演员ssize_t或size_t

In source files which I am using in my project, there is a comparison between ssize_t and size_t variables:

在我的项目中使用的源文件中，ssize_t和size_t变量进行了比较:

ssize_t sst;
size_t st;

if(sst == st){...}

I would like to get rid of the warning:

我想消除这个警告:

warning: comparison between signed and unsigned integer expressions

But I am not sure, which variable should I cast to the other?

但是我不确定，我应该把哪个变量赋给另一个呢?

if((size_t)sst == st){...}

或

if(sst == (ssize_t)st){...}

What is safer, better, cleaner? Thanks

什么更安全、更好、更干净?谢谢

2 个解决方案

#1

There is no one right answer to this question. There are several possible answers, depending on what you know a priori about the values that those variables may take on.

这个问题没有一个正确的答案。这里有几个可能的答案，取决于你对这些变量的值的先验认识。

If you know that sst is non-negative, then you can safely cast sst to size_t, as this will not change the value (incidentally, this is what happens if you have no cast at all).

如果您知道sst是非负的，那么您可以安全地将sst转换为size_t，因为这不会改变值(顺便说一下，如果您根本没有cast，这就是会发生的情况)。
If sst might be negative but you know that st will never be larger than SSIZE_MAX, then you can safely cast st to ssize_t, as this will not change the value.

如果sst可能是负数，但是您知道st永远不会大于SSIZE_MAX，那么您可以安全地将st转换为ssize_t，因为这不会改变值。
If sst might be negative, and st might be larger than SSIZE_MAX, then neither cast is correct; either one could change the value, resulting in an incorrect comparison. Instead, you would do the following if (sst >= 0 && (size_t)sst == st).

如果sst可能是负数，而st可能大于SSIZE_MAX，那么这两个cast都是不正确的;任何一个都可以改变值，从而导致不正确的比较。相反，您应该执行以下if (sst >= 0 && (size_t)sst = st)。

If you’re not absolutely certain that one of the first two situations applies, choose the third option as it is correct in all cases.

如果你不确定前两种情况中的一种适用，那么选择第三种，因为它在所有情况下都是正确的。

#2

Either will work fine as long as both values fit in the positive representable range of ssize_t.

只要两个值都符合ssize_t的正可表示范围，任何一个都可以正常工作。

If either value doesn't, you could end up in trouble - check those cases before testing for equality:

如果两个值都没有，您可能会陷入麻烦——在测试平等之前检查这些情况:

if ((sst >= 0) && (st <= SSIZE_MAX) && (sst == (ssize_t)st))
{
  ...
}

(I'm sure the C++ people will recommend you avoid the C-style cast entirely - I have no doubt someone will comment or answer and let you know the right way to do that in C++.)

(我肯定c++的人会建议您完全避免使用C-style cast——我确信有人会评论或回答，并让您知道使用c++进行这种操作的正确方法。)

#1