Question

When a compiler finds a signed / unsigned mismatch, what action does it take? Is the signed number cast to an unsigned or vice versa? and why?

Answer 1

If operand are integral and there is an unsigned value, then conversion to unsigned is done. For example:

-1 > (unsigned int)1 // as -1 will be converted to 2^nbits-1

Conversion int->unsigned int is: n>=0 -> n; n<0 -> n (mod 2^nbits), for example -1 goes to 2^nbits-1

Conversion unsigned int->int is: n <= INT_MAX -> n; n > INT_MAX -> implementation defined

If the destination type is unsigned,
the resulting value is the least
unsigned integer congruent to the
source integer (modulo 2^n where n is
the number of bits used to represent
the unsigned type).

If the destination type is signed, the
value is unchanged if it can be
represented in the destination type
(and bit-field width); otherwise, the
value is implementation-defined.