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Don’t blindly prefer emplace_back to push_back

(quuxplusone.github.io)
178 points todsacerdoti | 3 comments | 04 Mar 21 08:02 UTC | HN request time: 0.65s | source
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codeflo ◴[04 Mar 21 09:31 UTC] No.26340405[source]▶
I’ve personally never encountered the particular misunderstanding the author dispels here, but I’m sure the post is written from bitter experience. On the one hand, I shouldn’t be that surprised: Unfortunately, programming in C++ is really brutal for people who would rather learn by example than by carefully reading documentation.

But on the other hand, my instinctive reaction is: Come on, is this really something that confuses people? emplace_back is all about constructing something in-place, which is exactly the opposite of moving it there! How can any professional programmer be 180 degrees wrong about the purpose of a function they use regularly?

What I want to say here is that C++ is really hard, and that there are a million subtle things that one simply has to look up instead of making educated guesses. I don’t think people appreciate this difficulty enough.

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1. oconnor663 ◴[04 Mar 21 17:34 UTC] No.26345318[source]▶
> How can any professional programmer be 180 degrees wrong about the purpose of a function they use regularly

If you have a strong understanding of move semantics, then I agree it would be pretty weird to misunderstand the relationship between move semantics and emplace_back. But how common is it to have a strong understanding of move semantics, really? Can we confidently assume that even 50% of professional C++ programmers have a strong understanding of move semantics? I've known competent C++ devs, who write template metaprograms for fun, who were still surprised to learn basic facts about move semantics like "the destructor of a moved-from object will still be called." I know for certain that I don't have a strong understanding of move semantics, but I wouldn't call myself a C++ programmer either, so at least I'm not dragging down the statistics :)

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2. wnissen ◴[04 Mar 21 18:36 UTC] No.26346284[source]▶
Yes, I think his recommendation to prefer push_back is wrong for that reason. I can't imagine it's easier for the compiler to reason about std::move and destructors compared to inlining the constructor.
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3. ncmncm ◴[05 Mar 21 04:01 UTC] No.26352514[source]▶
Nope, he's right. He maybe shouldn't be right, but certain convenience features are still missing from Standard Library components and the language that could someday make him wrong.

It is sometimes right to hold your nose and use std::piecewise_construct.

But most usually it doesn't matter, and worrying about it will distract you from what does matter. So, push_back() unless you know you have a good reason not to. And, you might never encounter one.

Good C++ and naïve C++ are not so different. Both are much better than fake-smart C++.