Testing for a maximum unsigned value

Question

Is this the correct way to test for a maximum unsigned value in C and C++ code:

if(foo == -1)
{
    // at max possible value
}

where foo is

Answer 1

Here's an attempt at doing this in C. It depends on the implementation not having padding bits:

#define IS_MAX_UNSIGNED(x) ( (sizeof(x)>=sizeof(int)) ? ((x)==-1) : \
                             ((x)==(1<

Or, if you can modify the variable, just do something like:

if (!(x++,x--)) { /* x is at max possible value */ }

Edit: And if you don't care about possible implementation-defined extended integer types:

#define IS_MAX_UNSIGNED(x) ( (sizeof(x)>=sizeof(int)) ? ((x)==-1) : \
                             (sizeof(x)==sizeof(short)) ? ((x)==USHRT_MAX) : \
                             (sizeof(x)==1 ? ((x)==UCHAR_MAX) : 42 )

You could use sizeof(char) in the last line, of course, but I consider it a code smell and would typically catch it grepping for code smells, so I just wrote 1. Of course you could also just remove the last conditional entirely.