std::move and the Move Assignment Operator in C++
Summary
TLDRIn 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
- đ The video is part of a C++ series focusing on move semantics, continuing the journey from previous episodes.
- đ The host encourages viewers to watch prior videos on move semantics and L values for better understanding.
- đŻ The main topics for this video are `std::move`, the move assignment operator, and their applications in C++ programming.
- đ ïž The `std::move` function is introduced as a utility to efficiently move resources from one object to another without copying.
- đ `std::move` is used to cast an object to an r-value reference, enabling the use of move constructors or move assignment operators.
- đ The move assignment operator is explained as a special assignment operator used to move resources into an existing object.
- đĄ The video demonstrates how to implement the move assignment operator in a custom class, emphasizing the importance of handling self-assignment.
- â ïž 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.
- đ 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.
- đ The video includes a practical example of implementing and testing a move assignment operator in a custom string class.
- đ The host promotes Skillshare as an online learning platform offering a wide range of creative classes, with a special offer for the viewers.
- đ 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
đ§ 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.
đ ïž 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.
đŠ 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.
đĄ 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.
Mindmap
Keywords
đĄMove Semantics
đĄR-value references
đĄL-values and R-values
đĄstd::move
đĄMove constructor
đĄMove assignment operator
đĄSkillshare
đĄRule of Three
đĄDestructor
đĄSelf-assignment
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]
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