c++ - Using -1 as a flag value for unsigned (size_t) types -

i using -1 flag value function return type size_t (an unsigned type).

i didn't notice @ first, particularly because wasn't causing errors in code (i checking x == -1, not x < 0).

are there subtle reasons shouldn't leave is? when might behave unexpectedly? commonly used?

ptrdiff_t less common, takes longer type, , anyway it's not appropriate type since function returns index array.

-1 convert max unsigned value, due section 4.7 integral conversions:

if destination type unsigned, resulting value least unsigned integer congruent source integer (modulo 2n n number of bits used represent unsigned type). [ note: in two’s complement representation, conversion conceptual , there no change in bit pattern (if there no truncation). —end note ]

the same quote c99 6.3.1.3:

otherwise, if new type unsigned, value converted repeatedly adding or subtracting 1 more maximum value can represented in new type until value in range of new type.⁴⁹⁾

so end with:

-1 + (umax + 1) 

which is:

umax