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L-Value and R-Value Expressions | Microsoft Docs |
11/04/2016 |
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b790303e-ec6f-4d0d-bc55-df42da267172 |
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mikeblome |
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ghogen |
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Expressions that refer to memory locations are called "l-value" expressions. An l-value represents a storage region's "locator" value, or a "left" value, implying that it can appear on the left of the equal sign (=). L-values are often identifiers.
Expressions referring to modifiable locations are called "modifiable l-values." A modifiable l-value cannot have an array type, an incomplete type, or a type with the const attribute. For structures and unions to be modifiable l-values, they must not have any members with the const attribute. The name of the identifier denotes a storage location, while the value of the variable is the value stored at that location.
An identifier is a modifiable l-value if it refers to a memory location and if its type is arithmetic, structure, union, or pointer. For example, if ptr is a pointer to a storage region, then *ptr is a modifiable l-value that designates the storage region to which ptr points.
Any of the following C expressions can be l-value expressions:
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An identifier of integral, floating, pointer, structure, or union type
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A subscript ([ ]) expression that does not evaluate to an array
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A member-selection expression (-> or .)
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A unary-indirection (*) expression that does not refer to an array
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An l-value expression in parentheses
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A const object (a nonmodifiable l-value)
The term "r-value" is sometimes used to describe the value of an expression and to distinguish it from an l-value. All l-values are r-values but not all r-values are l-values.
Microsoft Specific
Microsoft C includes an extension to the ANSI C standard that allows casts of l-values to be used as l-values, as long as the size of the object is not lengthened through the cast. (See Type-Cast Conversions for more information.) The following example illustrates this feature:
char *p ;
short i;
long l;
(long *) p = &l ; /* Legal cast */
(long) i = l ; /* Illegal cast */
The default for Microsoft C is that the Microsoft extensions are enabled. Use the /Za compiler option to disable these extensions.
END Microsoft Specific