int x = 4<5;
Completely portable. Standard conformant. bool to int conversion is implicit!
§4.7/4 from the C++ Standard says (Integral Conversion)
If the source type is bool, the value
falseis converted to zero and the valuetrueis converted to one.
As for C, as far as I know there is no bool in C. (before 1999) So bool to int conversion is relevant in C++ only. In C, 4<5 evaluates to int value, in this case the value is 1, 4>5 would evaluate to 0.
EDIT: Jens in the comment said, C99 has _Bool type. bool is a macro defined in stdbool.h header file. true and false are also macro defined in stdbool.h.
§7.16 from C99 says,
The macro
boolexpands to _Bool.[..]
truewhich expands to the integer constant1,falsewhich expands to the integer constant0,[..]
Section 6.5.8.6 of the C standard says:
Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.) The result has type int.
You tagged your question [C] and [C++] at the same time. The results will be consistent between the languages, but the structure of the the answer is different for each of these languages.
In C language your examples has no relation to bool whatsoever (that applies to C99 as well). In C language relational operators do not produce bool results. Both 4 > 5 and 4 < 5 are expressions that produce results of type int with values 0 or 1. So, there's no "bool to int conversion" of any kind taking place in your examples in C.
In C++ relational operators do indeed produce bool results. bool values are convertible to int type, with true converting to 1 and false converting to 0. This is guaranteed by the language.
P.S. C language also has a dedicated boolean type _Bool (macro-aliased as bool), and its integral conversion rules are essentially the same as in C++. But nevertheless this is not relevant to your specific examples in C. Once again, relational operators in C always produce int (not bool) results regardless of the version of the language specification.
There seems to be no problem since the int to bool cast is done implicitly. This works in Microsoft Visual C++, GCC and Intel C++ compiler. No problem in either C or C++.
Source: Stackoverflow.com