问题描述
#include <stdio.h>
int arr[] = {1,2,3,4,5,6,7,8};
#define SIZE (sizeof(arr)/sizeof(int))
int main()
{
printf("SIZE = %d
", SIZE);
if ((-1) < SIZE)
printf("less");
else
printf("more");
}
用gcc
编译后的输出是"more"
.为什么即使 -1
-1
条件也会失败?8
?
The output after compiling with gcc
is "more"
. Why the if
condition fails even when -1 < 8
?
推荐答案
问题在于你的比较:
if ((-1) < SIZE)
sizeof
通常返回一个 unsigned long
,所以 SIZE
将是 unsigned long
,而 -1
只是一个 int
.C及相关语言的提升规则是-1会在比较前转换为size_t
,所以-1
会变成一个非常大的正值(最大值unsigned long
).
sizeof
typically returns an unsigned long
, so SIZE
will be unsigned long
, whereas -1
is just an int
. The rules for promotion in C and related languages mean that -1 will be converted to size_t
before the comparison, so -1
will become a very large positive value (the maximum value of an unsigned long
).
解决此问题的一种方法是将比较更改为:
One way to fix this is to change the comparison to:
if (-1 < (long long)SIZE)
尽管这实际上是一个毫无意义的比较,因为根据定义,无符号值总是 >= 0,并且编译器很可能会就此警告您.
although it's actually a pointless comparison, since an unsigned value will always be >= 0 by definition, and the compiler may well warn you about this.
正如@Nobilis 随后指出的那样,您应该始终启用编译器警告并注意它们:如果您使用例如编译gcc -Wall ...
编译器会警告你你的错误.
As subsequently noted by @Nobilis, you should always enable compiler warnings and take notice of them: if you had compiled with e.g. gcc -Wall ...
the compiler would have warned you of your bug.
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