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.
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."
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.
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
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.
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.
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 !
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.
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.
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.
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!)
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()
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! 🤩😍❤️🥳
@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.
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.
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!🥰😍🥰😍🤗❤️
FOR LINUX USERS OR PEOPLE WHO IS HAVING ERRORS IN THE CODE: JUST DO THAT IN THE BEGINING class String { private: /* data */ char *m_Data = nullptr; uint32_t m_Size = 0; AND THAT ON THE PRINT void Print() { if (m_Size != 0) { for (uint32_t i = 0; i < m_Size; i++) { printf("%c", m_Data[i]); } } printf(" "); }
can you explain why my default constructor cannot provide value to m_data and in terminal showing free(): invalid size error when I try to delete m_data
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?
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.
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.
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!!!
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?
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.
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
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.
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?
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.
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 .
9:42 instead of doing a manual delete[], isn't it better to simply transfer ownership of this->m_Data to the temporary? That way it will get deleted when the temporary is destructed? const auto otherSize = other.m_Size; const auto otherData = other.m_Data; other.m_Size = m_Size; other.m_Data = m_Data; // Will get deallocated on other's destruction m_Size = otherSize; m_Data = otherData;
Fix the String destructor for the love of c++. It just hurts me to see this memory leak. He even says "Trying to delete an array" in 12:57 and doesn't notice the leak.
Are you talking about the missing [ ]. If this is what you mean, then I think (but I am not sure) that dropping the [ ] is not an enough reason for memory leak in this case because the deleted array consists of trivially-destructed elements of type 'char'.
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.
Your string(string&&) constructor assigns to members instead of initialising. Was that on purpose? (4:20) Haven't heard the rest - thanks for making the video :-)
Hi, Cherno, i have some questions regarding to the std::move : please see the code below: //start of code int a[5] = {1,2,2,2,2}; a[0] = std::move(a[3]); // is this std::move useless? will it steal the resource of a[3] cout
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?
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.
I ran into a free() error while following along. Just in case someone else also gets something similar, here's how I fixed it. Error: free() invalid memory Compiler: gcc OS: Pop OS (Linux) Solution: Replace `String() = default` with a properly initialized default constructor. ``` String() : m_Data {}, m_Size {} { } ``` This problem might happen intermittently because it is entirely dependent upon how the memory at the location assigned is configured at the time.
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.
No because std::move does not move anything. It is just casting a variable to a rvalue reference (type&&). Casting is not moving. Ok, the name is misleading, but that is it.
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
9:04 have we really created a memory leak? We reference m_data to new memory location. Which means the previous memory location is not referenced anymore and so that memory must get auto flushed/removed/ garbage collected right?
Have seen the use of uint32_t in the past few videos in the C++ series. Is this a good practice as compared to using unsigned int/ unsigned long? I might have missed/ forgot something if this was mentioned in a prior video...
It removes ambiguity from variables based on compiler or system. For example, on my system using visual studio "int" is a 32 bit value. Annoyingly "long" is also a 32 bit value which makes it basically useless. If I want an unsigned 64 bit integer, I would need to type "unsigned long long" using the default syntax. It's way cleaner to use "uint64_t" instead. I've since adopted the convention for all integers just to make my overall syntax more uniform.
When I write delete[] m_Data; Am I not eliminating the pointer? How is it possible I can assign data after that at m_Data = other.m_Data? If the pointer still exists... does that mean that when I call a destructor to delete any pointer, like: ~Obj(){delete[] ptr;} this pointer (ptr) still exist? Is it not a memory leak? How is it different from the operation above?
Hi Yan..what your thought on Rust..alot of people love Rust but honestly, the only things i like about Rust is its package manager ( Cargo )..other than that, i dont see any benefit from using it...its syntax is maddness...
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.
You think they are. Do your course work.
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'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
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.
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.
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 😭
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 !
Great video! This is my favorite C++ series. I hope you make one on perfect fowarding soon.
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.
you definitely made a cool explanation of the std::move functionality - thanks, this is going to help many developers I think
Daym, my man just explained everything very clearly. Thank you, it's usually explained very complicated
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" 🤣
I never realized the difference between a constructor being called with '=' vs an assignment operator being called with '=', nice bonus information!
perfect forwarding ~
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.
after he said "implicit conversion and call into this specific string constructor" my furnature started floating, help
Your examples make move semantics really easy to understand. Please explain copy and swap idiom too.
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?
Move semantics is a topic difficult to comprehend. Your in depth explanation made it very clear.
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!)
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
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()
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.
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.
First time somebody explained this and i managed to understand it, thank you!
Incredibly helpful and enjoyable video -- thousand thanks Cherno!
You're a legend, learning c++ with you is exciting !
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
Dude, you're amazing. Your videos are helping me a lot!
thanks for the difference between = and move constructor part, really never thought about it!
FOR LINUX USERS OR PEOPLE WHO IS HAVING ERRORS IN THE CODE:
JUST DO THAT IN THE BEGINING
class String
{
private:
/* data */
char *m_Data = nullptr;
uint32_t m_Size = 0;
AND THAT ON THE PRINT
void Print()
{
if (m_Size != 0)
{
for (uint32_t i = 0; i < m_Size; i++)
{
printf("%c", m_Data[i]);
}
}
printf("
");
}
can you explain why my default constructor cannot provide value to m_data and in terminal showing free(): invalid size error when I try to delete m_data
@@ankitbhandari5222 How is your code:
is this way?
private:
/* data */
char *m_Data = nullptr;
uint32_t m_Size = 0;
public:
String() = default;
String(const char *string)
{
printf("Created
");
m_Size = strlen(string);
m_Data = new char[m_Size];
memcpy(m_Data, string, m_Size);
}
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.
Well done the Cherno, excellent lesson to be learnt!!!
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.
I've learned a lot from your vids, thanks for posting!
Excellent presentation. Well organized and full of important details.
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!!!
Shall get there and will overcome
Thank you, Cherno
Please never stop making videos.
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?
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.
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
Hi Cherno, can you please do a video about std::forward too ? Thanks for this.
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
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
Cherno finally a new video for C++ series, thanks ♥️
this is really well explained! Wish I would have watched your series before my interview...
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?
Dude you are a live saviour
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.
Could you do a video on streams? iostream and fstream class heirarchy and some stream handling functions?
Thank you much! very understandable!
thank you
Great video!
Got a question though, is it possible/recommended avoiding copying when the object is returned?
i literally have been waiting and researching this the last few days. nice !
Destructor -> delete[] m_Data instead of delete m_Data
Wow! Really Nice! Many Thanks 4 this concept!
You are a legend man!!
This is insane, back to my simple Java world
Cant wait till next video 😍
A video on lifetime extension, return value optimization, forwarding...
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 .
Very insightful, thank you. 🙏🏿
Great Videos you make! Keep it up!
9:42 instead of doing a manual delete[], isn't it better to simply transfer ownership of this->m_Data to the temporary? That way it will get deleted when the temporary is destructed?
const auto otherSize = other.m_Size;
const auto otherData = other.m_Data;
other.m_Size = m_Size;
other.m_Data = m_Data; // Will get deallocated on other's destruction
m_Size = otherSize;
m_Data = otherData;
Fix the String destructor for the love of c++. It just hurts me to see this memory leak. He even says "Trying to delete an array" in 12:57 and doesn't notice the leak.
Are you talking about the missing [ ]. If this is what you mean, then I think (but I am not sure) that dropping the [ ] is not an enough reason for memory leak in this case because the deleted array consists of trivially-destructed elements of type 'char'.
@Jayanam Develop
thats why you should use malloc/free
c > c++
@@MatkoFaka that's why you should use neither and leave it to RAII/smart pointers
@Jayanam Develop that's why i said RAII/smart pointers, and not just smart pointers.
thanks for this amazing series
awesome!! Thanks buddy
What about the copy assignment operator? Do you ever use it?
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.
Why don't you share these example codes so that we can execute and experiment easily ? Thanks
Your string(string&&) constructor assigns to members instead of initialising.
Was that on purpose? (4:20)
Haven't heard the rest - thanks for making the video :-)
Hi, Cherno, i have some questions regarding to the std::move :
please see the code below:
//start of code
int a[5] = {1,2,2,2,2};
a[0] = std::move(a[3]); // is this std::move useless? will it steal the resource of a[3]
cout
I think moving primitive types (int, long long, float, double) does nothing, I believe this is because they have no move constructor.
Rule of five.. rule of five!
Please, explain this and do the string class with copy-ctor && swap() idiom! :)
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?
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.
I ran into a free() error while following along. Just in case someone else also gets something similar, here's how I fixed it.
Error: free() invalid memory
Compiler: gcc
OS: Pop OS (Linux)
Solution:
Replace `String() = default` with a properly initialized default constructor.
```
String()
: m_Data {},
m_Size {}
{
}
```
This problem might happen intermittently because it is entirely dependent upon how the memory at the location assigned is configured at the time.
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 ?
Should the deconstructor not use delete[] just like the operator as as new[] is used?
@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.
confused here.. so you can use move method as a swap method to swap two variables? like std::move twice between the two variables, will it work?
No because std::move does not move anything. It is just casting a variable to a rvalue reference (type&&). Casting is not moving. Ok, the name is misleading, but that is it.
how can you Print the uninitialized m_data member without an error?
UA-cam getting ridiculous with the ads now, I mean 4? wft
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
@@PflanzenChirurg Yes, like here when you must create specific constructors and operators :)
Great! keep going!
thank you very much!
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
9:04 have we really created a memory leak? We reference m_data to new memory location. Which means the previous memory location is not referenced anymore and so that memory must get auto flushed/removed/ garbage collected right?
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
This is off topic but how do you use Italic font in VS?
Have seen the use of uint32_t in the past few videos in the C++ series. Is this a good practice as compared to using unsigned int/ unsigned long? I might have missed/ forgot something if this was mentioned in a prior video...
It removes ambiguity from variables based on compiler or system. For example, on my system using visual studio "int" is a 32 bit value. Annoyingly "long" is also a 32 bit value which makes it basically useless. If I want an unsigned 64 bit integer, I would need to type "unsigned long long" using the default syntax. It's way cleaner to use "uint64_t" instead. I've since adopted the convention for all integers just to make my overall syntax more uniform.
When I write delete[] m_Data; Am I not eliminating the pointer? How is it possible I can assign data after that at m_Data = other.m_Data?
If the pointer still exists... does that mean that when I call a destructor to delete any pointer, like: ~Obj(){delete[] ptr;} this pointer (ptr) still exist? Is it not a memory leak? How is it different from the operation above?
Maybe you can do a video about Hash Tables!! it would be very useful. Thanks for the videos btw, they help me a lot
Hi Yan..what your thought on Rust..alot of people love Rust but honestly, the only things i like about Rust is its package manager ( Cargo )..other than that, i dont see any benefit from using it...its syntax is maddness...
1:48 the classes are bytesized... that was a little nerdy but yeah i liked it :)