bool to int conversion

Question

int x = 4<5;

Completely portable. Standard conformant. bool to int conversion is implicit!

§4.7/4 from the C++ 11 or 14 Standard, §7.8/4 from the C++ 17 Standard, §7.3.9/2 from the 20 Standard says (Integral Conversion)

If the source type is bool, the value false is converted to zero and the value true is converted to one.

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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 bool expands to _Bool.

[..] true which expands to the integer constant 1, false which expands to the integer constant 0,[..]

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.

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.

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++.