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L-Value and R-Value Expressions

  • Artikkeli

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 can't 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:

  • An identifier of integral, floating, pointer, structure, or union type

  • A subscript ([ ]) expression that doesn't evaluate to an array

  • A member-selection expression (-> or .)

  • A unary-indirection (*) expression that doesn't refer to an array

  • An l-value expression in parentheses

  • 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 isn't lengthened through the cast. (For more information, see Type-Cast Conversions.) 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

See also

Operands and Expressions