JoelFernandes.org

Age is of no importance unless you're a cheese -- Billie Burke

Blog posts by category.
Archive of all blog posts.
Presentations and other work.

C++ rvalue references

| Comments

The author works in the ChromeOS kernel team, where most of the system libraries, low-level components and user space is written in C++. Thus the writer has no choice but to be familiar with C++. It is not that hard, but some things are confusing. rvalue references are definitely confusing.

In this post, I wish to document rvalue references by simple examples, before I forget it.

Refer to this article for in-depth coverage on rvalue references.

In a nutshell: An rvalue reference can be used to construct a C++ object efficiently using a “move constructor”. This efficiency is achieved by the object’s move constructor by moving the underlying memory of the object efficiently to the destination instead of a full copy. Typically the move constructor of the object will copy pointers within the source object into the destination object, and null the pointer within the source object.

An rvalue reference is denoted by a double ampersand (&&) when you want to create an rvalue reference as a variable.

For example T &&y; defines a variable y which holds an rvalue reference of type T. I have almost never seen an rvalue reference variable created this way in real code. I also have no idea when it can be useful. Almost always they are created by either of the 2 methods in the next section. These methods create an “unnamed” rvalue reference which can be passed to a class’s move constructor.

When is an rvalue reference created?

In the below example, we create an rvalue reference to a vector, and create another vector object from this.

This can happen in 2 ways (that I know off):

1. Using std::move

This converts an lvalue reference to an rvalue reference.

Example:

#include <iostream>
#include <vector>

int main()
{
    int *px, *py;
    std::vector<int> x = {4,3};
    px = &(x[0]);
 
    // Convert lvalue 'x' to rvalue reference and pass
    // it to vector's overloaded move constructor.
    std::vector<int> y(std::move(x)); 
    py = &(y[0]);

    // Confirm the new vector uses same storage
    printf("same vector? : %d\n", px == py); // prints 1
}

2. When returning something from a function

The returned object from the function can be caught as an rvalue reference to that object.

#include <iostream>
#include <vector>

int *pret;
int *py;

std::vector<int> myf(int a)
{
    vector<int> ret;

    ret.push_back(a * a);

    pret = &(ret[0]);

    // Return is caught as an rvalue ref: vector<int> &&
    return ret;
}

int main()
{
    // Invoke vector's move constructor.
    std::vector<int> y(myf(4)); 
    py = &(y[0]);

    // Confirm the vectors share the same underlying storage
    printf("same vector? : %d\n", pret == py); // prints 1
}

Note on move asssignment

Interestingly, if you construct vector ‘y’ using the assignment operator: std::vector<int> y = myf(4);, the compiler may decide to use the move constructor automatically even though assignment is chosen. I believe this is because of vector’s move assignment operator overload.

Further, the compiler may even not invoke a constructor at all and just perform RVO (Return Value Optimization).

Quiz

Question:

If I create a named rvalue reference using std::move and then use this to create a vector, the underlying storage of the new vector is different. Why?

#include <iostream>
#include <vector>

int *pret;
int *py;

std::vector<int> myf(int a)
{
    vector<int> ret;

    ret.push_back(a * a);

    pret = &(ret[0]);

    // Return is caught as an rvalue ref: vector<int> &&
    return ret;
}

int main()
{
    // Invoke vector's move constructor.
    std::vector<int>&& ref = myf(4);
    std::vector<int> y(ref); 
    py = &(y[0]);

    // Confirm the vectors share the same underlying storage
    printf("same vector? : %d\n", pret == py); // prints 0
}

Answer

The answer is: because the value category of the id-expression ‘ref’ is lvalue, the copy constructor will be chosen. To use the move constructor, it has to be std::vector<int> y(std::move(ref));.

Conclusion

rvalue references are confusing and sometimes the compiler can do different optimizations to cause further confusion. It is best to follow well known design patterns when designing your code. It may be best to also try to avoid rvalue references altogether but hopefully this article helps you understand it a bit more when you come across large C++ code bases.

Comments