Hacker News Reader

Comment by CyberRabbi | Hacker News Reader

Copy constructors are usually necessary when wrapping data on the heap or an OS handle. I agree this is relatively rare in user code but what’s not necessarily rare is having one of those types as members in an otherwise trivial type. std::string is a good example.

View on HN

> std::string is a good example

Yeah, so USE std::string in your classes, then no copy constructors written by you will be needed. If you find yourself heap-allocating things explicitly with "new", then you are almost certainly doing something wrong.

This seems mostly irrelevant to the example in the post.

Why does it matter who wrote the copy constructor? In the author's example, they did not write a copy constructor, they USEd std::string.

Yes but this article is not about the hazards of using new/delete. It’s about the hazards of custom copy constructors. Simply using a common type like std::string is sufficient to trigger that hazard even if you never write a custom copy constructor yourself.

Perhaps you could point out the hazards? The generated code is longer, but so what, so long as it is semantically correct - longer code is not a hazard. And without knowing how the code was built, with which compiler, with which options etc. it's very hard to comment.

Bigger ASM means you can fit less code in cache, which makes your code slower. If this happens in a hot loop, it may be the difference between "my code fits entirely in L1 cache" and "my code needs to be fetched from L2/L3".

This matters in high-performance scenarios. The fact that you can introduce a performance issue simply by creating a temporary is absolutely bonkers, and makes writing high-performance code more complicated than it needs to be (and god knows it's already very complicated).

Depending on the processor, it can also be the difference between instructions in a tight loop only needing to be decoded once (because of micro-op caching) vs. decoded each loop iteration.

> without knowing how the code was built

The article links out to Compiler Explorer (https://godbolt.org/), which shows the exact compiler version and options and lets you play with different ones. It shouldn’t matter which compiler — the standard prohibits optimizing this.

Reply on news.ycombinator.com