C ++数学比较使用函数返回值不起作用

Question

Consider the following code:

   char foo[32] = "123456";
   printf("strlen(foo) = %d\n", strlen(foo));
   if ((5 - strlen(foo)) > 0)
   {
      //This statement prints because the comparison above returns true, why?
      printf("String must be less than 5 characters, test 1\n");
   }

   int tmp;
   if ((tmp = 5-strlen(foo)) > 0)
   {
      //This statement does not print and makes since
      printf("String must be less than 5 characters, test 2\n");
   }

As the comments indicate, I do not understand why a temporary variable is needed to store the result of the mathematical calculation before comparisons to another value work.

Answer 1

The reason why you get a true there is because the result of the subtraction is unsigned . This, in turn, is because size_t , the return type of strlen() , is unsigned, and it's large enough that int got converted to its type rather than the other way around.

When you assign the result of the subtraction to a signed int variable tmp , you make the result signed again, so the comparison works as expected.

In general, you should be very careful with subtraction of unsigned values if you suspect that the result might become negative. If you are not 100% sure, use a signed type, or avoid subtractions in the first place. For example, a legitimate replacement for your condition above would be

if (strlen(foo) <= 5)
    ...