Condition assignment to boolean

Question

Emulating booleans in C can be done this way:

int success;
success = (errors == 0 && count > 0);
if(success)
   ...

With stdbool.h included following could be done:

bool success;
success = (errors == 0 && count > 0) ? true : false;
if(success)
   ...

From what I understand logical and comparison operators should return either 1 or 0. Also, stdbool.h constants should be defined so that true == 1 and false == 0.

Thus following should work:

bool success;
success = (errors == 0 && count > 0);
if(success)
   ...

And it does work on compilers that I have tested it with. But is it safe to assume it is portable code? (Assume that stdbool.h exists)

Is the situation different on C++ compilers as bool is internal type?

Answer 1

It is safe to assume. In C99, upon conversion to the _Bool type, all non-zero values are converted to 1. This is described in section 6.3.1.2 in the C99 standard. The equality and relational operators (e.g. ==, >=, etc) are guaranteed to result in either 1 or 0 as well. This is described in section 6.5.8 and 6.5.9.

For C++, the bool type is a real Boolean type where values are converted to true or false rather than 1 or 0, but it is still safe to assign the result of an == operation etc. to a bool and expect it to work, because the relational and comparison operators result in a bool anyway. When true is converted to an integer type it is converted to 1.