CODEWITHSUNDEEP
c++visual-studio-2012
PortMan
PortMan

Reputation: 4523

What does it mean when a char or wchar_t == -1?

In Visual Studio why would one compare the value of a character (previously a char, now a wchar_t) to the value of -1?

I have a project with multiple invalid checks against -1:

TCHAR ch;
...
if(ch==-1)

Coverity is telling me that ""ch == -1" is always false regardless of the values of its operands."

Now, this was originally written to use char values instead of wchar_t, and was upgraded when unicode support was needed. I need to figure out why the "char ch;" was being checked against -1 so that I can make a similar check against the wchar_t.

Upvotes: 0

Views: 467

Answers (3)

Cubbi
Cubbi

Reputation: 47448

The integer constant EOFis typically #defined to -1. If that is inded what your code is checking (depends on where ch came from), then the corresponding wchar_t comparison would be to WEOF, and the appropriate Windows TCHAR macro seems to be _TEOF.

PS: As absolutely correctly pointed out in comments, if a value was converted to char or to wchar_t, it is too late to compare it to EOF or WEOF: plenty of encodings use the char '\xff' as a valid letter, and that becomes indistinguishable from EOF after such conversion. The code is simply wrong: the variable ch needs to have type int/wint_t/_TINT for any such comparison to be valid.

Upvotes: 1

James Kanze
James Kanze

Reputation: 153967

What is the type of TCHAR? If it is an unsigned type really smaller than int, the expression can never be be true, since integral promotion will occur, resulting in a large positive integer, but still something less than UINT_MAX, where as -1 is UINT_MAX. (If TCHAR is something like unsigned short, for example, it can typically have values in the range 0...65365, which will convert to an int or an unsigned int without changing the value. -1, on the other hand, will typically convert to something like 4294967295 when converted to unsigned.)

If you're checking for end of file (e.g. after an std::iwstream.get()), then you should do so before converting the results from int. Even with char, it doesn't work; if plain char is unsigned, then it can never be -1, and if it is signed, then one valid character (ÿ in ISO 8859-1) will appear to be end of file.

Upvotes: 0

NewbiZ
NewbiZ

Reputation: 2508

Most of the time, -1/255 is associated to EOF

Upvotes: 0

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