Defning SIZE_MAX

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本文详细解析了在C语言中如何使用宏定义MY_SIZE_MAX来表示size_t类型的最大值。通过对表达式的逐步拆解，解释了为什么通过将-1转换为size_t类型可以得到该类型的最大值。

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http://www.velocityreviews.com/forums/t658743-defning-size_max.html

#define MY_SIZE_MAX ((size_t)-1)

(ozbear) writes:
> In another thread related to size_t and SIZE_MAX, a #define
> was suggested for SIZE_MAX where is it not predefined.
>
> The #define was:
>
> #define MY_SIZE_MAX ((size_t)-1)
>
> I have tried this on my system in a printf and it yields a
> reasonable value but I do not fully understand how it it
> evaluated. size_t is a /type/, defined on my system as
>
> typedef unsigned int size_t;
>
> The part I do not understand is the the meaning of subtracting 1
> from a /type/ rather than an expression? How does the compiler
> evaluate this?

As Ian Collins already posted, it's a cast, not a subtraction.

Looking at the expression in painful detail from the inside out:

``1'' is an integer constant.

``-1'' is an expression consisting of a unary "-" applied to ``1''.
It's value is -1, and its type is int.

``(size_t)-1'' is a cast expression, specifying that the result of the
expression ``-1'' is to be converted to type size_t. Given the the
rules for converting a signed int value to an unsigned type, the
result is the maximum value of type size_t.

Finally, we put parentheses around the whole thing because it's a
macro definition.

--
Keith Thompson (The_Other_Keith) kst- <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"