To watch the youtube video explaining this topic, click here.
Prerequisite: You Should Know About Pointers And References
Make sure you check out the blogpost on pointers and references, because this topic will be a continuation of that.
In this blog post, we'll be going over what R-Values (and L-Values) are, what R-Value References are, and what Move Semantics are. We'll also look at std::move. If you're a good, modern C++ programmer, then you should know this.
Note: It's possible to get more technical (and accurate) about the concepts I'll mention, but we'll keep it simple to start out with.
What Are L-Values and R-Values
L-Values and R-Values are category types that describe variables, function returns, and objects in C++.
An L-value is stored in memory and will exist on the next line of code.
An R-Value is temporary and won't exist (in memory) on the next line of code.
Let's go through some code examples.
A simple rule of thumb (that isn't entirely accurate) is "if it can be on the left side of an equal sign, it's an L-Value. Otherwise, it's an R-Value."
Okay, so remember references from the last blog post? The & sign that means a variable refers to something that already exists in memory (instead of making a copy)? Well, the & sign only works with L-Value references. If you use the && sign, then you mean you want an R-Value reference, specifically.
Let's look at some code to see how this works.
Make sure you compile with C++ version 11 or higher.
The copy way works like this:
The R-Value reference way works like this:
This type of reference is powerful: we no longer have to spend time and space with copies, we can just keep the same variables (or objects) in memory that would have been removed.
std::move is a standard library function that takes an L-Value or R-Value, and casts it to an R-Value.
In the following code, move will convert the L-Value to an R-Value, which means that the R-Value will go into the overloaded function meant for R-Values.
So why use std::move? Use it when you want to convert an L-Value to an R-Value (presumably to call an overloaded function). The primary use case is when you don't care about what happens to the value of the variable/object after you use it, as seen in move semantics.
Move Semantics is a simple concept, but you need to understand and use the stuff from the previous sections to really make use of it: Copying is expensive. If we don't care about what happens to the value of a variable/object after using it, then use R-Value References.
The way you actually use move semantics is through a special move constructor/assignment operator, that will be called instead of a copy constructor/assignment operator. The difference will be that the move constructor/assignment operator will have a && sign.
(This is also what makes the Rule of 3 into the Rule of 5.)
It's also possible to have special move functions in a class implemented depending on certain situations.
One very easy example to see who move semantics is good is with a swap function. With a normal "copy" version of swap, there's an extra copy being used.
What's really going on is something like this:
You see that an extra T (which could be a variable, an object, a container, etc.) named tmp must be generated in RAM for the swap to occur. If we're dealing with a large container like a vector, this is bad. Move semantics is better.
The following code will be more efficient (assuming that class T has a move constructor defined).
The following code is explained in the following picture:
The above code generally changes depending on what T's move constructor will do, but in a simple case, we can see that tmp temporarily points to the exact place in memory for item a, let's a point to b's item, and then let's b point to a's item.
This is much more efficient, as we don't need to allocate memory for an extra temporary variable/container/object.
In this blog post, we covered the topic of R-Values, R-Value Reference, std::move, and the concept of Move Semantics in C++. However, it's a bit hard to really grasp the concept without actually writing code and thinking about how to define a move constructor and a move assignment operator.
In a future blog post, we're going to implement the Rule of 5 by making our own Vector class, and it'll teach us how to actually make and code with Move Semantics in mind.
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Hi, I'm srcmake. I play video games and develop software.
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