c - Comparing int with long and others

Question

I am wondering if things like this :

int a = ...;
long b = ...;

if (a < b)
     doSomethings();

always works (excepted for unsigned)

I just tested with a few values, but I want to be sure. I assume a is cast to long in the comparison, what about others type ?

Answer 1

int/long compare always works. The 2 operands are converted to a common type, in this case long and all int can be converted to long with no problems.

int ii = ...;
long ll = ...;
if (ii < ll)
   doSomethings();

unsigned/long compare always works if long ranges exceeds unsigned. If unsigned range was [0...65535] and long was [-2G...2G-1], then the operands are converted to long and all unsigned can be converted to long with no problems.

unsigned uu16 = ...;
long ll32 = ...;
if (uu16 < ll32)
   doSomethings();

unsigned/long compare has trouble when long ranges does not exceed unsigned. If unsigned range was [0...4G-1] and long was [-2G...2G-1], then the operands are converted to long, a common type that does not encompass both ranges and problems ensue.

unsigned uu32 = ...;
long ll32 = ...;

// problems
if (uu32 < ll32)  
   doSomethings();

// corrected solution
if (uu32 <= LONG_MAX && uu32 < ll32)  
   doSomethings();

// wrong solution
if (ll32 < 0 || uu32 < ll32)  
   doSomethings();

If type long long includes all the range of unsigned, code could use do the compare with at least long long width.

unsigned uu;
long ll;
#if LONG_MAX >= UINT_MAX
  if (uu < ll)  
#if LLONG_MAX >= UINT_MAX
  if (uu < ll*1LL)  
#else 
  if (uu32 <= LONG_MAX && uu32 < ll32)  
  // if (ll < 0 || uu < ll)  
#endif