In source files which I am using in my project, there is a comparison between ssize_t and size_t variables:
ssize_t sst;
size_t st;
if(sst == st){...}
I would like to get rid of the warning:
warning: comparison between signed and unsigned integer expressions
But I am not sure, which variable should I cast to the other?
if((size_t)sst == st){...}
or
if(sst == (ssize_t)st){...}
What is safer, better, cleaner? Thanks
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.
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).
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.
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).
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.
Either will work fine as long as both values fit in the positive representable range of ssize_t.
If either value doesn't, you could end up in trouble - check those cases before testing for equality:
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++.)
