l-value and r-value（左值和右值）

搞了这么久程序了，对这个一直都没有搞的特别清楚，这种态度真可悲。

以下摘自MSDN。

L-Value and R-Value Expressions

("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.")。

指向内存位置的表达式被称为左值（l-value）表达式。一个左值代表着一个存储区域的定位符（locator）的值，或者一个“左边”的值，暗示着它可以出现在等号的左边。左值经常是标识符。

（"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.")。

表达式指向可修改位置的，被称为“可修改的左值”。一个可修改的左值不能是一个数组类型，一个不完整的类型，或者一个带有const属性的类型。structure和union可以是可修改的左值，它们不能有任何带有const属性的成员。当变量的值是那个位置存储的值，标识符的名字指定了一个存储地址。

（"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."）

如果一个标识符指向一个内存地址，和如果它的类型是arithmetic，structure，union，或者一个指针，那么它就是一个可修改的左值。例如，如果一个ptr是一个指向存储区域的指针，那么*ptr就是一个可修改的左值，指定了这个存储区域。

 

("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 does not evaluate to an array

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

• A unary-indirection (*) expression that does not 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 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.")