Why is this comparison always true?

Question

I have the following code in my file:

unsigned char * pData = new unsigned char...

...

if(pData[0] >= 160 && pData[0] <= 255)

When I compile it, I get a warning from the compiler (gcc):

Warning: comparison is always true due to limited range of data type

How can this be? Isn't the range of an unsigned char 0-255? I'm confused.

Answer 1

If the range of unsigned char is from 0 to 255 and pData[0] is a char then pData[0] <= 255 will always be true.

Answer 2

The expression pData[0] <= 255 is always true since the range of unsigned char is 0..255 (in your particular implementation).

It's only complaining about that bit of the expressions since pData[0] >= 160 can be true or false.

Keep in mind that the range of an unsigned char need not be 0..255 for all implementations (ISO C standards do not mandate this).

Answer 3

The second part of the comparison is redundant. It is always less than or equal to 255.