异常状况评估
以下代码：
#include <stdint.h>
int main() {
uint8_t Byte;
if (Byte < 0) { }
}
发出以下编译警告：
main.cpp: In function `int main()':
main.cpp:6: warning: comparison is always false due to limited range of data type
这很好。但是，当我将条件更改为：时，
(1) if (true || (Byte < 0)) { }
我仍然收到警告，而我希望收到类似“比较始终为真...”的警告:)
如果我将字节声明更改为：
(2) uint32_t Byte;
警告就会消失。
我该如何解释这种行为？
我的系统是 RHEL 5.3 64 位，附带 gcc 4.1.2。
编辑：
(1) 不是问题，我只是误解了编译器警告。 它并没有说整个 if 是假，而是说“Byte < 0”。
所以问题只是（2）——为什么Byte类型会触发编译器警告。 常量“0”是 int 类型，因此它有 4 个字节。所以它一定与 uint8_t 与 int 的比较有关
The following code:
#include <stdint.h>
int main() {
uint8_t Byte;
if (Byte < 0) { }
}
emits the following compilation warning:
main.cpp: In function `int main()':
main.cpp:6: warning: comparison is always false due to limited range of data type
This is fine. But when I change condition to:
(1) if (true || (Byte < 0)) { }
I still receive the warning, while I expect to receive warning like "comparison is always true ..." :)
If I change Byte declaration to:
(2) uint32_t Byte;
warning disappears.
How can I explain the behavior?
My system is RHEL 5.3 64 bit shipped with gcc 4.1.2.
EDIT:
(1) is not a problem, I just misunderstood compiler warning.
It doesn't says that the whole if is false but rather "Byte < 0".
So the problem is only (2) - why Byte type triggers compiler warning.
Constant "0" is of type int, so its 4 bytes. So its must be related to comparison if uint8_t with int
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评论（1）
将无符号值与 0 有符号 int 进行比较时，该值首先（隐式）转换为 int。由于 uint8_t 介于 0 到 255 之间，因此转换为 32 位 int 时它为正。
另一方面，uint32_t 介于 0 和 2^32-1 之间，因此当转换为 32 位 int 时，它可能会换行并变为负值（所有大于或等于 2^31 的值都将转换为负 int事实上）。因此，您的比较并不总是正确的，编译器是正确的。
When comparing an unsigned value to your 0 signed int, the value is first (implicitly) cast into an int. As an uint8_t is between 0 and 255, it is positive when cast into a 32-bit int.
On the other hand, you uint32_t is between 0 and 2^32-1, so when cast into a 32-bit int, it may wrap and become negative (all values higher than or equal to 2^31 will be cast into negative int values, in fact). So, your comparison isn't always true, and the compiler is right.