c - 为什么 sizeof 类型与整数比较返回 false

Question

这个问题已经有答案了:

sizeof() operator in if-statement (5 个回答)

Why is sizeof(int) less than -1? [duplicate] (3 个回答)

void main() { if(sizeof(int) > -1) printf("true"); else printf("false"); ; [duplicate] (3 个回答)

Why does this if condition fail for comparison of negative and positive integers [duplicate] (6 个回答)

已关闭 4 年前。

我在下面的代码中注意到一个奇怪的行为。

#include<stdio.h>
#include <stdbool.h>

int main()
{
int x = sizeof(int) > -1;

bool z = sizeof(int);

printf("x is %d \t z is %d \n",x,z);
if(sizeof(int)>-1)
{
printf("true\n");
}
else
printf("false\n");
}

当 sizeof(int) > -1 为 true 且预期输出应为 1 时，为什么 int x 为零。

Answer 1

GCC 编译器警告说:

prog.c: In function 'main':
prog.c:6:21: warning: comparison of integer expressions of different signedness: 'long unsigned int' and 'int' [-Wsign-compare]
 int x = sizeof(int) > -1;
                     ^
prog.c:11:15: warning: comparison of integer expressions of different signedness: 'long unsigned int' and 'int' [-Wsign-compare]
 if(sizeof(int)>-1)

因此，您正在比较signed int 和unsigned long int。当 -1 转换为 unsigned long int 时，结果是最大可能的 unsigned long int 值(与 ULONG_MAX 相同) >)。

C $6.3.1.3，第 2 段:

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

有关更多信息，请阅读 cppreference .