rvalue

Rvalue is a programming term used to describe an expression that yields a value and is typically not associated with a persistent storage location. In many languages, rvalues can appear on the right-hand side of an assignment but cannot generally be assigned to themselves. This contrasts with lvalue, or locator value, which denotes an expression that refers to an identifiable storage location and can appear on the left-hand side of an assignment.

In languages such as C and C++, an rvalue includes literals (for example, the number 5), the result of most arithmetic expressions (such as x + y), and temporary objects returned by functions. An lvalue, by contrast, refers to a named object or a location in memory, such as a variable or a dereferenced pointer. For example, in the statement int a = 3; the variable a is an lvalue, while the literal 3 is an rvalue.

In C++11 and later, the concept evolved into value categories, clarifying how values can be used and moved. The categories include glvalue (generalized lvalue), prvalue (pure rvalue), and xvalue (expiring value). These distinctions underpin move semantics and resource management, allowing efficient transfer of resources via rvalue references (type&&). For instance, objects can be moved from when passed as rvalues, enabling constructors and functions to take ownership of resources without unnecessary copying.

Across other languages, the formal terminology may vary, but the core idea remains: rvalues denote values or temporaries, typically not tied to a modifiable storage location, in contrast to lvalues which designate concrete storage locations.