1 children)

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.

[–] robinm 1 points 1 year ago (1 children)

In C++ integer overflow is UB so this edge case cannot exist

[–] kornel 3 points 1 year ago (1 children)

Only signed overflow. size_t is unsigned.

[–] robinm 1 points 1 year ago* (last edited 1 year ago)

That's totally right but I thought you were talking about signed numbers since you said “integer overflow”. I forgot that len is usually unsigned in C++.

[–] [email protected] 0 points 1 year ago (1 children)

Ok, but why did you not use emplace?

[–] robinm 1 points 1 year ago

emplace controls 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”.