std::move and the Move Assignment Operator in C++

Summary

In this C++ tutorial video, Deshawn continues the series on move semantics, focusing on two crucial aspects: `std::move` and the move assignment operator. He explains `std::move` as a function that converts an object into an r-value, enabling move operations. Deshawn then demonstrates how to implement the move assignment operator for efficient resource transfer between objects. The video also highlights the difference between move constructors and assignment operators, emphasizing the importance of these concepts in modern C++ programming. Viewers are encouraged to explore the topic further through the series and take advantage of a special Skillshare offer for learning new skills.

Takeaways

• 📚 The video is part of a C++ series focusing on move semantics, continuing the journey from previous episodes.
• 🔗 The host encourages viewers to watch prior videos on move semantics and L values for better understanding.
• 🎯 The main topics for this video are `std::move`, the move assignment operator, and their applications in C++ programming.
• 🛠️ The `std::move` function is introduced as a utility to efficiently move resources from one object to another without copying.
• 🔄 `std::move` is used to cast an object to an r-value reference, enabling the use of move constructors or move assignment operators.
• 📝 The move assignment operator is explained as a special assignment operator used to move resources into an existing object.
• 💡 The video demonstrates how to implement the move assignment operator in a custom class, emphasizing the importance of handling self-assignment.
• ⚠️ A key point is made about the difference between using `std::move` for an existing variable versus a temporary, and its implications on resource management.
• 🔑 The concept of the 'rule of fifths' or 'rule of thirds' is mentioned, indicating a set of best practices including move constructors and move assignment operators.
• 📈 The video includes a practical example of implementing and testing a move assignment operator in a custom string class.
• 🎁 The host promotes Skillshare as an online learning platform offering a wide range of creative classes, with a special offer for the viewers.
• 👋 The video concludes with a summary of the move assignment operator and `std::move`, and an invitation for viewers to engage with the content and ask questions.

Q & A

What is the main topic of Deshawn's C++ series video?

The main topic of the video is move semantics in C++, specifically focusing on std::move, the move assignment operator, and how to implement them in custom classes.

Why is understanding move semantics important for C++ programmers?

Understanding move semantics is important because it allows for efficient transfer of resources between objects without unnecessary copying, which can improve performance and resource management in C++ programs.

What is the purpose of std::move in C++?

The purpose of std::move is to efficiently transfer the contents of an object to another, effectively 'moving' the resources and leaving the original object in an unspecified but valid state.

What is the difference between a move constructor and a move assignment operator?

A move constructor is used to initialize a new object with the resources of a temporary object, while a move assignment operator is used to transfer the resources from one existing object to another, modifying the existing object in the process.

Why is it necessary to check if the move assignment is happening to the same object in the move assignment operator?

It is necessary to check because moving an object into itself would lead to self-assignment, which can cause issues like memory leaks if not handled properly by first nullifying the current object's resources.

What does the rule of five in C++ include, and why is it important for classes that manage resources?

The rule of five includes the destructor, copy constructor, copy assignment operator, move constructor, and move assignment operator. It is important for classes that manage resources to ensure proper handling of object lifetimes and resource management.

How does the move assignment operator differ from the copy assignment operator in C++?

The move assignment operator transfers ownership of resources from one object to another, potentially leaving the source object in a valid but unspecified state, while the copy assignment operator creates a copy of the resource, which is generally more expensive in terms of performance.

What is the role of 'noexcept' in the move assignment operator?

The 'noexcept' specifier in the move assignment operator indicates that the function is not expected to throw exceptions. This can help with optimization and exception safety in C++ programs.

Why is Skillshare mentioned in the video, and what does it offer to its users?

Skillshare is mentioned as a sponsor of the video. It offers an online learning community with thousands of classes on various creative topics, allowing users to learn new skills at their own pace with bite-sized video lessons.

What is the significance of the 'rule of thirds' and 'rule of fifths' in C++ programming?

The 'rule of thirds' refers to the three special member functions (constructor, copy assignment operator, and destructor) that need to be properly defined for classes managing resources. The 'rule of fifths' extends this to include move constructors and move assignment operators, which are important for efficient resource management in modern C++.

Outlines

00:00

🔧 Introduction to Move Semantics

Deshawn welcomes viewers back to his C++ series and emphasizes the importance of understanding move semantics. He references previous videos on the topic and introduces the concepts of the STD move function and the move assignment operator, promising to cover both in this video.

05:01

🛠️ Understanding STD Move

Deshawn explains the STD move function, demonstrating its purpose in converting objects to temporary r-value references. He contrasts manual casting with the more elegant and flexible STD move function, highlighting its role in move semantics.

10:02

📦 Implementing Move Assignment Operator

The move assignment operator is introduced as a crucial component of move semantics, particularly for assigning existing objects. Deshawn shows how to define this operator, ensuring proper memory management and avoiding self-assignment issues.

15:03

💡 Practical Example and Conclusion

A practical example using the move assignment operator is presented, demonstrating its effectiveness in transferring resources without copying. Deshawn emphasizes the importance of including both move constructors and move assignment operators in classes. He concludes by encouraging viewers to explore more on move semantics and inviting them to check out Skillshare for learning new skills.

Keywords

💡Move Semantics

Move semantics is a feature in C++ that allows for efficient transfer of resources from one object to another by 'moving' them, rather than copying. In the video, the concept is central to understanding how to efficiently manage resources, especially when dealing with temporary objects or when reassigning resources to existing objects. The script discusses how to implement move semantics in custom classes to prevent unnecessary copying and to improve performance.

💡R-value references

R-value references in C++ are a type of reference that can bind to temporary objects. They are denoted by an ampersand prefix followed by two ampersands (&&). The script uses r-value references in the context of move constructors and move assignment operators, which are designed to take advantage of r-values to perform moves instead of copies.

💡L-values and R-values

L-values and R-values are classifications of expressions in C++. An L-value refers to an object that persists beyond a single expression, allowing its address to be taken, while an R-value represents temporary objects that do not have a persisting address. The video script refers to these concepts to explain the conditions under which move semantics can be applied.

💡std::move

std::move is a function in C++ that is used to cast an object to an r-value reference, indicating that the object can be safely moved from. In the script, std::move is discussed as a utility function that enables the move constructor or move assignment operator to be invoked, facilitating the transfer of resources from a temporary or existing object.

💡Move constructor

A move constructor is a special constructor in C++ that takes an r-value reference as its parameter, allowing the transfer of resources from a temporary object to a newly constructed object. The script explains how to implement a move constructor in a custom string class to optimize the transfer of data without unnecessary copying.

💡Move assignment operator

The move assignment operator is a member function of a class that overloads the '=' operator to handle the transfer of resources from one object to another when the right-hand side is an r-value. The script demonstrates how to implement this operator to move resources from one string object to another, leaving the source object in a valid but unspecified state.

💡Skillshare

Skillshare is an online learning community mentioned in the script as a sponsor of the video. It offers a wide range of classes on creative topics, and the script highlights its value for learning new skills. The mention of Skillshare serves as an example of how educational platforms can complement technical learning, such as C++ programming.

💡Rule of Three

The Rule of Three in C++ is a guideline that suggests that if a class requires a custom copy constructor, copy assignment operator, or destructor, it should also provide the other two. The script refers to an extension of this rule, the Rule of Five, which includes move constructors and move assignment operators, emphasizing the importance of considering all special member functions for resource management.

💡Destructor

A destructor in C++ is a special member function of a class used to free resources and perform cleanup when an object's lifetime ends. The script discusses the role of the destructor in the context of move semantics, ensuring that no resources are leaked after a move operation by properly deallocating memory.

💡Self-assignment

Self-assignment occurs when an object is assigned to itself. The script mentions the importance of handling self-assignment in the move assignment operator to avoid unnecessary operations that could lead to undefined behavior or resource leaks. It suggests checking if the source and destination objects are the same before performing the move.

Highlights

Introduction to move semantics and the importance of previous videos in the series.

Explanation of STD move and its usage.

Introduction to the Move assignment operator.

Difference between move constructors and move assignment operators.

Example of using STD move with a string object.

Importance of making an object temporary for move semantics.

Code example showing move assignment operator implementation.

Explanation of deleting old data to avoid memory leaks.

Importance of checking for self-assignment in move assignment operators.

Testing move assignment with a string example.

Difference between assignment operators and constructors.

Use of STD move to convert an existing variable into a temporary.

Rule of fifths and its relevance to move semantics.

Distinction between move constructors and move assignment operators.

Encouragement to use STD move for converting objects to temporaries.

Transcripts

as of guys my name is Deshawn and

welcome back to my C++ series so today

we're gonna be continuing on with our

whole move semantics journey if you guys

have not seen the other videos in this

kind of a miniseries and definitely

check them out I'll have one linked up

there that's going to be about move

semantics in general but there's also

another video about L values and our

values for sure you know this whole

moving journey make sure that you check

those videos out because they're going

to be important for what we'll be

discussing today so so far we've we've

basically learned the the gist and all

of the basics of move semantics what it

means to actually be able to move one

object into another however we haven't

covered a crucial part of that or should

I say two crucial parts and instead of

making two videos one on each topic I

figured I'd combine them into this video

because they're not particularly

complicated topics and I think we can

just get them done in one video and that

is STD move so what is that function

what does it do when do we use it when

do we need to use it and then also

something called the Move assignment

operator and this is specifically an

assignment operator that we use when we

want to remove an object into an

existing object so we've covered move

constructors or a move constructor

that's what we did in the last episode

however what we haven't covered is what

happens if we instead of constructing a

new object we want to move an existing

object into another existing object and

that's what we're going to be talking

about today but first this video is

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learn a new skill speaking of learning

new skills let's get back to the move

assignment operator and as city you move

as always the best way to show this is

gonna be with an example so let's dive

into the code from last from the last

episode where we looked at move

semantics in general and let's upgrade

our string and NC class to be able to

handle move assignments and we'll also

be talking about SUV move while doing

that since that's an important part so

this is the string class from the last

video and here's our NC class as well

let's first talk about STD move and what

it does I'll actually use string as an

example here just because we seem to

have this kind of we where we have two

classes we have string we have empty and

entities like an extra level of

indirection entity itself in fact is an

example showing how to use string which

is the real thing we're interested in

moving so what happens here when we

decide to actually take in a string as a

temporary here is we're using STD move

and this is from the last video we had

to use it because this was a way for us

to move this name temporary which were

providing over here into this kind of

more or less permanent spot for it to

live in if I simplify this by writing it

over here I'll create a string and I'll

just set it to hello over here so now I

have a brand new string what happens if

I want to remove it into here maybe this

is the destination for my move well I

could assign it like this however what

that's doing is copying the string into

this new destination variable of course

so what do I do if I want to

specifically move it into this new

variable well I have to make it into a

temporary because that's what the move

constructor takes right if we look at

the string constructor here we have this

one which is our move constructor this

takes in and our value reference which

is a temporary object so in other words

it's really simple I have to make it use

this constructor because this little

blob of code here is what actually

steals the resources from other so in

other words that's the thing that

actually does the move so in order for

it to use that can

tractor I need to actually make sure

that this string this hollow string

becomes a temporary now and how do we

convert from one type into another

generally as he was bus we cast so what

I could do is I could cast this to be a

string temporary like this and what's

happening here is it's now constructing

a new string with a constructor that

takes in a string r-value reference here

so in other words this is the same as if

I had just written code like this

because of course that assignment

operator is just doing an implicit

conversion and call into this specific

string constructor so this is the code

that I end up with I'm now moving this

string into this new destination however

this is a little bit well it's not the

most elegant way of doing this and it

also doesn't work with every single type

for example if we just had something

that was order and the type couldn't be

deduced by ourselves writing the code

static like this it's just not not not a

great way to actually write code like

this so what we do instead is we use a

slightly more flexible function that

basically at compile time figures out

what type is coming in and that function

is called sv move so what this is is

just a utility function provided to us

by the standard C++ library that will

basically do what I just did but in a

slightly more elegant and flexible way

if we hit f12 over here individual

studio to get to it you can see what's

actually happening here it's a

consecration it returns a remove

reference T type of T Y and you can see

there's that our value reference here so

it takes in an argument here and it will

actually return that double reference

for us but it does so in a nicely

templated way and it will work properly

with all types including Const and all

of that stuff so instead of doing what I

did you should use STD move and it also

looks a little bit more elegant in my

opinion because it also gives a little

bit of context as to what's going on in

this case we're moving this string into

this destination now I want to emphasize

here that it doesn't really matter how

you write this code I could just have an

assignment operation going on over here

the thing is we're creating a new object

here this is not a move assignment that

is taking place here because as you can

see we've got the variable type out

front here and then the variable name

this is a brand new object that is being

constructed and as such it will actually

use the move

octa now this leads into the movie Simon

operator which is what I talked about an

assignment operator which I don't think

we've covered in general in this A+ for

Syria so that's definitely something

that will do soon but the assignment

operator only gets called when we

actually assign a variable into an

existing variable so in other words if

we now wrote something like desc equals

string or something like that or more

specifically STD move string to actually

move it in this case what would actually

happen is the assignment operator would

get called because this is already an

existing variable don't forget that an

operator is really just like a function

and so when you actually call the equals

kind of operator here which is the

assignment operator on an existing

variable it's kind of the same as if you

just had like a little assigned function

and did something like that clearly this

is happening on an existing variable

whereas over here like it wouldn't

really work because we're creating a new

variable so let's go up and define this

assignment operator so a regular

assignment operator is something that

we'll save for a different video of

course because is about the move

assignment operator but basically it's

going to be very very very similar to

the move constructor so this is going to

be a good starting point what I'll do is

I'll just copy all of this stuff here

and then I'll go down here and write

string reference so this is going to

return a string reference will write

operator equals which is our assignment

operator and then it's going to take in

that temporary object here we'll also

mark it as no except I'm gonna paste in

all of our code here and for the most

part that is basically it however there

is one very important thing missing here

what we're doing is we're not actually

constructing a new object like we were

in the move constructor we're actually

moving another object into ourselves

into this current object so clearly we

need to actually overwrite the current

object because the current object might

have some memory allocated and obviously

if we just do this if we just set m data

equal to another pointer we've created a

memory leak because there's no way for

us to actually delete that old data so

what we need to do is first of all

delete our current data because we're

about to move another object into our

cell

and then we can reassign all of our

variables and also make sure that the

other object is left in a valid state so

this is basically the most important

thing that we can add now typically in

assignment operators you also want to

make sure that this object that this

current object is not equal to another

object so in other words if we try to do

something somewhat silly such as

assigned destination to itself using

something like this right or I guess

more specifically we'd have to put it

into here if you try and do something

like this then this will not really work

because obviously what we're doing here

is we're deleting data and then we're

just reassigning everything I mean

what's gonna what what's gonna end up

happening is we just delete the data and

we've lost the data and that's it sighs

we'll still be kind of correct though so

it's in in general it's just gonna mess

everything up what we could do and

specifically the right answer here is to

just not do anything in this case

because if these objects are equal then

there's no removing necessary I mean

you're trying to move this object into

itself it's already there

there's no removing necessary and the

way we would typically implement this is

just by saying if this equals other just

like that and if you do something like

that and specifically does not equal

what we want if you if this is not equal

to other so in other words these are in

fact different objects then nothing

needs to be done and we can just return

this which is a reference to the current

object so in other words we only do the

move if they're not equal if they are

this same object literally and we don't

need to do anything if they're different

objects but the data is different you

still need to do the move because

they're still moving some other object

into yourself so this isn't really like

the assignment operator where you doing

a copy and if the data happens to match

you don't need to do anything so you

still need to do it and it's very easy

to just implement a little a little god

that does something like this so there

we go that is a complete move assignment

operator let's test it out so what I'll

do is I'll make and to actually make

this work we do have a default

constructor for string which is good

we're gonna kind of ignore the entity

class for this example I think and what

I'm going to do is basically come over

here and I'll make a string here called

well this will just be like our first

I'll just call it Apple and I'll set it

equal to Apple then I'll make another

string called our kind of destination

which is just going to be an empty

string here and then what I'll do is

I'll try and actually assign destination

to the Apple now obviously this is going

to cause an error here because we don't

have an actual copy assignment operator

so what we need to do is make sure this

is wrapped in a move otherwise it's not

even going to compile and what should

happen here is we should move the

contents of Apple in to destination and

also leave Apple in a good State in a

valid State so let's print the value of

both of these that I'll print Apple

child call Apple dot print and then

destination dot print here and I'll also

do so after we do the move and we'll see

if the values have basically switched so

in other words since we're stealing

everything from Apple now it should be

left with nothing and we shouldn't get

any crashes or anything obviously upon

destruction okay so and there's some

other stuff going on here so I'm gonna

quickly get rid of all this other noise

okay so we have created them we we have

Apple and this is I just realized that

what I should also do is make sure that

I'm actually printing the value of these

very or rather the name of these

variables because the moment we're only

seeing the actual values okay so now if

we run this we create the object Apple

is set to Apple destination is set to

nothing at all then we move it and now

Apple is set to nothing at all and

destination is set to Apple so we've

basically transferred ownership of that

entire array of characters without doing

any copying or any allocating or

deallocating or anything like that we've

switched these two variables which is

really good and amazingly powerful and

that's it you can see both of the

objects gets to get destroy successfully

we don't get any crashes because in this

case the Apple destructor is doing

absolutely nothing since it's trying to

delete an array over here that is set to

null pointer and then the the

destination destructor is is destroying

our character successfully so that is

the end of the move assignment operator

and SCD move so in summary the move

assignment operator is something that

you want to include in your class when

you include a move constructor because

of course it might be desirable to

an object into an existing variable it's

basically part of something called the

rule of fifths or something called the

rule of thirds and then there's the rule

of fifths which basically just includes

all the new move semantics things we'll

talk about that more in depth in later

episodes of this equals law series as

well one more thing I quickly want to

show is that I did mention the

difference between destination and the

assignment operator versus if we had

just used the constructor like this keep

in mind that by using this operator it

is literally the same as if we had

written dot operator equals and called

it like a function which is why clearly

that would not work here because we're

not actually calling the assignment

operator we're just we're just

constructing a new string that happens

to use a temporary value which is why

the move constructors being called

whereas here we're actually calling a

function so that's that that's an

important distinction and one of the

things that used to confuse me in the

past because as you can see they both

look like assignment operators but

they're actually different this is the

move constructor this is the assignment

operator and STD move is what you want

to do when you want to convert an object

to be a temporary so in other words if

you need a existing variable to become

temporary in the sense that you're

basically marking it as hey you can

still resources from this particular

variable this is what enables us to

actually do the moves on an existing

variable if you're creating a new

variable and it's like in a function

parameter or in the move constructor or

something like that then it's already a

temporary it's fine but if you've got an

existing variable like with this Apple

example that was an existing variable

you need to make sure that you use sv

move to basically convert it into a

temporary and then that way you can

actually use the move constructor or the

move assignment operator to take the

resources from that variable and

actually do the move I hope you guys

enjoyed this video please don't forget

to hit the like button if you did if you

have any more questions about move

semantics in general hopefully these

kind of past videos have cleared up the

general complexity and the I mean I

guess the the fact that it seems

possibly more complicated and actually

is hopefully all that has been cleared

up for you but if there are still things

that you want to know leave a comment

below and I will try and help you out as

well as the community but that's pretty

much all there is to it

smooth semantics I hope you guys enjoy

don't forget to check out the free two

months of Skillshare by clicking the

link in the description below and I will

see you guys next time goodbye

[Music]