Lvalues

An lvalue (locator value) is an expression that designates a memory location and an object stored at that location. In C and C++, lvalues have identifiable storage and can typically be used on the left-hand side of an assignment, provided the object is modifiable. Expressions that denote objects, such as a variable name or a dereferenced pointer, are commonly lvalues; expressions that do not designate a persistent object, such as the result of an addition, are usually rvalues.

Not all lvalues are assignable. A const object, or an object that is otherwise non-modifiable, cannot be assigned to. The concept of lvalue interacts with the language’s value categories: C++ adds refinements such as prvalues (temporary values) and xvalues (expiring values) to distinguish different kinds of expressions. In C++, lvalues designate objects and can be bound to lvalue references; prvalues can be used to initialize objects or to be moved from, while xvalues represent certain expiring resources.

Examples help illustrate: in int x = 5; x is an lvalue, and you can refer to its address with &x. The expression x + 1 is not an lvalue; it yields an rvalue. If you have int* p; then *p is an lvalue (assuming the pointed-to object is modifiable), whereas the result of a function returning int is typically an rvalue unless that function returns an lvalue reference. Understanding lvalues is important for correct use of assignments, references, and operator overloading.