std::move and the Move Assignment Operator in C++
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- čas přidán 1. 08. 2024
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I really wish I had've just watched your entire C++ series instead of listening in any of my lectures. This has been miles more helpful and easy to understand than any programming class I've had.
I've just aced a C++ technical interview thanks to this video. Your ability to explain is unparalleled. Keep doing the good work!
i know this is 7 months old lol but congrats! Hope the job is going well and I hope to one day soon ace a C++ interview myself using what i learn from Cherno
A good ways to understand std::move() is that you are basically saying "I don't need the object that I am passing anymore. I am allowing someone to steal from it."
Voluntarily letting your stuff be stolen.
It basically steals the pointers, or takes ownership of them, very powerful, like Cherno says "no new allocation". I can only think of using it when I have a temp variable.
@@glee21012 The fundamental Computer Science concept here has to do with Type Theory, and in particular linear (more correctly, affine) types: any object can only be owned by a single owner at any given time. Because C++ is the way it is (i.e. a mess) it's harder to enforce. But it (i.e. C++) is a necessary evil.
@@Evan490BC c++ is only a mess when your bad code makes it seem like a mess.
I now realize I still dont know the basics of c++
I thought the same thing after watching the video but honestly, it more so made me realize what a crazy and confusing language C++ is. Stuff like this is why I prefer C or Rust.
Move constructors/assignment is basic C++
@@1vader or C#, or Python...
@@1vader That's not even the difficult part of the story of move semantics. Research perfect forwarding and reference collapsing
@@1vader Same here.
I like the sentence "steal everything from Apple" 🤣
This is something I did so many times in C, but my teachers never told me it was a concept (move). It's much clearer when to do what now and the class abstraction we can do in C++ helps the code to be cleaner.
I think what you must have been doing is a simple typecasting in school. This is just a specific version of typecasting which works specifically with rvalue references.
Great video! This is my favorite C++ series. I hope you make one on perfect fowarding soon.
I did not understand std::move() until I watched this. To give some background, I was first exposed to C++ in 1994 but moved away from it before r-value references were a thing.
I like the examples he provides. The problem I have with most explanations is that they use words like "tells the compiler it can cannibalize the object" or some such nonsense. That leaves the impression that the compiler is doing some type of cannibalizing or other magic behind the scenes.
Saying it just turns the object to an r-value reference and as a side effect can cause the move constructor or move assignment operator to be called. The person cannibalizing is not the compiler, it is the implementor of those operations.
After seeing this I wrote a simple class like he had and played around with it. It is not nearly as complicated as I thought.
14:05 distinction between constructor and assignement operator, that's why I rather prefer to use {} brackets with new object creation and use the = only for assignement operator :)
(For instance : String name{"Cherno"};)
By the way, huge and awesome work, thanks !
you definitely made a cool explanation of the std::move functionality - thanks, this is going to help many developers I think
Not a clue what most of what you said means on a technical level. But still couldn't stop watching. Great video.
C++ gods help me 😭
Incredibly helpful and enjoyable video -- thousand thanks Cherno!
First time somebody explained this and i managed to understand it, thank you!
That's amazing, thank you so much Mr Cherno!
By the way, a lot of what I know of C++ I've learned from you, so a huge thanks, this has been very helpful!
(Shoutout to Corey Schafer too for teaching me Python!)
perfect forwarding ~
Dude, you're amazing. Your videos are helping me a lot!
Move semantics is a topic difficult to comprehend. Your in depth explanation made it very clear.
I've learned a lot from your vids, thanks for posting!
Your examples make move semantics really easy to understand. Please explain copy and swap idiom too.
Excellent presentation. Well organized and full of important details.
after he said "implicit conversion and call into this specific string constructor" my furnature started floating, help
Thank you for the video. I think what "clicked" hardest for me was when I learned that the assignment operator, by default, only moves the values inside an object to a different object, if the object that it is being moved into pre-existed - which is why we overload the assignment operator, so that we can do this, even when the object that the data is being moved into is being initialized. Or at least, this is how I think it works. If I'm misunderstanding, I would love some clarification.
Good point! In a way the moving is the best thing that can happen to m_Data inside that operator. And we're talking the move assignment operator and the moving as opposed to copying which is what memcpy does in the copy constructor.This line says it all:
m_Data = other.m_Data;
Don't get disillusioned, however, thinking the data is being copied here. It's only a pointer being copied to another pointer. Imagine m_Data (in other) points to a char array the size of like billion. The array needn't to be copied, it stays where it was in memory. That's when we like assignment by move better than by copy. And we for this reason overload the move assignment operator. You got the gist.
thanks for the difference between = and move constructor part, really never thought about it!
great video! well explained.
would be great having a video regarding Variadic functions :D
Thank you Cherno. I enjoyed learning every second of this video and I realized that the pace you spoke today in this video was quite perfect! Not too fast! I appreciate that!🥰😍🥰😍🤗❤️
Amazing video! You taught me how to create a move constructor and move assignment operator for my matrix class! Thank you!
If you still have that code can you send me the github repo coz I'm interested. Thank you
Very insightful, thank you. 🙏🏿
Shall get there and will overcome
Thank you, Cherno
The move command is extremely useful when using smart pointers. Moving vectors of unique pointers around without having to copy a bunch of stuff is extremely useful.
Well done the Cherno, excellent lesson to be learnt!!!
this is really well explained! Wish I would have watched your series before my interview...
Wow! Really Nice! Many Thanks 4 this concept!
You're a legend, learning c++ with you is exciting !
i literally have been waiting and researching this the last few days. nice !
1. Instead of casting using
(T &&)var you can use std::move(var)
2. A good ways to understand std::move() is that you are basically saying "I don't need the object that I am passing anymore. I am allowing someone to steal from it."
3. We need to always delete the current data before shallow copying the other data. Also we need to point other data to nullptr.
4. Before moving the other object into ours we need to ensure that we are not moving the same object to itself. Otherwise data may be lost.
Maybe covering std::forward here as well would've been a good idea, since they're somewhat related.
Yes, I was going to suggest the same.
Would you recommend any particular resource to learn that?
thanks for this amazing series
Cant wait till next video 😍
Great Videos you make! Keep it up!
Thank you much! very understandable!
You are a legend man!!
The more I learn about C++, the more I realize that I know NOTHING about this complicated but amazing language! I could learn C++ for the rest of my life and still there would be much to know! 🤩😍❤️🥳
that's not a good thing
@Prochy it is just the matter of preferences☺️ I would love to be challenged and C++ is a REAL programming language! C++ helps me to realize what is happening behind the hood, C++ gives insight into things I can’t see in other language! C++ is a miracle in the world of computer science! C++ being huge and big, makes you keep learning and learning! There will be always something new and challenging for you! That’s what I want! Never stop learning! 🙏❤️
@@__jan Some people like dead languages that never change, others like new features that improve their code clarity and performance. I'm of the opinion that C++ being an actively improved language is a very good thing, and having to continually learn new things is part of the job description of any software developer.
awesome!! Thanks buddy
Dope, some time ago I was looking for a nice explanation of move assignment operator and exisiting stuff was quite confusing, I could have used a tutorial like this.
I believe explanation in chapter 5 in "Effective Modern C++" by Scott Meyers is one of the best source I've read on this topic. I have a paper copy, but it seems you can find it for free on the internet.
Great! keep going!
I don't earn right now so can't really support you on patreon but the least I can do is watch all the ads on your videos :)
Man now I just feel bad.. I also should pause adblocker on this channel
thank you
Nice video. I like to think to std::move as a way to have an rvalue reference. I remember that Stroustrup wrote in "The C++ Programming Language" that the best name for std::move() would have been rval()
Please never stop making videos.
This is insane, back to my simple Java world
Thank you very much
I'd like to add one more thing.
In order to create an empty object (in this case String 'dest'), we need a default constructor which sets m_Size = 0 and m_Data = nullptr.
Dude you are a live saviour
Awesome work you’re doing here!!, do you plan on making videos on error handlings and file system read and write in C++?
file read and write is rather simple, use , open a file and use it as cout or cin
Awesome!
great vid
Cherno finally a new video for C++ series, thanks ♥️
SuperawesomethankyouYan!
Maybe you can do a video about Hash Tables!! it would be very useful. Thanks for the videos btw, they help me a lot
So one you’re an absolute amazing teacher and thank you for doing us, too maybe give use case examples for this I still factor five years don’t completely understand the best used cases for this. Is this more conducive for embedded systems or extremely low latency code?
Muchos gracias
A video on lifetime extension, return value optimization, forwarding...
Hi Cherno, can you please do a video about std::forward too ? Thanks for this.
nice video
Could you do a video on streams? iostream and fstream class heirarchy and some stream handling functions?
Very humbling video.. I don’t know shit ... 😅 I have so much to learn
you are great
Knowing what to type is the complicated part.
Everything was clearly explained, only place I could fully accep the use of std::move was in the Entity constructor with "String&& name" parameter, isn't name already a r-value reference of the type "String&&" in the member initializer list, why do we again have to convert it to an r-value reference using std::move?
Rule of five.. rule of five!
Please, explain this and do the string class with copy-ctor && swap() idiom! :)
Great video!
Got a question though, is it possible/recommended avoiding copying when the object is returned?
Can you make a video on push_back vs emplace_back used in move semantics when calling constructors.
perhaps would be correct to first say, that move is for performance optimization. there is nothing wrong with copying in most cases. performance optimizations must only be done based on performance requirements. there is no use of doing that where its not needed at all, because performance requirements are met anyways. it would make your product worse, because it would get more expensive, without getting perceivably and practically better. its like if I would put 30% more work into the car, by giving decorative painting to all the screws inside the car.
wow the way i never knew that std::move is just a small template function which just simply does the casting thing for us...and all that casting does is calls the move constructor...the lazy and dumb me never tried to look up the definition of that std::move function....now i know...thanks cherno!!! but actually have a question...in order to use the move constructor, do we always explicitly have to tell the compiler by casting the type into a rvalue ref??? or there are cases where the compiler does it implicitly??? thanks in advance!!!
Please cover the forwarding and arguments decay too Thanks
Now I realize that Rust is basically doing move semantics by default. And to do a deep copy, you have to clone() the variable.
let mut dest = String::new();
let simon = String::from("Simon");
let other = String::from("other");
dest = simon; // This is a move; simon is now gone
println!("{}", simon); // Compilation error
dest = other.clone(); // Both dest and other are available
println!("{}", other);
println!("{}", dest); // Works
In C++ is the other way around. By default deep copying is used.
No one:
Voice recorder: blinking green as Cherno say something
I can't believe he is still making C++ videos. This just shows how much there is to know about C++ and how little I know ... :P
it's because this language has become extremely bloated and a pain in the ass to work with
@@lordmushroom723 Words of wisdom. C++ has too many features to solve the issues that are there because of C++ itself.
@@alpyre not really, it has just its rules and every new standard makes it easyer to create decent code
@@greatbullet7372 Yes, like here when you must create specific constructors and operators :)
Thank you so much. Can you do it for std:: forward please?
Cpp devs learning basic rust concepts
very good explanation, thanks. i used to struggle understanding move semantics. one question though: in the string destructor you used "delete m_data;" while in the move operator you used "delete[] m_data". since you allocated the memory using new[], i think the destructor should use delete [] too. however it's not crashing. does it make a difference really which one you use to delete?
It is undefined behavior. As you point out, he should be using operator delete[].
In the case of POD types there are no destructors so new and new[] behave the same in this case. For none POD classes/destructable classes the compiler/implementation does do some trickery like encode the number of objects belonging to the array so it can later call the destructors on them all. It is best practice to delete[] though as some compilers/implementations may not simplify array allocations (most compilers do, but that's not a guarantee!)
@@SerBallister It is undefined behavior and thus is illegal C++ and shouldn't be relied on. It might work on your machine, but not other machines, and not your machine tomorrow as compiler and standard library implementer figure out better ways to optimize your program.
A simple example, an allocator might pool all allocations for a single byte into a pool of memory such that a call to `delete charptr;` would be assumed to come from this pool whereas `delete [] charptr;` would expect the object to be allocated in a different pool (perhaps a large object pool). As the implementer knows it is illegal to use `operator delete` for memory allocated with `operator new[]` they can avoid storing extra information that isn't needed for a legal program and are under no obligation to properly cleanup programs relying on undefined behavior.
Being a primitive type or not having a constructor isn't a reason to accept undefined behavior in a program.
1:48 the classes are bytesized... that was a little nerdy but yeah i liked it :)
Hi Cherno, would you please make some videos about c++ windows form / desktop application.
Just swap the values in move assignment instead of setting to default. You can skip the whole "what if they're the same object" problem
You are the best teacher on CPP topics ;)
Please do a video on memory allocators
This is off topic but how do you use Italic font in VS?
I understand the basic idea. But I can't really figure a realistic case where I need to implement these explicitly. Usually you want to avoid heap allocation and people use a lot the stl library. Unless you're writing your own library I can't really picture a situation where you need to implement move constructors and move assignment explicitly. You also have default move constructors, but I don't know how they works.
2 years late. You’re right that the STL encapsulates functionality. In cases where your class is composed of STL containers and/or smart pointers, then the default move constructor generated by the compiler would suffice. It does a member-wise move of all the members. If you use modern C++ and are more of a consumer than a creator, then you would likely never have to explicitly define those functions. Still, it’s good to be aware of the specifics of move semantics to allow you to leverage them well.
Hi, Awesome explanation, 😀
one question, why not pass objects by reference if want to give the ownership of the object to the function?
I know that move will invalidate the first object and literally move it in the function space.
but in a case, where we just want to avoid copy while passing to the function and the application is single-threaded even the pass by reference should work right?
That 's what he is doing isn't it. He is using the rvalue reference i.e (String&&) to move the object, similar to what he did in the Entity class in the previous video. And instead explicitly casting to the object, he is using the std::move. Furthermore, just passing it by reference will only be valid for lvalues, now the work around for that is const reference but for that also you have to copy the data. I hope this helps.
While overloading the move assignment operator, should we not allocate memory for m_Data? If the destination string length is greater than the original string we are moving into, there would be a memory access violation.
String orig = “Hello”;
String dest = “LongerString”;
orig = std::move(dest);
can somebody explain?
For me the code crashes in debug mode. On line 96 dest is constructed with default constructor, but it doesnt initialize the private members on the lines 60 (char* m_Data) and 61 (uint32_t m_Size), so when dest.Print() is called on line 101, the for loop on line 54 actually starts - because unitialized m_Size is a garbage value, and m_Data[i] (which is a nullpointer) causes the crash.
So i think char* m_Data should be set to nullptr uint32_t m_Size to 0.
any idea why it worked for him? crashed on mine too
there is a problem here . you dont initiate the m_data value in default constructor so when you try to use move constructor and try to delete there is nothing to be deleted so why this still works ? not for me though. i use gcc so probably i'm missing something .
HI Thank you for the amazing video. At 10:17 , you mention that "If they are different objects, then nothing needs to be done !", this should be rather "If they are the same objects, then nothing needs to done", am i correct ?
@The Cherno, please explain me why don't you have a return statement after line 46 (in the final version). The move assignment operator is returning *this when it's the same object, but nothing otherwise.
Hi, Cherno. Thank you very much for your course. I have a question. Both `delete[] m_Data;` and `delete mData;` exist in your code, which I don't know the difference. Can you kindly tell me when to use which one.
Inside the ~String() method he should've written delete[] m_Data instead.
The delete keyword alone deallocates a single object from memory, but the delete with [] deletes a whole heap allocated array.
Still not exactly sure WHY you would use the move constructor if you have to still instantiate a new temporary variable (which would likely use the copy constructor anyway, I'd imagine). What is the main use-case for the move constructor in those cases? (I can imagine it being helpful for updating the data pointer for large objects to point to a new memory address, but that point seems moot if you have to copy it anyway to a temporary variable)
derp. I just read through std::move docs and re-watched and realized that the temporary variable is just for the reference and not the data, which makes a lot more sense. lol. Guess I was thinking it would work similar to C where you can just yolo pointers with enough forceful casting :D
Do you think a video on std::forward is possible?
what's the benefit over usage of pointer/refs here? In other words why move if we can just refer to the data?
I have learnt basic components of C++ as a beginner and have tried to search 1000 ways of learning C++ as I am a self-taught person. The next step many people suggest is to solve the problems but I have solved basic problems such as calculater and some basic problems. Please could someone could help me with what projects should I consider which are not too advanced. Also some examples would do. (I am reading Effective C++ as Cherno suggested in one of his videos)
I'd be so so glad if someone could help me as mentor
That weird feeling when you move into yourself and refuse to do anything