Rvalues and Lvalues

Rvalues are

• Temporary objects
• Objects without a name
• Objects from which we can not get the address

If one of these characteristics holds for an object, it is an rvalue. On the other hand, values with a name and an address are lvalues.

• Lvalues can be on the left side of an assignment operator.
• Rvalues can only be on the right side of an assignment operator.

A few examples of rvalues:

int five= 5;
std::string a= std::string("Rvalue");
std::string b= std::string("R") +  std::string("value");
std::string c= a + b;
std::string d= std::move(b);

As previously stated, Rvalues are on the right side of an assignment. The value 5 and the constructor call std::string("Rvalue") are rvalues since we cannot determine either the address or the name of the value 5. The same holds for the addition of the rvalues in the expression std::string("R") + std::string("value").

The addition of the two strings a + b is notable. Both strings are lvalues, but the addition creates a temporary object. A special use case is std::move(b). The new C++11 function converts the type of the lvalue b into an rvalue reference.

A Few Exceptions

Earlier, we stated that rvalues are on the right side of an assignment, and lvalues can be on the left side of an assignment. It is important to note that this is not always true:

const int five= 5;
five= 6;

Even though the variable five is an lvalue, it is also a constant, so it cannot be used on the left side of an assignment operator.

Lvalue references are declared by one & symbol. Rvalue references are declared by two && symbols. Lvalues can be bound to lvalue references, and rvalues can be bound to rvalue references or constant lvalue references.

MyData myData;

MyData& lvalueRef(myData);

MyData&& rvalueRef(MyData());
const MyData& constLValueRef(MyData());

The binding of rvalues to rvalues references has higher priority.