Return Value Optimization and Copy Elision in C++
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- Опубліковано 10 лип 2024
- Embrace the copy-free lifestyle!
― mCoding with James Murphy (mcoding.io)
Source code: github.com/mCodingLLC/VideosS...
Cppref on copy elision: en.cppreference.com/w/cpp/lan...
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CHAPTERS
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0:00 Intro
0:40 Return Value Optimization
3:06 Where do the copies come from?
5:23 When can copies be elided?
7:46 C++17 mandatory copy elision
9:38 Thanks - Наука та технологія
I was just learning more about this! Perfect timing and perfect tutorial as always. Please do more C/C++ content!
I'm glad to see another C++ video from you. I hope there are more of these in the pipeline!
This reminds me of tail recursion in functional languages: a function calling itself recursively as its *last* action can have its context safely removed from the stack, and replaced by the context of the callee.
This isn’t just in functional languages. Java, C++, C probably do it too depending on the optimization level.
Please continue creating C++ content. Incredibly resourceful and interesting. Would like longer videos also!
C++ is like piloiting a jet engine naked with an open cockpit.
Thank you for that imagery 😂
the thing I like the most most about C++ is how simple and straight forward it is, no gotchas or special cases, what you see is what you get, it's amazing really
LOL, what a load of crap -- it is literally the exact opposite -- did you not see the copy constructor that was marked as deleted and how it behaves differently in 11 and 17 ?
@@firstname4337 thatsthejoke.jpg
amazing indeed 😂
Thanks for share your knowledge with us!
Your video cleared something about C++ i couldnt stop think that it's not exacly like good old C would normally do.
It really helped me. Thank You ❤
Your calm voice and intonation is great for these kinds of videos! I really appreciate these clean and concise explainations of concepts.
Thanks for the great videos James, you're the best programming channel on UA-cam. Always interesting and useful content explained clearly with no filler.
I understood practically nothing about copy elision from a lot of resources until I came across this video, and in ten minutes I was able to understand it fully. Thank you!
The world would be a better place if C++'s constructor rules didn't make my stomach twist in anxiety. Non-default constructors invite the devil into your home, and you'd never rest easy knowing you may have accidentally missed a common gotcha and now there's 5 heap allocated std::strings under your bed.
Love your C++ videos
Thank you for your video
I love how flat the delivery of the humor is ("devolve into our primal spaghetti nature"). Very well done 👌
and here i am juggling with move assignments, and returning references to internal members, and complicating life cycles like madman
What a great video.
Wish for more videos on C++ from you.
Yesss, more c++!
awesome!
Nice video. I was a bit stumped on 7:06 thinking it should be possible to move the value given both s1 and s2 are ephemeral values local to the function but then I remembered this is a compiler optimization, not a runtime optimization. So then I wondered if a simple design change to the code would work around such a case by doing the branching before calling the function. It's a interesting consideration that I wouldn't have thought to do before this, but it has performance implications.
Sean Parent in one of his brilliant talks went through this kind of copy behavior (but in code that actually doesn't allow compiler to do it magic, being a code smell). Fascinating stuff.
Woah, I've never been this early!
I think I just experienced a bug at work in C# that was due to this copy elision. I was doing something other than copying in the copy constructor, and it wasn't being done! Thanks for opening my eyes!
Good vid.
Hi James, what books\courses can you recommend on c++? Apart from Bjarne's books. Thank you!
C++ videos 🙏
Now how does the function now the address of the variable? is this another case of a hidden function argument, just like this in classes?
"Haven't said anything about move ops, but listing out more cases doesn't do anything good" - well, actually, every single one of your examples were calling MOVE, and not COPY constructor just your example S class' move constructor is the copy constructor (because you didn't implement a move, move is using the copy ctr).
I'm not sure what exactly your objection is here (perhaps you could elaborate?). Yes, the compiler will move returned local variables if a move constructor is available. No move was provided so it called the user-defined copy constructor. Are you saying I should still refer to this as a move even though the copy constructor was called and no move constructor is provided or generated by the compiler? I'm not sure I would agree with calling it a move in that situation, especially not colloquially. Colloquially, when one speaks of eliminating copies and eliminating moves, one is typically referring the act of reducing the number of times the copy constructor or move constructor actually gets called, i.e. an empirical reduction in copies or moves, not a reduction in places where the standard would allow a copy/move. In any case, it is not correct to say that the move constructor "is the copy constructor" if one is not provided. The definition of copy/move constructor in the standard is based on the signature of the function alone. It is *not* based on whether it is called when initializing from a prvalue. Instead, the compiler may chose to copy *instead of* move due to the fact that no move constructor is available.
So it's basically auto reference for all the function call?
It'd be nice if that rule were more clearly and concretely defined. Ideally it should always optimize it away, and in the case of the two values caught by control flow, while it couldn't always catch that, I hope modules mitigate some of the cases where it wouldn't. Obviously when taking user input to decide that control flow it will never optimize it away, but with a constant as in the example I hope it does.
afaik, the compiler can’t do that - if a function is called ( explicitly ), it must be called.
That optimization would mean you would be calling a different function.
7:12 could the compiler inline the no_elusion function and then remove the redundant "if(27>0)" and then do copy elusion?
If you use the inline keyword or up the optimization level it should, but I'm not an expert on the C++ standard, so don't quote me on that.
A great one to test on compiler explorer! godbolt.org/z/rYq1ejzKW
Even in C++23 at -O3 with "if (true)" we still see the copy.
Note however that if you move the instantiations of s1, s2 inside the scope of the if block that they are actually returned in, then the copy can be elided because they will no longer interfere with each other.
6:55 couldn't you arbitrarily choose one of either s1 or s2 to be constructed in the return value space, and then in the worst case if it ends up being the other one, you have to copy and overwrite it (shouldn't make performance worse if I'm not missing something) and in the best case you guessed correctly and you can elide the copy. The compiler could even try to figure out which branch is more likely and choose which variable to construct in the return value space based on that. I'm guessing that this optimisation isn't mentioned in the language standard, but would a compiler still be allowed to do it, at least if the elided copy constructor has no side effects?
spaghetti code is more about Goto continue and break (loop type of thing !)
this was an awesome video -- i learned C++ WAY BACK before the "modern age" and i really hated it
but these required optimizations make it sound much better than i remember
don't be fooled, it's still horrible to actually code with
@@slpwrm filtered, LMAO
@@slpwrm not true for me. I actually enjoy writing modern C++ than C.
@@drnoob13 I mean yes, but that bar is very low.
Thanks for this video, it reminds me how lucky I am, having left behind C++ for now to work in a sane language.
Hahaha what is your language of choice now?
@@mCoding In my case, I've ditched C++ after learning about Rust, at least for me, Rust is just C++ minus the enormous headaches that came with the language. I was using C++ for only 2-3 months or so, it's kinda funny how I switched languages so quickly.
Nice way to tell the world you're too bad for real languages.
@@mCoding Python.
@@Tibor0991 LOL. I've spent more than a decade with C++, I just got tired of dealing with the pointless obstacles that it puts in the developer's way for no good reason. You've got an "object oriented language" with a "standard library", but you can't use actual objects or types from the standard library in the public interface of your library unless the user happens to use the same compiler? Get outta here.
What about the parallelism between Windows 10 (latest update) and C++ (20) compilers?
What are you implying here?
UA-cam subtitle generation algorithm wasn't optimized enough to understand “copy elision”
james murphy? like from lcd soundsystem?
Same names, different James!
discord gang 🤙🤙
hi mod
Surprised there isn’t a comparison to const byref
I'm not that great at c++.If someone can help me, one of the problems I face often is the hash for a few template types just aren't defined. For example if I want to put a pair of ints or a tuple of ints in an unordered_map as keys, I can't do it. Any easy ways to get around this?
The easiest way is to just define your own hash function, for example, for strings you could do:
#include
#include
#include
typedef std::pair pair;
struct pair_hash
{
template
std::size_t operator() (const std::pair &pair) const {
return std::hash()(pair.first) ^ std::hash()(pair.second);
}
};
int main()
{
std::unordered_map unordered_map =
{
{{"C++", "C++11"}, 2011},
{{"C++", "C++14"}, 2014},
{{"C++", "C++17"}, 2017},
{{"Java", "Java 7"}, 2011},
{{"Java", "Java 8"}, 2014},
{{"Java", "Java 9"}, 2017}
};
for (auto const &entry: unordered_map)
{
auto key_pair = entry.first;
std::cout
Extend std::hash, if nobody else is going to use your code.
I miss Python content
What does “vroom vroom” mean?
Sounds cute, though.
It's the sound a car makes revving its engine!
For people saying that Rust is now as complex as C++
Huh? 2:33
HELP
C++ being c++
5:20 Stolen from Kaze Emanuar!
You caught the reference! I love his videos!
@@mCoding I knew it was a reference because I saw your comment on his video. 😉
and thats why cpp is considered as BS language
C forever 🤣🤣
Honestly, what he showed is quite irrelevant to how you code in Cpp. Why? Because you would never do what he did here. I can not think of any context this knowledge would be useful.
Why should you make copies, if you did not need one? You understand that some objects might be really heavy to copy right?
@@Kurkkulimu I think he means because you don't have to jump through hoops to avoid unnecessary copying as well as to make the compiler generate optimized code.