What is rvalue reference in C++11

In this article we will discuss what is rvalue reference and how it is different from lvalue reference.

lvalue references

Prior to C++11 we only had references i.e. A Reference variable is an alias that always points to a an existing variable i.e.

int x = 7;

int & refX = x; // refX is a reference

With C++11 this reference has become lvalue reference and it can refer to lvalues only i.e.

int & lvalueRef = x; // lvalueRef is a lvalue reference

Here, lvalueRef is a lvalue reference pointing to a lvalue x. A lvalue reference can not point to a rvalue i.e.

int & lvalueRef2 = (x+1); // COMPILE Error - lvalue Reference Can't point to rvalue

Here lvalueRef2 is a lvalue reference, so it cannot point to a rvalue.

rvalue Reference

rvalue references are introduced in C++11 and rvalue references can do what lvalue references fails to do i.e. a rvalue reference can refer to rvalues.

Declaring rvalue reference

To declare a rvalue reference, we need to specify two & operator i.e. &&.

int && rvalueRef = (x+1); // rvalueRef is rvalue reference

Here, rvalueRef is rvalue reference which is pointing to a rvalue i.e. (x+1).

Let’s see an another example,

int getData()
{
	return 9;
}

getData() is a rvalue. So, only rvalue reference can refer to it. If lvalue reference will try to refer to it then will result in compile error i.e.

int & lvalueRef3 = getData(); // Compile error - lvalue Reference Can't point to rvalue

Although const lvalue reference can refer to temporary object returned by getData() but as its a const reference, so we can not modify this temporary.

const int & lvalueRef3 = getData(); // OK but its const

But a rvalue reference can refer to rvalue without const i.e.

int && rvalueRef2 = getData();

Now, as we understand the rvalue references, the next question that comes in mind is, What was the need of rvalue references in C++11 ?

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Well, we will discuss it in next articles.