Rvalue and Lvalue References

Lvalues and Rvalues

In C++, lvalues and rvalues are two basic categories of expressions:

• Lvalue: an object or memory location that has a name and can appear on the left side of an assignment;
• Rvalue: a temporary value or literal that does not have a persistent memory address and usually appears on the right side of an assignment.

Examples:

int x = 10;      // x is an lvalue, 10 is an rvalue
x = 20;         // x (lvalue) can be assigned a new value (rvalue 20)

Lvalue References

An lvalue reference allows you to create an alias for an existing variable (an lvalue). You declare it using a single &:

int a = 5;
int& ref = a;   // ref is an lvalue reference to a
ref = 8;        // changes the value of a to 8

• Lvalue references cannot bind to rvalues (temporaries or literals).

Rvalue References

An rvalue reference allows you to bind a reference to a temporary value (an rvalue). You declare it using &&:

int&& temp = 42; // temp is an rvalue reference bound to the temporary value 42

• Rvalue references cannot bind to lvalues (named variables).
• They are useful for optimizing code, such as moving resources instead of copying them.

Reference Binding Rules

• Lvalue references (&) bind to lvalues.
• Rvalue references (&&) bind to rvalues.

Example:

int x = 7;
int& lref = x;      // valid: lref binds to lvalue x
// int& lref2 = 8;  // error: cannot bind lvalue reference to rvalue

int&& rref = 8;     // valid: rref binds to rvalue 8
// int&& rref2 = x; // error: cannot bind rvalue reference to lvalue

Understanding these concepts helps you write safer, more efficient C++ code, especially when working with modern features like move semantics.