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Incomputer science andsoftware programming, avalue is the representation of some entity that can be manipulated by a program. The members of atype are the values of that type.[1]
The "value of a variable" is given by the correspondingmapping in theenvironment.[2] In languages withassignable variables, it becomes necessary to distinguish between ther-value (or contents) and thel-value (or location) of a variable.[3]
Indeclarative (high-level) languages, values have to bereferentially 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.[citation needed]
Despite its name, in the C++ language standards this terminology is used to categorize expressions, not values.[4]: 8.2.1
Some languages use the idea ofl-values andr-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.[5]
The notion of l-values and r-values was introduced byCombined Programming Language (CPL). The notions in an expression of r-value, l-value, and r-value/l-value are analogous to theparameter 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.
In many languages, notably theC family, l-values havestorage 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 orde-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 expression4 + 9. 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 expressionx 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 wordconst (constant) was added to the language, the term is now 'modifiable l-value'. InC++11 a special semantic-glyph&& exists ( not to be confused with the&& operator used for logical operations ), to denote theuse/access of the expression's address for thecompiler only; i.e., the address cannot be retrieved using the address-of& operator during therun-time of the program (seethe 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.[6]
This type of reference can be applied toall r-values including non-l-values as well as l-values. Some processors provide one or more instructions which take animmediate 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 theopcode, 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 [e.g., index register address] to 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" (see above) and consequently not addressable.
A value can be virtually any kind of data by a givendata type, 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 asASCII,decimal,hexadecimal,octal, orbinary data. Thus, the ASCII character'A' is the same as65 or0x41. Thebyte order of strings may differ between processors, depending on the assembler and computer architecture.