Rvalues and Lvalues

Rvalues vs. Lvalues

Rvalues are

• temporary objects.
• objects without a name.
• objects from which we cannot get an address.
• always on the right side of an assignment operation.

The rest are lvalues. They can be on either side of an assignment operator.

An lvalue can be on the left hand side of an assignment operation. (and of course on the right hand side).

A const lvalue or an rvalue can not be on the left hand side of an assignment. They can only be on the right hand side of an assignment.

int lValue = 1998; // 1998 is an rvalue
lvalue = 2011; 
const int lValue2 = 2011;
lvalue2 = 2011; // ERROR

int defInt = int{};
int res = 2000 + 11;
auto func = []{std::cout << "2011" << std::endl;};

Lvalue and Rvalue references

• Lvalue references are declared by one & symbol.

• Rvalue references are declared by two && symbols.

Lvalues can only be bound to lvalue references. However, 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 rvalue references has higher priority.

Rvalue references: applications

Move semantics

• Cheap moving of objects instead of expensive copying.

• No memory allocation and deallocation.

• Non-copyable but movable objects can be transferred by value.

Perfect forwarding

• Forward an object without changing its rvalue/lvalue nature. This helps in function templates.

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In the next lesson, we’ll study an example of rvalue references.