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In computer science and software programming, a value is the representation of some entity that can be manipulated by a program. The members of a
type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * Ty ...
are the values of that type. The "value of a variable" is given by the corresponding mapping in the environment. In languages with
assignable variable In computer programming, a variable is an abstract storage location paired with an associated symbolic name, which contains some known or unknown quantity of information referred to as a ''value''; or in simpler terms, a variable is a named cont ...
s, it becomes necessary to distinguish between the ''r-value'' (or contents) and the ''l-value'' (or location) of a variable. In declarative (high-level) languages, values have to be referentially transparent. This means that the resulting value is independent of the location of the expression needed to compute the value. Only the contents of the location (the bits, whether they are 1 or 0) and their interpretation are significant.


Value category

Despite its name, in the C++ language standards this terminology is used to categorize expressions, not values.


Assignment: l-values and r-values

Some languages use the idea of l-values and r-values, deriving from the typical mode of evaluation on the left and right-hand side of an assignment statement. An l-value refers to an object that persists beyond a single expression. An r-value is a temporary value that does not persist beyond the expression that uses it. The notion of l-values and r-values was introduced by Combined Programming Language (CPL). The notions in an expression of r-value, l-value, and r-value/l-value are analogous to the parameter modes of input parameter (has a value), output parameter (can be assigned), and input/output parameter (has a value and can be assigned), though the technical details differ between contexts and languages.


R-values and addresses

In many languages, notably the C family, l-values have storage addresses that are programmatically accessible to the running program (e.g., via some address-of operator like "&" in C/C++), meaning that they are variables or de-referenced references to a certain memory location. R-values can be l-values (see below) or non-l-values—a term only used to distinguish from l-values. Consider the C expression . When executed, the computer generates an integer value of 13, but because the program has not explicitly designated where in the computer this 13 is stored, the expression is a non l-value. On the other hand, if a C program declares a variable x and assigns the value of 13 to x, then the expression has a value of 13 and is an l-value. In C, the term l-value originally meant something that could be assigned to (hence the name, indicating it is on the left side of the assignment operator), but since the reserved word (constant) was added to the language, the term is now 'modifiable l-value'. In C++11 a special semantic-glyph exists ( not to be confused with the && operator used for logical operations ), to denote the ''use/access of the expression's address for the ''compiler'' only''; i.e., the address cannot be retrieved using the address-of operator during the ''run-time'' of the program (see the use of move semantics). The addition of move semantics complicated the value classification taxonomy by adding to it the concept of an xvalue(expiring value) which refers to an object near the end of its lifetime whose resources can be reused (typically by moving them). This also lead to the creation of the categories glvalue(generalized lvalue) which are lvalues and xvalues and prvalues(pure rvalues) which are rvalues that are not xvalues. This type of reference can be applied to ''all'' r-values including non-l-values as well as l-values. Some processors provide one or more instructions which take an immediate value, sometimes referred to as "immediate" for short. An immediate value is stored as part of the instruction which employs it, usually to load into, add to, or subtract from, a register. The other parts of the instruction are the
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operat ...
, and destination. The latter may be implicit. (A non-immediate value may reside in a register, or be stored elsewhere in memory, requiring the instruction to contain a direct or indirect address .g., index register addressto the value.) The l-value expression designates (refers to) an object. A non-modifiable l-value is addressable, but not assignable. A modifiable l-value allows the designated object to be changed as well as examined. An r-value is any expression, a non-l-value is any expression that is not an l-value. One example is an "immediate value" (look below) and consequently not addressable..


In assembly language

A value can be virtually any kind of data by a given
data type In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
, for instance a string, a digit, a single letter. Processors often support more than one size of immediate data, e.g. 8 or 16 bit, employing a unique opcode and mnemonic for each instruction variant. If a programmer supplies a data value that will not fit, the assembler issues an "Out of range" error message. Most assemblers allow an immediate value to be expressed as ASCII,
decimal The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...
,
hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...
, octal, or binary data. Thus, the ASCII character is the same as or . The byte order of strings may differ between processors, depending on the assembler and computer architecture.


Notes


References

* * {{cite journal, first=Christopher, last=Strachey, author-link=Christopher Strachey, title=Fundamental Concepts in Programming Languages, journal= Higher-Order and Symbolic Computation, volume=13, pages=11–49, year=2000, doi=10.1023/A:1010000313106


External links


Value Object
Computer data Programming language concepts Type theory