Value (computer science)

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Template:Mergefrom In computer science, a value may be a number, literal string, array and anything that can be treated as if it were a number. In other words, label, subprogram, datatype, inheritance and control flow are not values in general. The exact definition varies across programming languages. Variables and subprogram calling sites are mostly treated as values.

The distinction from object is subtle but one can say that objects include the private memory storage referred to by targets of references while values are independent of storage and only contain public parts. For example, references are commonly expressed as values, notably a pointer in C programming language. In C, object references can be represented as a struct that contains a void pointer whose target structure is unknown to client code; the void pointer would refer to an object.

The use of a term value is useful in clarifying the exact meaning of assignments, copying, and comparison of objects. In the value model, for instance, objects itself are copied instead of references to it, they are treated as if they were numbers.

L-value and r-value

Some languages use the idea of l-value and r-value. L-values are values that have addresses, meaning they are variables or dereferenced references to certain place. R-value is either l-value or non-l-value — a term only used to distinguish from l-value. In C, the term l-value originally meant something that could be assigned (coming from left-value, indicating it was on the left side of the = operator), but since 'const' was added to the language, this now is termed a 'modifiable l-value'.

An lvalue is an expression that designates (refers to) an object. A non-modifiable lvalue is addressable, but not assignable. A modifiable lvalue allows the designated object to be changed as well as examined. An rvalue is any expression that isn't an lvalue, it refers to a data value that is stored at some address in memory.

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